Heliconius butterflies show a continuum of geographic divergence and speciation; they are unpalatable and exhibit inter and intraspecific diversification of colour and patterns. Bates classic paper (Bates, 1862), reflecting observations during his stay in the Amazon, showed a geographical pattern for the different colour forms: similar between species within any one area of the Amazon basin, but the mimetic colour patterns themselves changed every 100-200 miles. Beside this geographic divergence, closely related species within an area often belonged to mimicry rings (groups of unpalatable species, together with some palatable species, that have converged on the same warning colour pattern) (Mallet and Gilbert, 1995). Bates system has all the intermediate stages between local varieties, geographic races, and sympatric species that make it an excellent biological model to study selection at the species boundary.
Co-mimetic species such as H. erato (top) and H. melpomene (bottom) have frequently evolved a diversity of geographic races or sub-species. The two species look identical in any one locality but their patterns change in concert across their geographic range; Localities left to right: Zamora, Ecuador; Puyo, Ecuador; Tarapoto, Peru; Guayaquil, Ecuador; Yurimaguas, Peru. James Mallet
Multiple forms of selection are acting in the divergence of Heliconius species, a good example being the sister species H. cydno and H. melpomene. They are broadly sympatric, from Central America to Andean South America, but they differ in habitat, host plant use and mimicry (Mallet et al., 1998). In mimicry, although both species are unpalatable and warningly coloured, they belong to two different mimetic rings, H. cydno is typically black with yellow and white and usually mimics species from the group sapho/eleuchia, that inhabit deep forest interiors. H. melpomene is black with red and yellow and is a mimic of H. erato, which inhabits open areas. The two species occasionally hybridise and backcross in nature. Hybrid females are sterile, but males are fertile. The few hybrids produced are expected to have low fitness because they are non- mimetic (Naisbit et al., 2002).
Recent work shows that a shift in colour pattern mimicry was crucial in generating pre-mating isolation between those two species (Jiggins et al., 2001b). To test whether males use mimetic colour pattern as a cue in choosing mates, the response of males to moving model made with either natural wings or coloured paper was investigated. H. melpomene males approached H. cydno colour patterns about half as frequently as those of their own type. Similarly, H. cydno males were a third as likely to court a H. melpomene pattern as their own type. The results suggested that colour is important for choosing mates and that assortative mating is greater in sympatry. Assortative mating reduces hybrid mating and the few hybrids produced are poorly adapted because they are non-mimetic. Thus, the mimetic shift can cause both pre-mating and post-mating isolation (Jiggins et al., 2001b). This study is a clear example of speciation facilitated by ecological adaptation.
Bates, H. W. 1862. Contributions to an insect fauna of the Amazon Valley. Lepidoptera: Heliconidae. Trans. Linn. Soc. Lond. 23:495-566.
Jiggins CD, Naisbit RE, Coe RL, Mallet J. 2001. Reproductive isolation caused by colour pattern mimicry. Nature 411, 302-305.
Mallet, J. & M. Joron. 1999. Evolution of diversity in warning color and mimicry: polymorphisms, shifting balance, and speciation. Annual Review of Ecology and Systematics 30: 201-233.
Mallet J, McMillan WO, Jiggins CD. 1998. Mimicry and warning colour at the boundary between races and species. In: Endless Forms: Species and Speciation (eds. Howard DJ, Berlocher SH), pp. 390-403. Oxford University Press, New York.
Mallet J, Gilbert LE. 1995. Why are there so many mimicry rings? Correlations between habitat, behaviour and mimicry in Heliconius butterflies. Biological Journal of the Linnean Society 55, 159-180.
Naisbit RE, Jiggins CD, Linares M, Salazar C, Mallet J. 2002. Hybrid sterility, Haldane’s rule and speciation in Heliconius cydno and H. melpomene. Genetics 161, 1517-1526.