So far we've been talking about natural selection that occurs as a result of differences in the probability of survival, viability selection. There are, of course, other ways in which natural selection can occur:
In fact, most studies that have measured components of selection have identified far larger differences due to fertility than to viability. Thus, fertility selection is a very important component of natural selection in most populations of plants and animals. As we'll see a little later, it turns out that sexual selection is mathematically equivalent to a particular type of fertility selection. But before we get to that, let's look carefully at the mechanics of fertility selection.