Rapid Evolution of Character Displacement in Myzomelid Honeyeaters

Abstract

We consider a possible example of rapid evolution of character displacement, involving two honeyeater species on Long and neighboring islands off New Guinea. Long''s caldera collapsed about three centuries ago in one of the largest volcanic explosions of recent millennia. The present trees, mammals, and people of Long suggest recent overwater origins. The avifaunas of Long and the two nearest islands, Tolokiwa and Crown, also betray recent origin: Long has a deficit of the expected large-island species, and all three islands have an excess of supertramps (small-island specialists) and other species noted for overwater dispersal. Among the nine pairs of congeners on Long and its neighbors, two supertramp honeyeaters, the larger Myzomela pammelaena from the northern Bismarcks and the smaller M. sclateri from the southern Bismarcks, constitute the sole pair of species that are sympatric nowhere else and that presumably first met on the Long group. There are also the sole two bird populations of the Long group that differ significantly in morphology from their relatives (putative ancestors) elsewhere. We measured all available specimens of both species, from the allopatric ancestral populations as well as from the sympatric populations of the Long group. The larger M. pammelaena is even larger on Long than in ancestral populations, whereas the smaller M. sclateri is even smaller. The weight ratio for the two species shifted to a value of 1.52 in sympatry from a value between 1.24 and 1.43 in allopatry. The two species occur abundantly together in all habitats and at all altitudes of the Long group, often in the same flowering tree. We interpret the ecological significance of their size divergence by analogy with studies on other sympatric honeyeaters, among which size affects coexistence through effects on dominance, productivity requirements, prey size, perch position, and the economics of hovering. In particular, on all 13 known island groups where two or more myzomelid species occur sympatrically on the same island, the species differ either by habitat (including altitude) or by size; they occur syntopically in the same habitat only if their weight ratio is 1.5 or larger. Thus, evolution of character displacement was prerequisite to the observed syntopy (co-occurrence in the same habitat) on the Long group. After evaluating alternative interpretations (continuation of clines existing in allopatry, Bergmann effect, response to an environmental factor, nongenetic response), we conclude that the size shifts in myzomelids on the Long group probably represent character displacement in response to each other''s presence and that the shift evolved in the approximately three centuries since Long''s eruption.