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

Abstract

Convergent evolution provides a good test of theory of natural selection. If two species are subject to similar environmental conditions, then natural results in convergence of two with respect to 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 well enough, it should be possible to predict its outcome, no matter what species. Visual 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 background. Any deviation from background in distribution of color patch size, color frequency, or brightness distributions will make an animal's pattern conspicuous, and degree of conspicuousness is proportional to deviation from background distributions. As predation intensity increases, 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 by predators (favoring crypsis) and sexual (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 predictions about color patterns and predation intensity. As predation intensity increases, 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 effects are not peculiar to a particular system. If 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 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 similarities and differences between guppies and Phalloceros in their response to various predation intensities.