Traits arise due to variations which occur due to sexual reproduction of inaccuracies during DNA copying or environmental factors. Now, the individuals with a particular trait may increase in a population due to the following factors :
(i) Natural selection: Those variations which give survival advantage to an organism are selected in nature and such traits increase in population.
(ii) Genetic drift: It occurs due to change in gene frequency due to accumulation of particular type of genes.
(iii) Geographical isolation: It leads to change in gene frequency leading to expression of one type of traits in a geographically isolated population.
(iv) Migration: It leads to flow of a particular type of gene in a specified population.