Speciation, Raciation, and Color Pattern Evolution in Heliconius Butterflies: Evidence from Hybrid Zones

Abstract

Hybrid zones often involve either morphological traits such as color patterns of ver-tebrates with poorly understood genetics or genetic traits such as chromosomes, allo-zymes, and mitochondrial DNA, which tell us little about selection. In neotropical Heliconius butterflies, hybrid zones for warning, mimetic wing patterns are known in which both genetics and selection can be comprehended. While learning to avoid unpalatable prey, predators cause frequency-dependent selection against rare color-pattern morphs. Good evidence for this evolutionary constraint on color pattern change comes from both sympatric Mullerian mimicry between Heliconius butterflies and narrow hybrid zones between color pattern races within Heliconius species. Given selection against rare morphs (which occurs even though a new morph might be advantageous if common), it is difficult to explain the rampant geographic variation we see in Heliconius color patterns (Fig. 9-1). Once divergence has occurred, it is pre-served by local selective pressures, even in parapatry, but the explanation of the initial divergence remains elusive. It has been generally accepted that Heliconius races differentiated in allopatric Pleistocene refugia and that differentiation was in response to divergent mimetic pres-sures within each refugium (Turner, 1965, 1971a; Brown et al., 1974; Sheppard et al., 1985; Brown, 1987a). In this chapter, it is argued that allopatry is not necessary for divergence, and that de novo warning color evolution must have been partially respon-sible for raciation in Heliconius. I develop hypotheses of divergence and test them against data from hybrid zones and from distribution patterns of the races and species of Heliconius, which they separate. Little evidence is found for the importance of refugia in divergence once one admits that a null hypothesis of allopatric divergence is invalid on current theoretical grounds. The implications of these studies for those of other systems of hybrid zones and refugia are discussed. Conservation strategies that employ refugium theory as a means of choosing conservation areas should be urgently reexamined; it would be better to conserve areas that house particularly endangered EVOLUTION OF HELICONIUS: HYBRID ZONE EVIDENCE 227