The mesoderm is proliferated by primitive streak. It is formed as two layers. In front of the primitive streak an area without mesoderm is present. It is called proamnion. After 48 hours of incubation the proamnion is also occupied by mesoderm. The mesoderm is divided into dorsal and intermediate and lateral mesoderms.

The notochordal cells arrange themselves to form a cylindrical, rod called notochordal process. It will begin at hensen's node and it slowly grows. Because of its growth the primitive streak is slowly reduced. By the end of gastrulation the primitive streak is reduced and incorporate into tail bud.
The dorsal mesoderm is located on either side of notochord. It is divided into segments. They are called somites. The first pair of somites will form after 21 hours of incubation. Afterwards, for every one hour one pair of somie will add. The 24 hours old embryo contains four pairs of somites.
The intermediate mesoderm connects the dorsal mesoderm with lateral mesoderm as a stalk. Afterwards it undergoes segmentation and give kidneys.
The lateral mesoderm extends on periphery of embryo, it is divisible into extra embryonic and embryonic mesoderms. This lateral mesoderm will split into two layers. The upper layer is called somatic mesoderm and inner layer is called splanchnic mesoderm. Ectoderm and somatic mesoderm will be called somatopleure. The splanchnic layer and endoderm will be called splanchnopleure. In between the two layers of mesoderm the space is called coelome.
Thus at the end of gastrulation specific organ forming areas started to develop.
The second step in gastrulation is the formation of primitive streak. At the posterior region of area pellucida in the mid dorsal line primitive streak will appear as a thickened area. It starts eight hours after incubation. The thickening is because convergence of cells of blastoderm towards the centre. Usually in the early stages the primitive streak is short and broad. It is called primitive streak. It gradually extends to the middle of blastoderm. At eighteen to nineteen hours of incubation, primitive streak is well developed. It is caikd definite primitive streak. Along the middle line of primitive streak a narrow furrow is develop called Primitive groove. The edges of groove are thick. They are called primitive folds. At the anterior end of groove a mass of closely packed cells will be present. It is called "hensen's node" or primitive knob. In the centre of this node a pit is present. It is called primitive pit. It represents the vestige of neurenteric canal. The primitive streak elongates along with this, area pellucida will also elongates. As the primitive streaks growing the cells from this region will invigilate into space between epiblast and hypoblast This process is called immigration. The immigrated cells will become prechordal plate, notochord, and mesoderm. Anterior to primitive streak the mesoderm cells will not migrate to primitive streak. This mesoderm free area is called proamnion. At this place head will develop.
Formation of endoderm: Endoderm of hypoblast develops as a single layer of cells in side of blastocoel. After the formation of endoderm, upper layer is called epiblast. There are different theories to explain the formation of endoderm.

Infiltration theory: This was proposed by Peter in 1923. According to this theory some cells in blastoderm which are loaded with yolk will fall into blastocoel. It starts from posterior end of blastoderm. From there the cells migrate forward one behind another and endoderm is formed.

Delamination theory : It was proposed by Spratt in 1946. Blastodenr. is two or three layered thick. The lower layer will separate-fcoijl the upper layer by splitting and the lower layer is called endoderm, upperr layers are called ectoderm. In between ectoderm and endoderm blastocoel is present.
Theory of involution: In 1909 Peterson Proposed this theory. According to this theory a slit like opening at the posterior side of blastoderm forms. Through this opening the blastoderm cells will role into the primary blastocoel. It forms an endoderm.
Theory of invaginaton: This was proposed by Jockobson in 1938. According to this theory the posterior end of blastoderm will invaginate in blastocoel as a small pocket. This becomes endoderm. In this way endoderm is formed.

Laying of the egg: Between 9 A.M. and 3 P.M., the egg is expelled from the cloaca of hen. At the time of laying formation of endoderm is completed. For further development it is to be incubated.

Incubation: When the egg is laid, the development is stopped. For further development it is to be kept at 38°C. This is done by hen by sitting over the egg. This is called incubation. Artificially eggs are incubated in incubators. For the hatching of egg 21 days are required.
In the upper region of oviduct fertilization will takes place. One sperm will penetrate into hens egg and fertilizes with the egg. The fertilized egg will travel through oviduct. It takes nearly 22 hours. Hence the early development of egg will take place in oviduct.

Cleavage: Cleavage is restricted to blastodisc. The presence of great quantity of yolk the cleavage is restricted to blastodisc. Such cleavage is called meroblastic or discoidal cleavage.
1. Cleavage: After five hours of fertilization the first cleavage will appear. It is confined to the centre of blastodisc. cannot completely divide the blastodisc. Blastomeres are not formed.
2. Cleavage: It takes place at right angles to first cleavage. Even because of second cleavage clear blastomeres are not formed.
3. Cleavage: It is vertical. It is in the two sides of first division. As a result of this division eight blastomeres are formed. But they do not show boundaries.
4. Cleavage: It takes place in such a way that eight central blastomeres and eight peripheral blastomeres will form. Only at this stage of division definite cells are formed. The central eight cells are completely separated from yolk.
After fourth cleavage the cleavages are irregular and a blastoderm is formed.
As the central cells are forming below these cells a horizontal cleft is formed. Because of this an upper layer of cells and a segmentation cavity is formed. This segmentation cavity is called blastocoel. These cells will undergo further division quickly. Hence above the segmentation cavity mass of cells will be present, in several layers. These cells have complete boundaries. The cells present towards the periphery are not separated from yolk. They are called marginal cells. This region is called zone of junction.
Area Pellucida & Area opaca:
The central cell mass of the blastoderm will be in four to five layers, they are lifted from the yolk. Hence the central part of the blastoderm is free from yolk. This region is transparent. It is called area pellicida. At the zone of junction the cells are in contact with that region is opaca. That region is called area opaca.
Shape: The 48 hours chick embryo will show this shape? (In mirror view). In this stage of development the head region shows prolific growth. Because of which the embryo shows this peculiar shape.
Flexure: In the head region bendings are seen. These bandings are called flexure. This bending is towards right and backwards. In 48 hrs. Chick embryo two flexures are seen. One is at the region of midbrain. Because of this fore and hind brain will become parallel. This is called cranial flexure.
In the posterior region of brain a cervical flexure is formed. This flexure turn the head towards right. This region will give neck of adult.

Torsion: The 24 hours chick embryo is flat. Its ventral surface is in contact with yolk. As the development proceeds the anterior end of embryo is turns towards right and hence the anterior part of embryo comes to lie on the left side of yolk. This twisting is called torsion. At the end of 48 hours chick twisting will reach the cervical flexure. It is nearly at 13th somite.
Brain: The brain, shows three regions. Fore brain shows telen­cephalon and diencephalon, the hind brain shows medulla oblongata.
Eye: The optic cup is two layered. Lens placode is started its development.

Ear: The invagination of optic vesicle is completed. It is connect­ed to ectoderm by endolymphatic duct.
Alimentary canal: Fore gut reaches 1/2 mm length.
Heart: It is 5' shaped. It is tubular. !t has not developed cham­bers sets.

Blood vessels: From the anterior end of heart two ventral aortae will arise. They run backwards and travel as dorsal aortae.

Kidney: Pronephrous is completely developed. Mesonephric tubules started development.

Somites: At 48 hours of incubations 25 pairs are seen. Mesonephric tubules will develop from intermediate mesoderm.
At 24 hrs. incubation period the chick embryo is oval in shape. The primitive streak is seen. From the hensen's node a notochord is developed.
In the 24 hrs. chick embryo four pairs of somities are present.
NEURALFOLDS: On either side of mid dorsal line the ectoderm becomes thick. It forms longitudinal folds. They are called neural folds. They include a space called neural groove, this neural groove is deep in anterior region, and the depth decreases gradually posterior region. Near the first somite region the folds will startrfusing. It will start after 24 hours of incubation. Posteriorly diverse. This area is called sinus rhomboidalis. In this region regressing primitive streak is developed.
GUT: At this incubation stage foregut is formed. Mid and hind guts are not formed. The opening of foregut into midgut is called anterior intestinal portal on either side of this the splanchanic mesoderm becomes thick.
MESODERM: It is present between ectoderm and endoderm. It occupies the entire blastoderm.

