CBSE Class 10 Science 2017 · Paper 31/2/1 · Question 11

Variations arise during reproduction in two ways: 1. The copying of DNA is not absolutely accurate, so the copies are similar to, but not always identical with, the original, and these small changes are passed on; this happens even in asexual reproduction. 2. In sexual reproduction each offspring receives DNA from two parents in a new combination, which produces much more variation. Example: in a population of bacteria living in temperate water, a few may, by variation, be able to tolerate more heat. If the temperature of the water rises, for example through global warming, most of the bacteria die, but the heat-resistant variants survive and multiply, so the population is not wiped out. Had all the bacteria been of the ordinary kind, the whole population could have died. Variation thus gives some individuals a chance to survive a change in their surroundings, so it is useful for the survival of species.