MATE SELECTION AND BALANCED POLYMORPHISM IN THE TROPICAL NYMPHALID BUTTERFLY, ANARTIA FATIMA.

Abstract

Polymorphism in adult color pattern is found in many Lepidoptera species, particularly in mimicry complexes where mimetic coloration confers protection from predators or in cryptic forms where concealing coloration serves the same purpose (Ford, 1957, 1964). Other factors promoting discontinuous diversity, usually leading to a balanced polymorphism, are the maintenance of a normal sex ratio, heterozygous advantage, relative abundance of morphs in relation to learning behavior of vertebrate predators, and differential mating preferences (Ford, 1965). Negative assortative mating is one of several systems that could maintain in addition a high level of heterozygosity within a population without the heterozygote necessarily being superior in fitness to either homozygote. However, preference for mating with unlike partners has only rarely been encountered in nature. Disassortative mating is reported between morphs of the adult tiger moth, Panaxia dominula (Sheppard, 1952, 1953), between certain mutant lines of Drosophila melanogaster (Rendel, 1951), and in wild populations of the polymorphic whitethroated sparrow, Zonotrichia albicollis (Lowther, 1961; Thorneycroft, 1966). Frequency-dependent disassortative mating has also been discovered in laboratory colonies of six Drosophila species (see Ehrman, 1969 for review). All of these reported situations involve a positive contribution of this type of mate selection to the maintenance of a balanced polymorphism in the population. The present paper reports a situation in a polymorphic species where assortative and disassortative mating both work against the maintenance of a balanced (stable) polymorphism. The neotropical nymphalid butterfly Anartia fatima Fabricius shows a striking color dimorphism in the adult. This study presents data that indicate the polymorphism is balanced, and offers evidence that both negative and positive assortative mating contribute significantly in a negative way against the maintenance of the two phenotypes in natural populations in Costa Rica. Selective forces are outlined that may contribute to counterbalancing this negative effect of mate selection. These two sets of opposing selective pressures influence the frequencies of the forms at different altitudes, geographic localities, and seasons in this neotropical area.