BLOOD ISLANDS: Around the area pellucida, area opaca is present. In this opaca region groups of mesenchyme cells will present. They are called blood islands. They produce embryonic blood vessels.
24hours chick embryo thumb18
AREA VASCULOSA & AREA VITELLINA: In the area opaca the region which contain blood islands, is called area vasculosa. The peripheral area without blood vessels is called area vitellina.
The fully formed and freshly laid hen's egg is large. It is 3cm. in diameter and 5cm. in length. It contains enarmous amount of yolk. Such egg is called macrolecithal egg. The egg is oval in shape. The ovum contains a nucleus. It is covered by yolk free cytoplasm. It .is 3mm. in diameter. It is seen on the animal pole. The entire egg is filled with yolk. This yolk has alternative layers of yellow and white layers. They are arranged concentri­cally arround a flask shaped structure called latebra. Below the blastodisc the neck of latebra expands. This is called nucleus of pander. Yellow yolk got its colour because of carotenoids White yolk layers are thin and yellow yolk layers are thick. Yolk is a liquid. It contains 49% water and 33% phospholipids 18% proteins, vitamins, carbohydrates.
The entire ovum is covered by plasma membrane. It is called plasmalemma. It is lipoprotein layer. This is ovum is covered by egg mem­branes.
Primary membranes: These membranes develop between oocyte and follicle. The primary membranes are secreted by follicle cells. It is called vitelline,membrane is come from two origins. Inner part is pro­duced by ovary. Outer part is from the falopian tube. (This is stated by Balinsky)

Secondary membranes: Oviduct secretes secondary mem­branes. Above vitelline membrane albumen is present. It is white in colour and it contains water and proteins. The outer layer of albumen is is called thin albumen. The middle layer of albumen is thick. It is called thick albumen, or dense albumen. The inner most albumen is very thick. It develops into chalazae. The chalazae are called balancers. They keep the ovum in the centre.

Shell membranes: Above the albumen two shell membranes are present. Towards the broad end of egg, in between the shell membranes an air space is present. This air space is formed when egg is laid cooled from 60°C to lesser temperature.
Shell: Above the shell membranes a shell is present, it is porous in nature. It is calcareous. This porous shell is useful for exchange of gases. In a freshly laid hen's egg shell is soft. Very soon it becomes hard.
Adaptive Convergence in Evolution
In this type of evolution, the organisms with completely different body organization possess superficial resemblance. It means that the organisms of different classes acquire similar characteristics independently and separately to avail similar environment. This is also known as ‘parallel evolution’.
Examples: The most common example of convergent evolution is the development of ‘wings’ in insects, flying reptiles (Draco), birds and flying mammals (Bats). All of them belong to different groups and possess one common character is the “development of wings” for flight.
The aquatic vertebrates - cartilaginous & bony fishes, aquatic reptiles (Extinct - Ichthyosaurs), and dolphins, porpoises and whales (aquatic mammals) are all superficially alike. All these are having stream-lined body with a median dorsal fin, a caudal fins and paired fin. In aquatic reptiles and mammals the forelimbs are modified into flippers. If a layman is asked, he says all are fishes because of their superficial resemblances and environment. But it is clearly evident that their, body organization is markedly different.
The burrowing mammals - Moles and gophers, forelimbs are modified for digging the soil. But these two belongs to two different orders (Moles- lnsectivora and gophers - Rodentia of the class mammalia.
Adaptive convergence Convergent Evolution Parallel evolution14
The convergent evolution leads to the production of analogous similarities among different groups of organisms. This indicates that evolution may lead to superficial resemblances. It could be concluded that in the origin of life from simple to more complex forms, both divergence and convergence have played a vital role. However, the divergence is more frequent at present than the convergence.
Adaptive Divergence

Evolution has been described as the process of gradual modification in the plants or animals (living organisms) basically two patterns are distinguished in the process of evolution. The minor changes in the gene pool of a population from one generation to the next may not produce new populations. The newly formed population is not genetically identical with its predecessor. This is called ‘Sequential evolution’. The changes occur in the newly evolved populations, species, families and classes is known as divergent evolution.

The animals of the same group are closely related groups exhibit great divergence in their morphology when they are found in different habitats. Prof. Osborn states that “each isolated region, if large and sufficiently varied in its topography, soil, climate and vegetation will give rise to a diverse fauna. The larger the region and more diverse the conditions, the greater will be varieties of animals found.” Therefore, the divergent evolution in specialized directions, starting from a common and generalised type or the entry of organisms of the original stock to new adaptive zones.

Example (1): The limb structure of placental mammals provides a classical example of divergent evolution. The ancestors of all the present day types of mammals can be traced back to a primitive insect eating five toed, short -legged creature walked with the soles of their flat feet. The pent dactyl limbs were not modified for any particular type of locomotion. These lived on land and formed as ancestors to the modern mammals. Now the modern mammals have occupied five different habitats. Therefore, divergence occurred in five lines for five different habitats with modification In their limb structure.
  1. The first line lead to “arboreal (climbing) modification “seen tree-dwelling forms like squirrels and primates.
  2. The second line acquired “aerial (flying) modification”, found in animals adapted for flight (Bat)
  3. The third line represents “cursorial (running) modification”. This type of mammals are adapted to fast running - Horse, Deer’s, Dogs etc.
  4. The fourth line acquired ‘fossorlal (burrowing) modification’, seen in moles.
  5. The last line lead to “aquatic (swimming) modification “found in seals, whales etc.
In all these lines, mammals exhibit the modified limb structure for some particular mode of locomotion, So these limb types are derived from one common type represented by short pentadactyl limbs of terrestrial mammals. This shows a relatively generalized ancestral group gives rise to many relatively more specialized descendents.
Example (2): Mammals possess’ heterodont dentition.

The incisors for biting, canines for tearing and grasping and the premolars and molars suited for grinding. The premolars and molars exhibit greatest structural modifications for different types of food.
  1. Insectivorous type Insect feeders - modified for crushing feeble prey.
  2. Carnivorous types: Meat eaters - modified by having high crowned with complicated cusps-carnasial.
  3. Herbivorous type modified for succulent vegetation & harsh grasses. Incisors are suited for cuffing the vegetation.
The toothed whales have become secondarily homodont with grasping teeth. In sperm - whales, the teeth are absent. This type evolution of group Is known as macro evolution.
A species is an ecological unit. The species is a group of potentially interbreeding natural populations which share in a common gene pool. But species are reproductively isolated from other such groups. A species comprises of several populations. There are two distinct ways in which new species arise from the pre-existing ones.
  1. Splitting of the species into two or more species is called Speciation.
  2. Transformations of the old species into a new one in due course of time is known as transformation in time.
It is very interesting to note that a species population usually has Discontinuous distribution. The species populations may in due course of time become separated by distance. Populations ‘A’ and ‘Z’ of a given species grow in size generation after generation. As such, the organisms radiate into progressively larger territory. After few generations these populations A & Z at the opposite ends of the territory may be too off. Thereby the direct gene flow among their members is not possible and they do not have reproductive contact directly.
The distance isolation is most common. These are other kinds of geographic isolations. The groups of related organisms become separated by some physical barriers like Sea1 mountain, desert, river. Moreover, homing instinct and territoriality oL animals also add to the isolation of individuals into groups populations.
Different types of speciation: There are three different types of speciation
i) Allopathic speciation or Geographic speciation:
The allopathic species have originated from the geographically isolated populations of a species.
  1. Geographic isolation of demes of widely spread out population of a species over a period of a number of generations.
  2. Genetic differences are accumulated independently in each deme.
  3. Genetic divergence leading to the establishment of reproductive isolation.
allopatric parapatric speciation19
So in alloptiric speciation, the population of a species divides and evolve into two or more new species. The basic factor in alloptric speciation is the geographic isolation. So it is also known as geographic speciation.

Example: In the Australian continent, a bird “Acanthiza pusilla” is widely distributed. It slightly differ from the Tasmanian species - ‘A. Ewingi’. It is presumed that during pleistocene glaciation, when the sea level is lower, Acanthiza entered Tasmania. With the rise of sea level it became isolated and differentiated into A. ewingi.
ii) Parapatric speciation:
White (1968-78) studied this type of speciation. He proposed that chromosomal aberration leads to partial reproductive isolation in the individuals of a population. Any chromosomal mutation which lowers fertility, gradually confers reproductive isolation in a very small population by genetic drift.

iii) Sympatric speciation:
This type of speciation arises due to some biological barrier for interbreeding. Polyploidy in plants leads to sympatric speciation.
If the hybrid between the two diploid species becomes tetraploid, it will be reproductively isolated from its diploid parents. Because the triploid offspring’s produced by back crossing with high ratio of aneuploid gametes which fail to survive.
As a general rule, a species is composed of a number of allopatric breeding populations, which are physically separated from one another to pursue independent evolutionary path. Each population is exposed to a different selection pressure. Therefore, random mutations along with genetic drifts and selection pressure establish genetic difference. The morphological and physiological variations are also present in the similar populations. These differences gradually accumulate and cause more and more divergence in the genetic constitution of populations.
Finally distinct sub-species is established. The geographical races or subspecies given an opportunity to interbreed or crossed artificially these produce fertile hybrids. The genetic modifications appeared so far have not reached to the e4ent of producing reproductive isolation. These may still exhibit preference of mating to the members of their own group.
Addition of certain more variations in their gene pool so as to affect their interbreeding, leads to reproductive isolation. The groups or subspecies or races become reproductively isolated and are ranked as species. The establishment of reproductive isolation is an event of biological significance.
The process of speciation occurs only in allopatric populations. It cannot act on sympatric populations inhabiting the same area. The sympatric species can arise either due to changes in the chromosome number or due to introgressive hybridization. The change in chromosome number may occur by polyploidy, aneuploidy, haploidy or translocation.
Example: Different species of ‘Drosophila’ have different number and Appearance of chromosomes.

