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.
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.


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