1. We all are familiar with modes of
reproduction
That depended on the involvement of two
individual before a new generation can be
created .
2. Sexual mode of reproduction
While DNA copying mechanism are not absolutely accurate, they are precise
enough to make the generation for variation a fairly slow process . If the DNA
copying mechanism were to be less accurate many of the resultant DNA copies
would not be able to work with cellular apparatus and would die.
Thus two different individual in a population would have quite different
pattern of accumulated variation.
3. DNA DNA
creates a major difficulty . If each new generation is to be the combination of
DNA copies from to pre-existing individuals, then each new generation will
be end up having twice the amount of DNA that the cellular apparatus by
the DNA. Besides, if each generation doubles its DNA, fairly soon there will be
only DNA on earth with no room for anything else.