3.2 Number of progeny
Female guppies begin to breed as soon as they become mature at about three months old; they then produce clutches of eggs, most of which become fertilized, at roughly one-month intervals until they die or become too old. Clutches vary in size from one to 40 eggs; the average clutch contains about 10 eggs. Thus, female guppies produce a large number of offspring during their lives, far more than can survive to maturity.
Suppose that, in a particular stream, the size of a population of guppies stays more or less stable over several years. How many of a given female's offspring, on average, must survive to reproductive age in such a population?
Two: one that 'replaces' her in the population when she dies and one that 'replaces' one of the males with whom she has mated during her life. If any more than two survive on average, then the population will increase.
Given the large number of fertilized eggs produced by female guppies, and the fact that, on average, only two survive to reproduce, it is clear that there is very high mortality among young organisms in this species. This obviously meets the first of Darwin's propositions. Guppies are fairly typical organisms and illustrate that mortality in nature is typically very high. This mortality provides the background against which natural selection acts.