The chromosomal composition of P. virilis is regarded to be the ancestral type. The chromosomal complement of D. pseudo obscura and D. persimilis could tiave derived from D. virilis by a translocation between X chromosome and one of the autosomes. The chromosome compliment of D. melenogaster could be derived by two translocations between the two pairs of autosomes.
Transformation of species in Time:
In the second method of evolution, in due course of time the transformation of a species into a new species takes place. in this type of evolution only one species exists at a time. For example, species ‘A’ evolves into ‘B’ and ‘B’ into ‘C’ and so on.
Sympson has recognized two types of Transformations
i) Phyletic evolution ii) Quantum evolution
i) Phyletic evolution: This type of evolution involves the sustained directional changes in the average characters of a population. This may be due to adaptations to a shifting environment or due to increasing specialization for a particular environment or improved adaptations in a constant environment. Thus the evolving forms present in a line of succession one being replaced by the other.It is presumed to lead to the origin of new genera and families.
Phyletic Quantum speciation thumb16
ii) Quantum evolution: It is related to the higher taxonomic groups like orders and classes. It involves rapid shift or sudden changes in the organization of a population to a new equilibrium. It is distinctly different from the ancestral forms and adapted to occupy new conditions. The quantum evolution is macro and mega evolutions operating above the species level.
The division of a single population into two or more groups because of some barrier for interbreeding is called Isolation. There are a number of processes by which two related populations living in the same area, can remain distinct These have been called Isolating mechanisms by Dobzhansky. According to him, the isolating mechanisms are classified into the following types.

i) Geographical Isolation: Two parts of one population are separated by some geographical barrier and arc prevented from interbreeding. Large bodies of water are barriers for land-dwelling animals. High Mountain ranges, deserts, dense forests and extremes of temperature serve as affective barriers. Such populations are completely ‘out of touch with each other genetically’ so that new mutations, genetic drift and the action of natural selection, in one population have rio effect on the other population. Thus, a new population may be developed.
ii) Environmental Isolation: Population living under different environmental conditions remains isolated from one another and are prevented from interbreeding. Environmental isolation depends upon differences in food habits and other physiological requirements of the animals.
For example, an insect which inhabits only coniferous frees, is environmentally isolated from an Insect which inhibits only the deciduous trees.
iii) Seasonal Isolation: The breeding season of two groups of animals or plants do not coincide..Th American toad. Bufo Americans, and the Fowler’s toad, B. fowleri have similar distribution and form fully fertile hybrids in the laboratory crosser. But in nature, they remain distinct because B. americanus breeds early in the season and B fowleri breeds late.
iv) Mechanical Isolation: The anatomy of the reproductive organs different from each other that copulation between males of one population and females of the other, is impossible.
The genitalia of a male will fit into those of a female of the same species as a key fit into a lock, but will not fit the genitalia of females of other species. Observations have not confirmed this theory.
Mechanicalisolation thumb
v) Physiological Isolation: Reproductive isolation may exist in those cases in which matings - between the males and females different populations take place. Patterson has shown that in some interspecific matings in Drosophila, the sperm fails to survive in the receptacles of the female of other species.

vi) Hybrid Sterility: Normal vigorous hybrids are formed but they are sterile and further exchange of genes is completely blocked. The Mule is a classical example of hybrid sterility.
Origin of Isolating Mechanisms
According to Muller, reproductive isolation is due to differences in genes that arise during the origin of sub-species and species in population.
According to Dobzhansky, reproductive isolation is the result of natural selection. Hybrids are either sterile or poorly adapted and are, therefore, eliminated by natural selection.
Isolation and Species Formation
Two populations become seperated from each other by means of geographic environmental barriers; each acquires new mutations and is acted upon by forces like genetic drift, natural selection, etc.
When a gene pool becomes divided by some geographic environmental factors, the allotrophic populations become differentiated so as to give rise to reproductive isolation. After the development of reproductive isolation, the populations may again come! into contact, still they remain distinct and &e said to be sympatric. Thus, geographic environmental isolation is this species formation. The development of the reproductive isolation brings new species formation.
Hardy Weinberg Law of Equilibrium:
The most fundamental idea in a population genetics was proposed by English-man G.H. Hardy and German W. Weinberg simultaneously in the year 1908. Later in 1929-30, the mathematical treatment of the distribution of gene and genotype frequencies in a population was developed principally by R.A. Fischer, JR. Haldane and Sewall Wright. The Hardy-Weinberg Law is the foundation of population genetics and of modem evolutionary theory.

This law can be defined as
‘The relative frequencies oi various kinds of genes in a large and randomly mating sexual panmictic population tend to remain constant from generation to generation in the absence of mutation, selection and gene flow”.
Hardy-weinberg’s law describes a theoretical situation in which a population is undergoing no evolutionary change. It explains that if the evolutionary forces are absent, the population is large and its individuals have random mating. Thus each parent produces equal number of gametes. Such gametes combine at random and the gene frequency remains constant. Finally the genetic equilibrium of the genes is maintained and the variability present in the population is preserved.
For example, suppose there is a panmictic population with gene ‘A’ will be the same is the frequency of gene ‘A’: Similarly the frequency of gametes with ‘a’ will be equal to the frequency of the gene ‘a’.

If the gametes unite at random, the total number of different genotypes will be.

There is a random union of the gametes with gene ‘A’ and ‘a’ at the Equilibrium state, the population will contain the following frequencies of the genotypes and genes ‘A’ and ‘a’ generation after generation.
AA + 2 Aa + aa (or) p2 + pq + 42 genotype frequency. in population of large size, the probability of receiving.
  1. The gent ‘A’ from both this parents will be p c p = p2, ii) for gene ‘a’ will be q x q = q2
  2. the probability of being heterozygote will be pq + pq = 2 pq.
As such the relationship between gene frequency and genotype frequency can be expressed as
p +2pq+q =1
- It is known as Hardy-Weinberg formula or binomial expression. it is clear that in a large and randomly mating population not only gene frequency but also the genotype frequencies will remain constant.
According to Hardy-Weinberg’ law:
  1. The gene and genotype frequencies of each allele in a population remain at an equilibrium generation after generation.
  2. In a population, the mating is a completely random manner.
  3. The equilibrium in the genotype and gene frequencies occurs on in large sized populations. But in small sized population gene frequencies may be un predict table.
  4. MI the genotypes in a population reproduce equally and successfully,
Example: In human populations, persons with gene T find weak solution of PTC (Phenyl -thio-carbamide) to be bitter in taste. But the homozygous ‘ft persons, the PTC is tasteless, Moreover, persons are unaware of their reaction to PTC and nobody selects his mate according to whether he or she can or cannot taste this substance. As such the marriages take place at random. Suppose, in a particular island or in a town the number of homozygous tasters (17) and of homozygous non tasters is equal, the probable marriages could occur as follows.

Therefore, the geno type frequency in the first generation will be TT-25% Tt = 50% and It = 25%. The homgous tasters (Ti’) and heterozygous tasters (Tt) are phenotypic ally alike. So the populations possess 75% tasters and 25% are non-tasters. The same results can be obtained if we consider the union of gametes at the time of fertilization.

There the genotype frequencies according to Hardy-Weinberg’s equation.
0.25 TT + 0.50 Tt.+ 0.25 ft (p - is frequency for gene 1)
p2 ÷2pq+q2 =l (q isfrequency for t )

This law provides a situation, where the genes in the population have reached the equilibrium and the gene pool is constant In such case, there will be no evolution. In nature, the mutations, natural selection, Non- random mating, genetic drifts and differential migration operate to change the genetic equilibrium actually can bring about organic evolution.


  • 11 Jun 2017
Genetic drift as one of the evolutionary force
The genetic architecture of small population changes irrespective of elective advantage or disadvantage. Analogously genes attain Hardy-Weinberg’s equilibrium in large populations only. The random changes in gene frequencies occurring by chance and not under the control of natural selection are called genetic drift. A series of steps and at each step the movement made is random, in directed which Is known as stochastic process.

The theory of genetic drift was developed by a geneticist SE WALL WRIGHT in 1930. It is also known as Sewall Wright effect or ‘scattering of variability’. It denotes that the random fluctuations in the gene frequencies In a small population from generation to generation.

