CBSE Class 10 Science 2018 · Paper 31(B) · Question 11(a)

Speciation is the formation of a new species from a pre-existing species: groups of one population change so much that they can no longer interbreed. Four factors responsible for speciation: (i) Geographical isolation: a barrier such as a river or a mountain range divides the population, so gene flow between the sub-populations stops. (ii) Genetic drift: chance changes in the frequency of genes, strongest in small sub-populations. (iii) Natural selection: different surroundings favour different variations in each sub-population. (iv) Mutation: severe changes in DNA, such as a change in the number of chromosomes, so that the germ cells of the two groups cannot fuse. Variation is also accepted as a factor.