When these are brought together, in fertilization , the cell which is produced will have some characteristics from each of the parent individuals. Flies may all look alike to you, but if you look closely enough, you'll find hundreds of differences. This means, of course, that any generation produced sexually will have any number of small, but possibly significant, variations . This allows a species to change slightly as the conditions they live in change. This change is called adaptation .
Most organisms reproduce sexually.
It is the way in which the species evolves to make new types more able to live in changed circumstances.