In demes of limited sizes, random genetic drifts arise by chance. These cannot arise in large population. For example if we compare two populations of two extreme sizes - Population ‘A’ consisting of 5,000 breeding individuals and population ‘B’ of only 50, the gene pool of each contains equal number of Land I. If their gene frequencies are represented by p & q
pL= qI -0.5

In the next generation, the gene frequency is expected to deviate from the original 0. 5 by an amount of equal to the ‘standard error’
The standard error is determined as the sequence root of the product of original frequencies (p & q) divided by the number of genes available. This number of genes will be double the number of breeding organisms.
In small population B with organisms, the standard error would be J(o. 5 x 0.5 + 1oo) = 0.05. So in the next generation the gene frequencies will change to 0.45 and 0.55 either way. This amounts to 10% change in the gene frequency. Thus the standard error in a large population’s) + 10,000 = 0.0005) is negligible and it significantly high in a small population.

Effects of Genetic Drift on Gene frequency
In small populations or demes, the genetic drifts have the following, effects on the gene-frequency.

i) Homozygocity: In small populations, due to genetic drift gene frequencies continue to fluctuate until one of the allele lost and other fixed. This leads homozygosity in small populations. It means the genetic drift reduces genetic variability by eliminating one of the two alleles ‘either new or old one.
ii) Fixation of new mutations: Since genetic drift tend to eliminate one allele and fix the other one, irrespective of its dominance or recessiveness or advantageous or non advantageous nature. So a new mutation has 50% chances of either being lost or be fixed in small population.

iii) Genetic divergence: The demes become progressively genetically different. In each sub population, the genes fixed and lost will be different. Thus, In due course of time, (each deme gradually diversifies from the other sister demes) lead to the establishment of new species.
Genetic Drift and Evolution:
The role played by the genetic drifts actually in the evolution of organisms in nature is doubtful. A widely ranging broad base population is isolated into small sub groups - ‘DEMES’. The causes for isolation may be either on account of ecological or geographical discontinuities, home instinct. The size of these small demes is such that they appear to be affected by chance of events underlying genetic drift. The limited size of small breeding populations, the gene pool of their new generations may not be the same of the parental gene pools due to the action of genetic drift. The changed gene pools gradually lead to the formation of new species.

Founder Effect: Whenever a few organisms from large population encroaches a new or isolated geographical region, these form the “founders or ‘founder members’. The founders carry only a limited portion of the parental gene pool. The descendants of the founder i.e. the founder population or marginal isolates in a new area will tend to have ratios similar to the founders. The resemblance of the descendants of the founders is called founders effect’ or ‘founder principle’ (Maw).

The diffusion of genes into populations through migrations and interbreeding is known as Gene flow. The gene flow links all the demes of a population. It tends to counteract the loss of variability due to genetic drift in small population.
Rh gene (r) was introduced into the Chinese population by American immigrants. This Rh factor is associated with erythroblastosis fetalis or hemolytic disease in new born.
The study of the distribution of animals on the globe is called Zoo geography. The different kinds of animals living in a particular area forms the fauna. The geographical distribution of animals provides a good evidence for organic evolution.

The earth inhabits various kinds of animals. But all kinds of animals are not present in all regions of the earth. Hence the distribution is not uniform. It is mainly due to the difference in climatic conditions of various regions. But even though the climatic conditions are same in southern hemisphere, the fauna varies. For example the fauna of Africa belongs to Ethiopian region differs from the fauna of Madagascar even though both the regions are present in subtropics. The African fauna consists of Lions, Elephants, Rhinos, Chimpanzees and Gorilla. Where as Madagascar consists of primitive primates Lemurs. This can be explained that both the regions had the same fauna i.e lemurs during the early tertiary period. Due to the separation of Madagascar from main land by sea water, Lemurs were unable to migrate into new places to evolve into new individuals. Where as the African fauna migrated into new regions hence they evolved into new individuals.
The fauna of Australia differs from that of South America even though the climatic conditions are same. The fauna of South America located d in neotropical region consists of bears,llamas, sloths, armadillos etc. This is because Australia was separated from the main land, when the globe consisted of prototherians and metatherians. As these individuals are unable to migrate into new regions, they are confined to Australian region only. Where as these individuals in main land migrated into new climatic conditions, hence evolved into eutherian mammals. The South America was seperated from main land when the land consisted of armadillos, slothes etc.
zoo geography evolution29

The south and north poles consist of same ecological conditions. The south pole consists of penguins, where as the north pole consists of polar bears. This difference is mainly due to adaptability.
Another peculiar fauna of distribution is that similar animals exist in widely separated regions. But in between these regions there are no such representatives. For example lung fishes are found in Australia, Africa and South America. Such distribution is said to be discontinuous distribution. It can be explained, that these individuals might have present continuously. But due to changes in climatic conditions between the above continuity they became extinct.
dipnoi lungfish20
The continents, North America and Eurasia of northern hemisphere consist of same fauna. It is mainly because these two continents are connected by land isthmus called bearing isthamus. Hence the animals from North America are migrated to Eurasia and vice versa. But now the 56 miled bearing land isthmus was recently converted into aquatic isthamus.

Thus the geographical distribution of animals supports the theory of organic evolution.
Study of the past lived organisms which are not now on the globe or fossils is called Paleontology. Normally hard parts like spicules, setae, chitinous exoskeleton, shells, spiny exoskeleton, scales, bones, feathers, teeth and hair of past lived organisms were preserved as fossils.
Paleontology fossil9
It is the process of formation of fossils of past lived organisms. Normally an animal becomes fossil due to sudden environmental change. Majority of the fossils are formed by Petrifaction. It is the process of replacement of organic matter by minerals like sand, lime, iron oxides etc. In petrifaction decomposers decompose the organic matter of dead organism. In such places minerals precipitate. The minerals become hard to form into a rock. This rock resembles to the dead organism.

