CBSE Class 10 Science 2017 · Paper 31/2/1 · Question 15

(1) Geographical isolation: a barrier such as a large river or a mountain range splits a population into sub-populations that cannot meet, so the gene flow that kept them alike stops and each group changes on its own. (2) Genetic drift: in each isolated group, especially a small one, the frequencies of genes change by chance from generation to generation, so different changes accumulate in the two groups. (3) Natural selection: the places the groups live in may differ in food, climate or enemies, so different variations survive and spread in each group, and the groups become adapted differently. Over many generations these differences build up in the DNA until the groups can no longer interbreed even if they meet again (for example, a change in the number of chromosomes can stop their germ cells from fusing). The separated group has then become a new species. Severe variation in DNA, such as a change in the number of chromosomes, is also accepted as a factor.