THE EVOLUTION OF APOSEMATIC COLORATION IN DISTASTEFUL PREY: AN INDIVIDUAL SELECTION MODEL.

Abstract

Many insects that have evolved distastefulness as a means of defence against predation, have also evolved aposematic or warning coloration (Cott, 1940; Rettenmeyer 1970; Edmunds, 1974). By definition aposematic coloration functions as a avoidance inducing signal to predators (Matthews, 1977), and this function has two components as evidenced by experiments. First, an initial reluctance to sample aposematic prey has been found in birds that have had no previous experience of such prey (Coppinger, 1969, 1970; Wiklund and Jarvi, 1982), and secondly, Gittleman and Harvey (1980) demonstrated that predators learn to avoid distasteful prey faster when they are aposematic than when they are cryptic. Different aspects of coloration can potentially induce an avoidance reaction in a predator, so that aposematic coloration can take many forms such as distinct markings and/or special colors like red, orange and yellow. One component, however, that is most often associated with aposematic coloration is brightness or conspicuousness (e.g., Harvey and Paxton, 1981). R. A. Fisher (1930) suggested that a prerequisite for the evolution of distastefulness as a means of defense against predation is the propinquity among closely related individuals, and currently kin selection is often used to explain the evolution of distastefulness (e.g., Benson, 1971; Harvey and Greenwood, 1978; Gittleman and Harvey, 1980). This view is predicated on the assumption that distastefulness does not confer any advantage to the individual because prey are always killed when sampled by unexperienced predators. However, there is now experimental evidence that distasteful prey can be sampled and released unharmed during a process of avoidance learning (Boyden, 1976; Jarvi et al., 1981a; Wiklund and Jarvi, 1982). Accordingly, these authors have concluded that a distasteful individual has a selective advantage over a palatable one, and that the evolution of distastefulness does not require kin selection. If prey are always killed in an encounter with a predator, the evolution of aposematic coloration in a distasteful species is even more difficult to conceive via individual selection because rare aposematic forms presumably would be more easily detected in a population of cryptic individuals. Therefore the evolution of aposematic coloration is often explained by invoking kin selection (e.g., Turner, 1971; Matthews, 1977; Harvey and Greenwood, 1978; Gittleman and Harvey, 1980; Harvey et al., 1982; Harvey, 1983). When distasteful insects can survive predator attacks, the evolution of aposematic coloration may occur through individual selection (Wiklund and Jarvi, 1982) but the conditions under which this may happen are unclear. Thus, in this paper we explore the conditions under which aposematic coloration can evolve solely through individual selection, without the need for kin selection. Specifically we ask whether, in a population of distasteful individuals, a mutation for aposematic coloration, which involves increased conspicuousness, can be selectively favored over a mutation for increased crypsis.