Ageing
However, later in life, inherited defects would have little or no effect on the population as a whole. In fact, during life our cells collect damage to their DNA which is random, but causes us to become gradually less fit as we age. And we have some cell types which never, or rarely, divide during adult life. Muscle cells and most neurons never divide in adult life. Obviously, they collect damage, and not just to the DNA. Also, we inherit various genetic conditions which have effect later in life, like Huntington's chorea. The twin effects of delayed heredity and buildup of damage is what makes us age. Protists do not age: they divide, and the next generation is just as good as the last. Why do even our dividing cells collect damage? The answer to that question is not known, but may be connected to a a cell division phenomenon.