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.