Fossils are exposed due to natural erosion of the soil or by excavation (digging of the soil). In India fossil collection and observation are being conducted by Birbal Sahni Institute of Palaeobotany, Lucknow. It observed three fossil parks in India.
A. Deccan plateau of Mandla district, M.P
The paleontologists collected 50 million years old fossil forest trees.
B. Rajmahal Hills, Bihar:
Scientists collected 100 million years old fossils.
C. Coal mines of Orissa:
Scientists collected 260 million years old fossils.
The fossils are four types. They are unaltered, altered, Moulds and coprolite fossils.
A. Unaltered fossils:
These are unchanged fossils. These are dead bodies of past lived organisms, preserved as fossils without any change. Examples: Wooly mammoths found in ice mountains of Siberia, Hexapod fossils found in the amber resin of California in U.S.A.
B. Altered fossils:
These are the changed fossils. These are formed by Petrification. Majority of the fossils are of altered type. Examples: Archaeopteryx fossil was collected from calcium mines of Bavaria state in Germany. Eusthinopteron and Ayshaea are also altered type.
C. Moulds:
These are the impressions of body, foot and leaves of past lived organisms. Normally plant fossils are of this type.
Moulds fossils6
D. Coprolites:
These are the faeces fossils of past lived organisms. The coprolites help to decide that the faeces is of a herbivore or carnivore or omnivore but not of a particular animal.
Coprolites fossils8
According to geologists, the approximate age of earth is about 5000 million years. The soil contains different layers(igneous,sedimentary, metamorphic). The layers are formed one after another So the bottom layer is the oldest layer and upper layer is the youngest layer Majority of the fossils are found in sedimentary rock layer The age of the rocks and fossils are calculated by using Carbonl4, Uranium238 and Potassium 40. Normally C is used. Hence this technique is called Carbon dating method. The age of the rock or fossil is calculated by the following formula.
The age of earth is divided into eras. Each era is divided into periods. Each period is divided into epochs. Name of the era, periods, epochs, origin and dominance of animals are represented in the form of a gelogical time scale.
geologica time scale27
A. Precambrain period:
The Archaeozoic and Proterozoic eras together called Precambrain era as they occured before cambrain period.
In the first eleven hundred million years of archaeozoic era there was no life. Hence the first part of archaezoic era is called Azoic era. The animals of precambrain did not contain hard parts. Hence fossils are very less in that period.
B. Palaeozoic era:
In this era, advanced invertebrates and primitive vertebrates (Fishes,Amphibians) arose.
C. Mesozoic era:
In this era, reptiles dominated and Archaeopteryx (first bird) and primitive mammals arose
D. Coenozoic era:
Paleontologists collected the complete ancestral history of various advanced mammals like horse, camel, elephant etc.
E. Fossil connecting links or Missing links:
Scientists collected the fossils that contain the characters of two groups. These are called missing links. These are
I. Archaeopteryx:
This missing link was collected from Jurassic period. The Archaeopteryx contains the structures of both reptiles and aves.
Archaeopteryx connectinglink9
II. Eusthinopteron:
It is the missing link between fishes and amphibians.
III. Avshaea:
It connects the annelids and arthropods
F. Documentary evidences:
These are the fossil evidences. The soil is a book. The pages of the soil are layers. The letters of the soil are fossils. The fossil evidences are the best evidences to support organic evolution.
G. Fossils complexity:
The fossils of lower layer are simple. Whereas the fossils of upper layers are complex and most complicated in the upper most layer. This observation indicates that all the layers of soil were not formed at one time. They are formed one after another from bottom to top. This also indicates that the organisms were formed one after another.
The Study of the developmental stages of an organism is called If we observe the embryos of different animals, there is a similarity. This similarity tells us that there is a relationship between the animals. The embryological evidences show support to organic evolution.
All multi cellular organisms begin their life as a single celled stage, namely Zygote. It undergoes cleavage to produce the first embryo, called morula. It develops into a single layered second embryo, called blastula. This embryo develops into a third embryo called gastrula. This embryo develops into adult. The Zygote represents the unicellular stage. Morula and blastula represent the colonial protozoan stages. Whereas gastrula represents the coelenterate stage. The embryos of organisms differ after the gastrula stage. The sequence of embryos shows that every multi cellular organism passes through the above stages representing their ancestors.
Zygote -------> Morula-------> Blastula -------> gastrula ------->adult.
(protozoan (colonial protozoan (coelenterate stage) stage) stage)
Von Baer proposed these principles by studying the embryology of Fish, Frog, Tortoise, Pigeon, Chimpanzee and Man. The early embryos of above animals resemble with each other closely. That it is impossible to separate if the embryos were mixed. But the embryos differ in the final stages clue to the formation of specialized characters. The similarity of early embryos tells that the above animals have common ancestors. The embryological principles are
  1. General characters appear in the early embryos.
  2. The special character appear in the last embryos.
  3. The embryos of closely related individuals are almost similar up to the end with small differences.
  4. The embryos of one organism resembles to the embryos o f its ancestors but not with adults.
It was proposed by Earnest Haeckel . The Study of the sequence of embryos or life history of an organism is called ontogeny. The evolutionary history of an individual is called phylogeny.
Biogenetic law states that every organism recapitulates its ancestors through the embryos. In other words the ontogeny of an individual repeats its phylogeny. The biogenetic law is also called recapitulation theory. Earnst Haeckel is considered as the father of embryology. Von Baer is considered as the father of modern embryology.
Biogenetic Law - Examples:
I. Tad pole larva of Frog:
The tad pole larva resembles fish both externally and internally. It contains two chambered heart, respires with gills like Fish. Later tad pole larva metamorphoses into adult Frog. Frog recapitulates its closet ancestor, fish through the tad pole larva. It also indicates that the Frog was evolved, from fish.
ii. Cater pillar larva of Butter fly:
This larva resembles to annelid. Butterfly recapitulates its closest ancestor, annelid through the caterpillar larva. It alsc tells us that the Butter fly was evolved from annelid.
iii. Development of ‘4’ chambered heart in the embryos of Birds and Mammals:
ontogeny repeats phylogeny18
In the embryos of Birds and Mammals, the heart is ‘2’ chambered, then ‘3’ chambered and incompletely ‘4’ chambered before developing into complete ‘4’ chambered heart. This shows that the Birds and Mammals recapitulate even the development of heart also.
Likewise the recapitulation is also observed in the development of brain, aortic arches, kidneys etc.
iv. Temporary embryonic nonfunctional organs:
The embryos consist of some temporary embryonic organs. These are present only in their ancestors but absent in adults. Such organs are called temporary embryonic vestigial organs. But these disappear when the embryo develops into adult. Some of the temporary embryonic vestigial organs are
  1. Formation of gill slits in the embryos of reptiles, birds and mammals.
  2. Formation of teeth in the embryos of pigeon and whale.
  3. Formation of limbs in the embryos of snakes.
  4. Formation of tail in the embryo of human beings.
These structures are of no use in the embryos. But their presence explains the biogenetic law.
These are the remains impressions of past lived organisms.

The theories which we have discussed are imaginary. There is no practical proof for them. The various animals which we are seeing today were gradually evolved from simple past lived organisms. It is not possible for anybody to observe a single change, because our life span is too short. Hence scientists collected evidences from different branches of biology. They are,
  1. Morphological and anatomical evidences.
  2. Embryological evidences
  3. Paleontological evidences
  4. Physiological evidences
  5. Zoo geographical evidences

In all the living organisms, the structural and functional units are cells. If all the living organisms were created by super natural power, why he created same cells in all living organisms? If we observe the shape of Chimpanzee and man, there is a great similarity. Why? Even if we observe the fore limbs of different vertebrates, there is a similarity. Why? If we observe the brain of different vertebrates, there is a similarity in structure and function. Why? All the above questions indicate that there is a relationship between the organisms.
The organs of different individuals are classified into three types. They are homologous, analogous and vestigial organs.
The organs which have common origin and structure are called homologous organs.
The homologous organs may differ in function and form (shape).
A. Homologous Organs:
Example 1: If we observe the fore limb of a Frog, wing of a Bird , paddle of a Whale and hand of a Man, all of them have same origin and same structure. Each of the organ has same bones, same blood vessels and same nerves.
Why the above organs (fore limb,wing,paddle,hand) differ in form (shape) and function? Because the above animals are living in different habitats and the organs have to perform different functions. In Frog, the homologous organs (fore limbs} perform leaping movement. In Birds, the homologous organs(Wings) perform flying movement. In Whale, the homologous organs (Paddles) perform swimming movement. In Man, the homologous organs(Hands) perform holding of objects.
Evolutionary explanation:
The above explanation tells us that the above animals (Frog,Bird,Whale,Man) had a common ancestrol group. Probably it may be osteolepid fishes. They lived in devonian period. One fish migrated to land. But this was not totally adopted for land life. Now and then it visited water. So one fish was evolved into Frog. Hence fore limb is adopted for leaping movement. One more fish first entered into land and finally entered into air. This lead to the origin of Bird. Hence forelimb is modified into wing for flying. One more fish first entered into the land and once again entered into water .This lead to the origin of whale. Hence fore limb is modified into paddle for swimming. One more fish first entered into the land and then acquired bipedal life. This resulted the origin of man. Hence fore limb is modified to hold the objects.
Adoptive radiation or Divergent Evolution:
The homologous organs explain adoptive radiation or divergent When the same animals (Osteolepid fishes) of same habitat entered into different habitats (land, air,water and land), they evolved into different individuals (Frog Bird ,Whale,Man). The formation of different animals of different habitats from the same animals of same habitat is called adoptive radiation or divergent evolution.
Example 2: Mouth parts of different insects (Cockroach, Housefly, and Butterfly). All of them have same labrum, mandibles, maxillae and labium. But they differ in form and function. It is because their mode of feeding habits is different.
Example 3: Vertebrae of different vertebrates.
Example 4: Hind limbs of different vertebrates.
Example 5: Thorn of Bougainvillea and tendril of Cucurbit.
Example 6: The brain of different vertebrates.
Example 7: The wings’ of Bat and Bird.
Example 8: The appendages of prawn explain serial homology.
evolution evidence9
The organs which have common form and function are called analogous organs. But they must differ in origin and structure.
A. Analogous Organs:
Example 1: The wings of Insect and Bird. In both the wings perform flying.But the wing of Insect is derived from ectoderm and it is supported by chitinous nervures. Whereas the wing of Bird is derived from mesoderm and it is supported by bones.
Evolutionary explanation:
The above explanation tells us that the Insect and Bird had different ancestors. Probably the acquatic Annelid when entered into air, it evolved into lnsect.The terrestrial Reptile when entered into air, it evolved into Bird.
It is explained by analogous organs.When the different animals (Annelid,Reptile) of different habitats (Water,Iand) entered into the same habitat (air), they evolved into different animals (lnsect,Bird). It is called Convergent evolution
Example 2: Scales of Fishes and Reptiles.
Example 3: Tubers of seweet potato and vegetable potato.
When different animals are living in the same habitat, they contain same shape. So, analogous organs have same shape. For example Fish and Whale contain same shape as both are living in water The shape of Earthworm and Snake is same as both live in burrows.
convergent evolution11
The organs which are non functional and reduced in an organism are called vestigial organs. But these organs were well developed and performed functions in ancestors. The organs become functionless when the animals enter into new habitat or when their function is taken up by another organ or when the habits are changed. Presence of vestigial organs is the most convincing evidence in favour of organic evolution and is also supported by the disuse principle of Lamarck.
A. Vestigial Organs of Man: In man there are nearly 180 different types of vestigeal organs. Hence Welder Sheim described human being as moving museum of variable antiquities.
vestigial organs28
The different vestigeal organs of man are Vermiform appendix Coccyx, Nictitating membrane (plica semilunaris), Muscles of Ear pinna, Clitoris of female human being etc.
B. Vestigeal Organs of Ratitee Birds: Wings
C. Vestigeal Organs of Python and Whale: hind limbs
D. Vestigeal Organs of Horse: splint bones of limbs.
E. Atavism: Formation of all ready disappeared organs suddenly in an individual is called atavism. Such organs are called atavistic organs. These support organic evolution. If human baby born with tail, the tail is considered as atavistic organ.
atavism tail4
If human baby born with many pairs of mammary glands, except one pair other pairs are considered as atavistic organs.
The organisms having the structures of two different groups are called connecting links. These explain the path of evolution. The connecting links are
A. Peripatus: It consists of nephridia like annelids and trachea like arthropods. Hence peripatus is considered as living connecting link between annelids and arthoropods. We can also say that annelids evolved into arthropods through the peripatus.
Annelids --------- peripatus ---------> Arthropods.
B. Monotremes (Prototherians-Echidna, Ornithorynchus): These lay eggs like reptiles and contain mammary glands like mammals. Hence monotremes are considered as connecting inks between reptiles and mammals. We can also say that the reptiles evolved into mammals.
Reptiles --------- Monotremes ----------> Mammals


