CONVERGENT AND DIVERGENT EFFECTS OF NATURAL SELECTION ON COLOR PATTERNS IN TWO FISH FAUNAS.

Convergent evolution provides a good test of the theory of natural selection. If two species are subject to similar environmental conditions, then natural selection results in convergence of the two with respect to the selected characters. Thus convergent evolution is "the production of a set of similar phenotypic characteristics in phylogenetically unrelated organisms subject to similar abiotic or biotic agents of natural selection" (Cody and Mooney, 1978). If we understand a particular process of natural selection well enough, it should be possible to predict its outcome, no matter what the species. Visual selection on cryptic color patterns is a well known system, and specific predictions have been made (Endler, 1978). This paper will show how specific predictions about color pattern characteristics work in two species of poeciliid fishes living with different predator faunas. In order to be cryptic, an animal's color pattern must appear to its predators as a random sample of the background. Any deviation from the background in the distribution of color patch size, color frequency, or brightness distributions will make an animal's pattern conspicuous, and the degree of conspicuousness is proportional to the deviation from the background distributions. As predation intensity increases, the match between animal and background should improve (Endler, 1978). Poeciliids are small fishes which live in streams in North, Middle, and South America (Rosen and Bailey, 1963). Guppies (Poecilia reticulata Peters) are ideal for color pattern studies. They are found in northeastern South America, and their ecology and behavior are well known (see Endler, 1978, and references therein). Color pattern frequencies at a particular locality represent a balance between visual selection by predators (favoring crypsis) and sexual selection (favoring conspicuous color patterns). The diet and number of predator species varies from place to place and allows sampling along predation intensity gradients. The color pattern variation of guppies fulfills many of the predictions about color patterns and predation intensity. As predation intensity increases, the color patterns become less conspicuous by a reduction in spot size and a shift from pigment and structural colors (which reflect strongly) to predominantly pigment colors (Endler, 1978, 1980). A good test for convergent evolution would be to examine another species living under similar conditions. It would be even better to have different predator species so that the effects are not peculiar to a particular system. If the observations made on guppies are generally true, then other species, living with different predators, should show similar effects of predation on color patterns. Another poeciliid, Phalloceros caudimaculatus (Hensel), represents a different tribe of the family (Rosen and Bailey, 1963), lives in southeastern South America, and is found with a different set of predators than guppies. This paper reports on the similarities and differences between guppies and Phalloceros in their response to various predation intensities.