How fast do crossbills speciate? On assortative mating and vocalizations

Abstract

The question of whether sympatric speciation (i.e. reproductive isolation of populations with overlapping distributions) is a source of biodiversity has always been controversial. So far, mounting evidence from only a few species, for example African lacustrine cichlids, African widowbirds and phytophagous insects, supports the notion that it occurs in nature. In the latter two, host shifts can lead to genetic isolation of the population that has shifted.A potential new example comes from Eurasian and North American crossbills. The North American red crossbill (Loxia curvirostra) can be divided into sibling species that differ in vocalizations, with positive assortative mating between males and females of the same vocal type. Moreover, crossbills with the same vocalizations share allozyme patterns, whereas other vocal types differ in allozyme patterns and bill morphology. Worldwide, some host race formation and subsequent speciation has obviously occurred allopatrically (i.e. when geographically isolated) in island populations such as those of Hispanolia (L. megaplaga), Scotland (L. scotica) and perhaps Corsica (L. curvirostra corsicana). However, in North America, red crossbills are continental sibling species that occur sympatrically over large areas and disperse over long distances, making a sympatric origin of these sibling species probable. For sympatric speciation and subsequent species coexistence to occur, two conditions must be met: genetic isolation and ecological diversification of the incipient species. The coevolution of American crossbills and conifer species has already been demonstrated, and provides evidence for ecological diversification. The numerous conifer species in North America provide a huge variety of different cone structures, which must have been a necessary condition for host–race formation. The incipient species can coexist because specialization reduces competition. But, how do incipient species come about? Crossbills are highly social; if songs are learned (as with other finches) cultural evolution is likely to occur within cohesive groups. The preference for mates of the same vocal type (i.e. positive assortative mating) can then ultimately lead to genetic isolation.Magnus Robb1xIntroduction to vocalizations of crossbills in north-western Europe. Robb, M.S. Dutch Birding. 2000; 22: 61–107See all References1 now publishes extensive evidence that European crossbills also fall into discrete vocalization patterns. Of the six types he observed in The Netherlands alone, four were still present at the start of the breeding season, and the males and females in all the observed pairs belonged to the same vocal type. Two vocal types were found breeding sympatrically in the area. Importantly, unlike North America, there is little evidence that European crossbill populations can be divided into groups that differ in bill morphology. In addition, specialist species feeding on pine (L. scotica and L. pytyopsittacus) often occur sympatrically with L. recurvirostra in Europe, excluding the possibility of specialization on pine for the latter. Therefore, in Europe, low conifer diversity and interspecific competition might have prevented the formation of host races and morphological differentiation.In both America and Europe dialect formation has evolved in association with assortative mating. In America there is differentiation both of the feeding niche and of morphology and sibling species can be recognized. In Europe there is no such differentiation both of the feeding niches, no apparent differentiation of the morphology and no evidence of sympatric speciation. The evidence presently available suggests, thus, that cultural evolution of vocalizations and assortative mating among birds of the same vocal type facilitate the generation of new species, provided that sufficient opportunity for ecological differentiation is present.