CBSE Class 10 Science 2019 · Paper 31/1/2/C · Question 21

(a) (i) The pea plant has many clearly contrasting visible traits, such as tall and dwarf plants, round and wrinkled seeds, and violet and white flowers, which are easy to observe and count. (ii) Its flowers are bisexual and normally self-pollinate, so pure-breeding varieties are available, and cross-pollination can be carried out by hand when needed. (Other correct reasons: a short life cycle; many seeds from each cross.) (b) When a geographical barrier such as a river or a mountain range splits a population of a species into two, the individuals of the two groups can no longer interbreed, so gene flow between them stops. Over many generations, each group accumulates different variations: genetic drift changes gene frequencies at random, and natural selection favours different variations in the two different environments. Eventually the groups become so different (for example, in their DNA or number of chromosomes) that even if they meet again they cannot interbreed to produce fertile offspring. They have become separate species, so geographical isolation can lead to the formation of a new species.