NATURAL SELECTION FOR AND AGAINST A POLYMORPHISM WHICH INTERACTS WITH SEX.

Abstract

The tropical American butterfly Heliconius numnata (Lepidoptera, Nymphalidae) shows an interesting and anomalous dimorphism; in this study the genetics of its forms are determined, and some of the selective forces roughly identified. The selective values of the genes appear to differ in males and females, and a brief outline of the mathematics of such selection is used to analyze the dynamics of H. numata; the paper ends with a discussion of the evolution of polymorphisms which interact with sex. Our knowledge of animal behavior and the theory of natural selection predict that in general (1) animal species which are Batesian mimics (i.e. which, although palatable, are protected from predators by their resemblance to an unpalatable species) tend to develop a stable polymorphism in their color pattern, but (2) species which are unpalatable and warningly colored tend to become monomorphic, as the behavior of visual predators makes the equilibria in polymorphic populations unstable and the polymorphism transient (Ford, 1953). Thus when we find a polymorphic Batesian mimic we infer that selection by predators is one of the factors maintaining the polymorphism, and this inference is used in interpreting the evolution of the species (cf. the work of Clarke and Sheppard on Papilio dardanus-Sheppard, 1961): when we find a polymorphic aposematic species or Miillerian mimic we must seek for other factors maintaining the polymorphism (unless it is transient), already knowing that there is selection by predators to remove the polymorphism; so