CBSE Class 10 Science 2019 · Paper 31/3/1 · Question 12(a)

(a) Speciation: It is the process by which new species arise from a pre-existing species. It happens when a population is split into groups that cannot interbreed, for example by geographical isolation (a large river or a mountain range), which stops the flow of genes between them. In each isolated group, genetic drift (random changes in the frequencies of genes, marked in small populations) and natural selection (different surroundings favouring different variations) bring about different changes over generations. When the differences become large, for example changes in the DNA or in the number of chromosomes, members of the two groups cannot reproduce with each other even if they meet, and a new species has formed. (b) Natural selection: It is the process by which, among the variations present in a population, those that give an advantage in a particular environment help their bearers survive and reproduce more, so that these variations become more common in the population over generations. For example, in a population of red beetles living on green bushes, a green variant is hidden from crows, survives and reproduces more, and over generations most of the beetles become green.