(a) The difference in the characters among individuals of a species is called variation. ‘ A basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA. The DNA in the cell nucleus is the information source for making proteins. If the information is changed, different proteins will be made. Different proteins will eventually lead to altered body designs. This causes variations among the population.
The great advantage of variation to a species is that it increases the chances of its survival in a changing environment. For example, the accumulation of heat resistant variation in some ‘ bacteria will ensure its survival even when the temperature in its environment rises too much due to heat wave. On the other hand, the bacteria which did not have variation to withstand heat, would not survive.
(b) In sexual reproduction, organisms produce gametes through a special type of division, meiosis : reductional division, in which the original number of chromosomes becomes half. These two gametes then combine to form the zygote and original number of chromosomes is restored as in the case of human beings.
