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 sources for making proteins. If the infromation is changed, different proteins will be made. Different proteins will eventually lead to altered body designs. This causes of 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 itws survival even when the temperature in its enviroment rises too much due to heat wave. On the other hand, the bacteia which did not have variation to withstand heat would not survive.
(b) In sexual reproduction , organisums produce gametes through a special type of division, meiosis- reductional division, in which the original number of chromo-somes becomes half. These two gemetes then combine to form teh zygote and original number of chromosomes is restored as in the case of humen beings.