  • 10 Jun 2017
Darwin’s theory of natural selection was accepted. The strong supporters of Darwinism are Wallace, Huxley, Haeckel, and Weismann. In the light of recent researches the theory was modified. Several experimental evidences have gone in favor of Darwinism. Basing on ‘those facts and statically data a synthetic theory of evolution was proposed. This is modified theory of Darwinism. This is called Neo-Darwinism. The ideas of Darwin were taken into consideration, But the meaning of those ideas were very much changed. Neo-Darwinism is the simple reconstructed Darwinism, like the old wine in a new bottle.

According to Neo-Darwinism the following factors operate for the formation of new species.
  1. Variations
  2. Mutations
  3. Natural selection
  4. Genetic drift
  5. Isolation of species.
Over production, struggle for existence, and universal occurrence of variation will take place as usual. But in the synthetic theory the formation of variations and mutations were discussed with experimental evidence for evolution which Darwin was unable to explain. Hence synthetic theory of organic evolution was more appropriate:
a) Variations: During Darwin’s time little was known about genetic variations. During Meosis and crossing over synapsis will take place. Because of this regrouping of genes will take place. Because of which genetic variation will appear or chromosomal aberrations will take place. The chromosomes may loose a bit or gain in a bit or order may be changed, or chromosomal bits may be exchanged between two chromosomes. These aberrations will become heritable variations.
Now and then the sets of chromosomes will increases or decrease. This is called ploidy. Because of this polyploidy heritable variations will arise they will be carried to number of generations This may result in the origin of new species.
b) Mutations: Any change in the nucleotide sequence of DNA and if one pair of nucleotides is replaced mutations will arise. These mutations are called point mutations. These are caused spontaneously in nature. They can also be brought by induction. Mustard gas, x-rays, gamma rays, electric shocks, temperature shocks etc. will bring mutations. These mutations arc rare. They are sudden and heritable. They may be harmful or beneficial. Most of the mutant genes are recessive. They can be expressed only in homozygous state.
Because of these sudden mutations new species are formed. For evolution, variations and mutations will be the raw material.
c) Natural Selection: Natural selection includes aft forces both physical and biotic factors and determine how and in what direction an organism is to change. Natural selection has no favoritism. But it is obvious that the organisms which are suited for environmental conditions will survive over power in the force of competition. Because of this better survivors are retained in the nature.
d) Genetic Drift: In small inter breeding population heterozygous gene pairs will tend to become homozygous. Because of this, disadvantage characters may be expressed and those organisms will be weeded out. Such genetic drifts are not theoretical. They operate in small populations of Islands. This genetic drift will provide a way to determine the line of evolution.
e) Isolation: In Darwin’s time nothing to known about isolation. Isolation is very important part in evolution. Usually the organisms of a population will be segregated into several populations because of physiological or geographical Isolation.
Mutations large stretches of water may separate a population in the separated groups one group may change. Because of this new species Will be developed. Thus geographical isolation will bring evolution.
The effects of natural selection in different environments will give different species.
Thus the old Darwin’s concept is re-organised with experimental proofs, New-Darwinism was proposed.

