CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 11(b)

Speciation: the process by which a new species arises from an existing (pre-existing) species, when a population splits into groups whose members can no longer reproduce with one another. Four factors responsible for speciation: (i) Geographical isolation: a barrier such as a river or a mountain range splits a population into sub-populations and stops the flow of genes (gene flow) between them. (ii) Genetic drift: random changes in the frequency of genes, marked in small sub-populations, make the isolated groups drift apart. (iii) Natural selection: different surroundings favour different variations, so each sub-population changes in a different way. (iv) Mutation (severe changes in the DNA), such as a change in the number of chromosomes, after which the germ cells of the two groups cannot fuse and the groups can no longer interbreed. (Variation within the population is also a correct factor, since drift and selection act on it.)