<P> The female widow bird chooses to mate with the most attractive long - tailed male so that her progeny, if male, will themselves be attractive to females of the next generation - thereby fathering many offspring that carry the female's genes . Since the rate of change in preference is proportional to the average taste amongst females, and as females desire to secure the services of the most sexually attractive males, an additive effect is created that, if unchecked, can yield exponential increases in a given taste and in the corresponding desired sexual attribute . </P> <P> It is important to notice that the conditions of relative stability brought about by these or other means, will be far longer duration than the process in which the ornaments are evolved . In most existing species the runaway process must have been already checked, and we should expect that the more extraordinary developments of sexual plumage are not due like most characters to a long and even course of evolutionary progress, but to sudden spurts of change .--Ronald Fisher, 1930 </P> <P> Since Fisher's initial conceptual model of the' runaway' process, Russell Lande and Peter O'Donald have provided detailed mathematical proofs that define the circumstances under which runaway sexual selection can take place . </P> <P> The reproductive success of an organism is measured by the number of offspring left behind, and their quality or probable fitness . </P>

What 2 criteria must be in place for natural selection to occur