Mutation Theory

  • 09 Jun 2017
It was proposed by Dutch botanist, Hugo devries in 1901.hugode-vries-mutation He was one of the rediscovered of Mendelism. Devries gave importance to those variations, which are discarded by Darwin. Though Devries thought that he proposed mutation theory in against to Darwin, but it was merged in Natural selection or rectified the detect of Darwin.
Sudden, Large, hereditable variations are called mutations. Organisms having mutations are called mutants.
Devries observed mutations in Oenothera Lamarckiana. He conducted breeding experiments in that plant. Devries collected the seeds of 0. lamarckiana and sowed the seeds in his field. After sometime, he observed four new varieties of plants in addition to parent variety.
They are oenothera-mutations
  1. 0. zygas: It is taller than the parent.
  2. 0. nanella: It is shorter than the parent.
  3. 0. livifolia: It contains more number of leaves than the parent.
  4. 0. brivistylis: It contains shorter style than the parent.
Devries cross pollinated the four new varieties and parent variety.
He got few more new varieties. Devries continued the experiment for about ‘7’ generations. He got 50,000 plants. These 50,000 are 800 varieties. When these varieties were self pollinated, Devries got the same plants.
Based on the above experiment, he proposed some observations. They are
  1. New species appear suddenly but not slowly.
  2. Large, sudden changes appeared in living organisms, these are called mutations.
  3. Mutations are hereditable.
  4. Mutations occur during gametogenesis or fertilization.
  5. Mutations may be harmful or beneficial.
The mutation causing chemicals are called mutagens. These are natural and artificial mutagens. The natural mutagens are ‘X’ rays, U.V. rays, temperature etc. The artificial mutagens are formaldehyde, colchicin, ‘X’ rays etc. The mutations caused by natural mutagens are called spontaneous mutations. The mutations caused by artificial mutagens are called induced mutations.
For the success of evolution, mutations must occur in germplasm chromosomes, germplasm genes or germplasm D.N.A. The mutations are physically expressed in the next generation.
Mutations are two types. These are chromosomal and gene mutations.
A. Chromosomal Mutations:
These are changes either in number or structure of chromosomes.
Normally individuals are diploid having two sets of chromosomes (paternal and maternal genome). Any change in the number of chromosomes of the gametes results the formation of mutations... The chromosomal number changes are polyploidy, haploidy and aneuploidy.
Chromosomal structure changes are of different types. They are duplication, inversion, translocation, deletion etc.
These are changes in the sequence of nitrogenous bases of D.N.A of gene. Gene mutations are also called Point mutations.
If gene changes, ‘m’ R.N.A is changed and finally sequence of amino acids in a protein is changed. The nutritional mutants or Auxotrophs were first observed in Neurospora by Beadle and Tatum.
Sometimes one gene mutation causes many changes phenotypically. Such mutations are called pleotrophic or polyphenic mutations.
  1. Albinism in man, rats, birds etc.
  2. Poly dactyly in human beings.
  3. Multi nipple condition in cattle.
  4. Presence of double flowers in Rose.
  5. Formation of Ancon sheep variety.
According to Devries the evolution is very fast like horse running movement, but not slowly like snail’s movement as said by Darwin. Devries also said that there are no middle forms between the ancestor and new species. In other words the new individual was evolved directly from ancestor without middle forms.
A. Evolution of Giraffe:
According to Devries, the Giraffe was directly evolved from short necked Antelope due to the mutation in neck length controlling gene.
B. Evolution of Snakes:
According to Devries the snakes were directly evolved from limbed lizards due to the mutation in limbs controlling gene.
C. Evolution of Duck:
According to Devries the duck was directly evolved froth terrestrial birds due to change in the skin of digits of hind limb.
Artificial Selection:
It is an additional factor. Here man (Animal Breeding and Plant breeding) acts as a source of selection. Man artificially selects two individuals having required qualities, breeds them and produces new variety. This method is applied in dairy, poultry, piggery etc.
The introduction of ‘Murrah cattle’ in Andhra pradesh leads to ‘white revolution’ by cattle breeding. By artificial selection many hybrid varieties in crops are developed. The artificial development of hexaploid ‘TRITICUM’ variety.
Darwinism Sexual Selection:
It is also an additional factor. Darwin proposed this to explain why certain characters are confined to one sex only. i.e., sexual dimorphism’. It does not play any role in competition for food and shelter.
His theory is mainly based on the following assumptions.
A. In general, males are more abundant than the females. So, competition develops among the males for copulation with females.
B. Female chooses the male having beautiful colour, appearance and physique.
E.g.: Female frogs initially selected the male frogs, which have small vocal sacs and amplexsuory pads. Hence these characters gradually increased in the successive male progeny. This ultimately lead to sexual dimorphism in frogs.
Pangenesis theory:
Darwin proposed this to explain the heredity. According to him, the every organ consists of pan genes. These are carried to the sex organs, and accumulated in gametes. Through the gametes, pan genes are transferred to the progeny.
Many objections were proposed by scientist to Darwinism. They are,
  1. Natural selection does not explain the origin of variations (arrival of the fittest). It explains only the presence of variations (survival of the fittest). Why Darwin was unable to explain the origin of variations? Because he did not know about genes.
  2. Darwin believed that the variations help in competition. At the beginning the wings in birds are in primordial state. Then how they fly to protect from enemies? Likewise in Torpedo [electric ray], the protective electric organ was in rudimentary stage at the beginning. Then how the Torpedo was protected before the full development of electric organ?
  3. Darwin was unable to explain the presence of vestigial organs.
  4. Darwin did not explain the overgrowth of antlers in Irish deer and tusks in Jefferson’s Mammoths.
It was proposed by Charles Darwin. He is an English naturalist. Darwin travelled on the ship H.M.S. Beagle for about five years from 1831 to 1835. He visited many islands of Atlantic and west Pacific oceans. Darwin was very much attracted by Finch Birds of Galapagos Island. He collected many fauna and flora from islands. While he was doing experiments on collected living Organisms, he studied essay on population, written by Malthus. This inspired Darwin to propose natural selection. Malthus said that the population increases geometrically --food and shelter increase-- arithmetically.
It was an accidental coincidence, that the Alfred Russell Wallace also got the same idea of natural selection. So, natural selection was actually proposed by Darwin and Wallace. But Darwin got credit as he produced more number of evidences for evolution. Darwin read his observations and the observations of wallace on July 1858 in London Linnaeus society. Darwin published the famous 19th century book, origin of species in 1859. Later he wrote two more books. They are Descent of man and Selection in relation to sex. Darwin is considered as the father of evolution.
Darwin proposed natural selection mainly on the basis of four facts and two inferences. The facts are
  1. Over production (prodigality of production)
  2. Constancy of population.
  3. Variations.
  4. Inheritance.
The inferences of Darwin are,
  1. Competition (struggle for existence)
  2. Natural selection (survival of the fittest)
It states that every living organism has the capacity to produce more number of young ones. The population increase is in geometrical ratio. This increase can be seen in many animals.
A. Paramecium: It undergoes binary fission three times for every two days. If all young ones survive and multiply at the above rate for five years. The volume of all paramecia would be ten times that of total earth.
B. Pearl Oyster: The female produces sixty million eggs per year. If all of them were fertilized, developed into young ones and reproduce at the above rate, for five years, the volume of all the individuals would be eight times that of the the size of earth.
C. Human Being: The population of human being is doubled for every ‘25’ years. If the same rate continues for about 1000 years, there would not be even standing place for some humans. The high rate of multiplication was observed in Frog, Fish, and Insects and even in plants also.
Even though the population increases enormously, the number of individuals in every generation remains constant. This constancy is mainly due to the following facts. They are
  1. Many eggs will not be fertilized,
  2. Many zygotes will not be developed into young ones,
  3. Some young once will die,
  4. Some will not reproduce and
  5. Some may not reproduce at the above rate.
So, there is high mortality at every stage. Even the small population may be increased; it is kept constant due to struggle for existence.
Even though the population increases enormously, the food and shelter will not be increased. Hence competition arises for their basic needs. The struggle is of three kinds. They are,
A. Intra specific struggle: It is the struggle between the individuals of same species. This struggle is highly severe as their basic needs are same. Highest severe struggle can be seen in the individuals of same species living in the same niche. E.g. Man and Man; Dog and Dog; Rat and Rat;
B. Inter specific struggle: It is the struggle between the individuals of different species. E.g. Rat and Cat; Cat and Dog; Herbivores and Carnivores. In inter specific struggle one kind of population regulates another kind of population [Rat population is controlled by cat population.
C. Environmental struggle: It is also called extra specific struggle. It is the struggle between the individuals and natural forces like earth quakes, volcanic eruptions, severe winter, serve cold etc. The environmental struggle causes high mortality. So competition acts as a natural check to limit the undue growth of a population.
The differences between the living organisms are called variations. These are raw materials for evolution. Presence of variations is the universal phenomenon. No two living organisms are identical except identical twins. Darwin recognized the presence of variations. But he did not explain the origin of variations. These are two types. They are (a) continuous, small, fluctuating and hereditary variations,(b) Discontinuous, large, sudden, non hereditable variations. Darwin said that the evolution is mainly due to small variations but not due to large variations. 
  • The organisms which have protective coloration are better protected from enemies than the others.
  • Sheep which have dense hair are better protected in north pole than the other sheep from severe cold.
It states that the nature creates struggle and selects those animals which won in competition.
The organisms which have useful variations will win in competition. Whereas the organisms which have no such useful variations will die.
Example: Imagine that there are two rabbits ‘A’ and ’B'. ‘A ’contains slightly longer legs and well developed muscles than ‘B’. Hence ‘A’ may be escaped easily from predator than ‘B’ by fast running. So, nature created struggle between predator and rabbits. The Nature selected ‘A’ to survive as it had useful variations.
Herbert Spencer called Darwin’s natural selection as survival of the finest.
Example of Natural Selection:
The best example is industrial melanism. The moth, Biston betularia is two coloured. They are grey and black coloured. The grey coloured files were abundant in England before industrial revolution. Because they were protected as they had protective colouration (mixed with the back ground grey colour of bark). But the black files were conspicuous to birds, they were eaten by birds.
After industrial revolution, black coloured moths are more than the grey coloured flies. Because the black coloured moths had protective colouration. So, nature before industrial revolution selected grey coloured flies and the nature now selected black coloured flies. 
6. Inheritances
It is the transmission of characters from one generation to successive generations. Darwin said that the useful variations are transmitted to the progeny with slight increase in every generation. Hence after many generations the useful variations become large. This lead to the formation of a new species.
Evolution means opening out or unfolding. It explains the formation of non living things evolution-of-man and living things.
The term organic evolution means continuous change or descent with modification or the developmental stages of an organism repeats the developmental stages of ancestors.
The organic evolution can be defined as the formation of complex organism from simple organism over a period of time. The raw materials for evolution are variations.
Many scientists proposed different theories to explain organic evolution. Lamarck, Darwin and Hugo devries tried their level best to explain organic evolution. Later scientists used the genetics to explain organic evolution. The different theories proposed by evolutionists are Lamarckism, Natural selection and Mutation theory.
It was proposed by French biologist, Jean Baptist Lamarck. He was the first scientist to Lamarck-Jean Baptiste explain the organic evolution. Lamarck published his principles in the book, philosophic Zoologique in 1809.
The principles of Lamarck are
  • Influence of the environment
  • Use and disuse of organs.
  • Inheritance of acquired characters.
The environmental factors are sunlight, temperature, humidity etc. These factors have influence on living organisms. Influence of the environment states that when the organisms of same species were living in different environments, they develop changes to suit their environment. The changes developed in the life time are called acquired characters.
Example:If we observe the sheep living in tropical (warmer) region have less hair. Where as the body of polar-sheep-wool polar sheep is covered by dense thick hair. The variation is mainly due to the difference in the environments. The dense thick hair on the body of polar sheep is mainly to protect from severe cold. The above example tells us that the thick hair on polar sheep is the acquired character
It states that the organs which are used continuously, they develop better.
Example 1: Ziraffe-lamarck
The ancestors of Giraffe were short necked antelopes. The ancestors stretched their neck continuously to grab the leaves. Hence neck length was gradually increased in every generation.
Example 2: duck-web
The ancestors of Duck were terrestrial birds. The terrestrial birds entered the water and continuously used the digits of hind limbs. Hence web was gradually formed.
B. Disuse Principle:
It states that the organs which are not continuously used they gradually decrease in size, first become vestigeal and finally disappear.
Example 1: The ancestors of Ratitae (flight less) birds were flying birds. As the flying birds did not use the wings, the wings gradually decrease in size and finally became vestigial . flightless-birds-ratitae.
Example 2: The ancestors of snakes were limbed lizards. As the limbed lizards did not use the limbs continuously, the limbs gradually decrease in size, first become vestigeal and finally disappeared.
The use and disuse of organs in animals lead to the formation of acquired characters.
ll. INHERITANCE OF ACQUIRED CHARACTERS: Inheritance is the transmission of characters from one generation to successive generations. Life time developed characters are called acquired characters. These are developed due to influence of the environment and use and disuse of organs. Inheritance of acquired characters states that the life time developed characters are transmitted to the progeny. Lamarck cited many examples to explain the inheritance of acquired characters.
A. Evolution of Giraffe:
The ancestors, short necked antelopes of Giraffe used the neck continuously to grab the food. Hence small increase in the neck length was occurred in every generation. This increase is called acquired character.
The acquired character is transmitted to the next generation. As the neck was used continuously, neck length was increased in every generation and the increased neck was transmitted to every generation, this lead to the formation of long necked Giraffe.
B. Evolution of Snakes:
The ancestors, limbed lizards of snakes continuously used the body and did not use the limbs. Hence in every generation body length was increased and limbs size was decreased. As the use of body and disuse of limbs occurred in many generations, body length was increased and limbs were disappeared. This lead to the formation of snakes.
C. Evolution of Ratitae Birds:
The ancestors of flight less birds (Ratitae)were flying birds. If we observe the distribution of flight less birds, they live in areas where there were no enmies. Hence flying birds did not use the wings. The wings gradually decreased in size and became vestigeal. This lead to the formation of Ratitae birds.These birds evidence evolution
Lamarck’s third principle, inheritance of acquired characters was highly criticised. But the remaining two principles (use and disuse of organs,influence of the environment) were accepted by many scientists.
Women wear Jewellary to the nose and earlobe. Gold smith bores (making holes) the nose and earlobe for wearing. Presence of holes in the nose and ear lobes is the acquired character. But the children of such women did not have hales.
A sports man develops well developed muscular body due to regular exercises. But his children will not inherit this acquired character.
A person may lose a limb in an accident, but his children are not limbless.
Decaudalisation experiment:
It was conducted by Augst Weismann to disprove the lamarck’s inheritance of acuired characters. He collected two rats and cut their tails. Weismann crossed such tail less (acquired character) rats and observed the progeny. He got the tailed progeny. He repeated the experiment (cutting of tails, crossing and observation of progeny) upto 22 generations. Weismann did not get not even a single rat with out tail. This showed him that the acquired character tail lessness is not transmitted to the progeny. So, the weismann is considered as a critic of Lamarck.
The above experiment resulted the weismann to propose Germplasrn theory.
Germplasm theory:
It was proposed by weismann. The body of every individual contains two plasms. They are wisemen-germplasm-theory somatoplasm and germplasm. Somatoplasm is found in all organs except gonads. Germ plasm is found only in gonads. Germplasm theory states that the changes of germplasm are only transmitted to the progeny. But somatoplasmic changes like tail less ness, holes in the nose are never transmitted to the progeny.
Germplasm theory-experiment:
It was conducted by Castle and Philips on Guinea pig. They transplanted the ovaries of homozygous black Guinea pig to the white female. Then Castle and Philips crossed such female white Guinea pig with white male. They got heterozygous black progeny. This indicates that the germplasm change of white female is transmitted to the progeny.
Even though the lamarckism was highly criticised, still some scientists belive lamarckism. They are called neo lamarckists. They are paul kammerer, Guyer, Smith and Mc Dugoll.
The origin of life on earth and organic evolution are two fundamental aspects in modern biology. Origin of life means the origin of first living organism. Organic evolution means the formation of complex organism from simple organism over a period of time.
It means the formation of life on the planet Earth. Both living and non living things contain the same chemical elements. These are carbon, hydrogen, oxygen and nitrogen. But living things differ from non living things in having energy flow in an ecosystem, metabolism, reproduction, etc.
Many theories were formulated to explain the origin of life before modern times. They are special creation, cosmozoic theory, spontaneous generation and biogenesis.
A. Special Creation theory:
It is the oldest theory. Special creation theory states that all living organisms are created by super natural power. It is a relegious view. Special creation theory was discarded as it did not stand in scientific arguments.
B. Cosmozoic theory:
According to this theory, life did not arise in our planet. But life was descended from other planet where life existed previously. The believers of this theory assumed that life was brought to the earth through some agencies like meteorite (mass of stone or metal that has reached the earth from outer space). Some meteorites do contain organic molecules. However, simple organic molecules do not constitute life. This theory explains the existence of life on earth. But the cosmozoic theory does not explain the formation of first life.
C. Spontaneous generation theory:
This theory states that life arose suddenly from non living matter. Hence spontaneous generation is also called abiogenesis (a - no; bio-Life, genesis - formation). It was proposed by Aristotle. According to this theory, the fishes and frogs arose from moist soil, maggots and worms arose from decaying organic meat, field mice arose from mud of Nile river, etc.
Why abiogenists was believed to be correct for such a long time? It is because the complex life cycles of animals were not observed and many microscopic stages were unobserved. So, scientific information was not available in that period.
D. Biogenesis theory:

