The four ways in which individuals with a particular trait may increase in a population are as follows:
(i) Sexual reproduction results into variations.
(ii) The individuals with special traits survive the attack of their predators and multiply while the other will perish.
(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.