It was proposed by Francisco Redi and Louis Pasteur. According to this theory life arose from pre existing life. Redi practically proved that life never arose from non living matter. Maggots will arise from decaying organic meat if it contains eggs only. If eggs are absent on such meat, never maggots will arise. 

Even though biogenesis explains the formation of living organism from pre existing life, it never explains the origin of first living organism. But it laid the foundation for proposing the theories of organic evolution.

The first modern theory of origin of life was proposed by A.I. Oparin and Haldane independently. Oparin explained the origin of life from non living materials through the chemical evolution of life. Oparin’s theory is called naturalistic theory or theory of coacervates.
The approximate age of earth is 4,600 million years. Where as the age of first life is 3,500 million years.
The origin of sun, earth and other planets is explained by cosmic evolution. It was proposed by Sir James Jeans. According to him, before 4,600 million years ago, the nebula was a ball like mass. It contained high temprature and everything is in the form of a gas. One star was attracted by the nebula. As the star speedily colloided with nebulous mass, the outer atmosphere was broken into nine pieces. These pieces were sprayed in the different distances. As these pieces moved away they gradually lost the temperature. These nine pieces were formed into planets. The remaining nebulous mass is being considered as the sun. As the earth began to cool, the cosmic dust was condensed. Hence on earth solids, liquids and gases are co existing.

The different steps in the formation of first living organism are explained below. 

A. Environment of early earth:
Before 3,500 million years, the earth contained six elements. They are hydrogen, oxygen, carbon, nitrogen, sulphur and phosphorus. These are present through out the solar system. Among these hydrogen is most prominent. According to Oparin, these elements react with one another due to lightinings and formed into methane, ammonia and water vapour. This hypothesis was correct as the scientists observed methane, ammonia, hydrogen sulphide and water vapour in Jupiter and Neptune. So early atmosphere of earth is a reducing atmosphere. Now the environment of earth is an oxidising atmosphere.
B. Production of polymers:
It is the formation of complex molecules, like glycogen, fats and proteins. According to Oparin all these were formed from the above simple molecules. The polymerisation of aminoacids lead to the formation of essential proteins. Like wise sugars united and formed into glycogen. Glycerol and fatty acids united and formed into fats. All these polymerisations were occured due to the ultraviolet rays and inorganic catalysts of marine water.
C. Origin of nucleic acids:
The nitrogenous bases, phosphates and sugars united and formed into nucleosides. The nucleotides were polymerised and formed into long poly nucleotide chains. This caused the formation of nucleic acids. The polymerisation may be due to inorganic catalysts of marine water.
Once nucleic acids were formed, transcription and translations were also formed. This caused the formation of essential proteins. The nucleic acids were also developed the property of self duplication. The early proteins and nucleic acids were formed by some non enzymatic processes.
The proteins, nucleic acids and other materials might have been formed into groups in marine water. Each group developed a small transparent membrane around. These water like globules are called coacervates. These developed a blue coloured pigment. This caused the &igin of photosynthesis. Now this is called first organism. This organism resembles to present day Cyanobacterium. So, the life first arose in marine water as microscopic, autotropic and anaerobic. This prokaryote later developed the membrane around the nucleic acids. This caused the formation of first eukaryotic cell. 
Miller and Urey experimentally proved the formation of complex molecules in the laboratory. They supported Oparin’s hypothesis.
Miller made an apparatus to synthesize complex materials, amino acids. Miller’s apparatus contains two rounded glass flasks and glass tubes. The rounded flasks are connected by glass tubes. In one flask, Miller took fresh water. He took C, H, N and phosphorus in one connected tube. In the another flask, Miller took marine water. He boiled the fresh water. Hence steam was produced. The steam was passed over the elements and finally mixture was sent into marine water flask. Miller and Urey observed ammonia, methane and phsophates in the marine water flask. These chemicals sent into a glass tube, which was connected to electrodes. The electrodes produced sparks. Then the chemical mixture was cooled and sent into the ‘u’ shaped tube. After one week, urey and Miller observed amino acids in the ‘u’ tube. •This experiment proved that the amino acids are produced from inorganic chemicals. It also proved the hypothesis of Oparin.


  • 08 Jun 2017
Zoo Geography is a branch of biological science studies about the different kinds of animals on different regions on the earth. The following list of contents clearly explains about the basic concepts of zoogeography.
Definition of Comparative Anatomy: is a one of  the branches of biology studying the similarities and differences of internal organs between different groups of organisms.
The following topics are detailed discussed in simple words.
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