The Causes of Evolutionary Radiations in Archipelagoes: Passerine Birds in the Lesser Antilles

Abstract

TO investigate why some lineages undergo evolutionary radiation, we compare the passerine avifaunas of the Hawaiian and Galapagos archipelagoes, which have supported well-known radiations of birds, with those of the Lesser Antilles, which have not. We focus on four steps required for the buildup of diversity through allopatric speciation and secondary sympatry: genetic divergence in isolation, persistence of island populations, recolonization of source islands, and ecological compatibility in secondary sympatry. Analysis of genetic divergence among island populations in the Lesser Antilles reveals evidence of both prolonged independent evolution and reexpansion of differentiated island populations through the archipelago but little evidence of secondary sympatry of divergent genetic lineages. Archipelagoes with high rates of colonization from continental or nearby large-island sources might fail to promote evolutionary radiations because colonists fill ecological space and constrain diversification through competition. However, morphological analysis demonstrated similar divergence between allopatric populations in species in Hawaii, Galapagos, and the Lesser Antilles, although the rate of divergence between secondarily sympatric species evidently is more rapid in Hawaii and the Galapagos. Alternatively, endemic buildup of diversity might be facilitated by the relative absence of pathogens in Hawaii and Galapagos that otherwise could prevent the secondary sympatry of populations owing to disease-mediated competition.

The influence of gene flow and drift on genetic and phenotypic divergence in two species ofZosteropsin Vanuatu

“…Tanna. Phylogenetic information will need to be used to determine if this remains true over longer evolutionary time frames, thus fulfilling the 'population persistence' requirement for divergence and potential adaptive radiation (Diamond & Marshall 1977;Ricklefs & Bermingham 2007).…”

“…The constraining effect of intraarchipelago gene flow is just one of the proposed reasons for the generally low levels of divergence across the archipelago (Diamond & Marshall 1977). Lack of population persistence is also thought to be responsible for limiting avian diversification (Mayr 1965;Diamond & Marshall 1977;Ricklefs & Bermingham 2007), and the higher numbers of endemic species on isolated islands have been attributed to less extinction and turnover (Diamond 1980;Price 2008). In Zosterops, the strong phylogenetic and population genetic structure among populations suggest that variation in population persistence is not a key limitation to divergence.…”

“…While the distribution and morphological variation of southwest Pacific avifauna provided the foundation for many early ideas about island speciation (Mayr 1954;Mayr & Diamond 2001), phylogeographic and population genetic studies of the avifauna of this region are only now beginning to accumulate (Kirchman & Franklin 2007;Smith & Filardi 2007;Phillimore et al 2008b), lagging behind other wellstudied island groups such as the Galápagos (Petren et al 1999), the Caribbean (see Ricklefs & Bermingham (2007) and references therein) and Hawaii (Fleischer & McIntosh 2001). In the Vanuatu archipelago, two Zosterops species, the endemic Z. flavifrons and the later colonizer Z. lateralis have largely coincident distributions, but are at different stages of divergence (Mees 1969;Phillimore 2006;Black 2010).…”

Host–pathogen coevolution, secondary sympatry and species diversification

“…In contrast to the songbird (Passeriformes) avifaunas of the Hawaiian and Galápagos archipelagos, each of which has supported a spectacular radiation of species, the New Hebrides (Vanuatu) have supported no such radiation (Diamond & Marshall 1976, 1977 and the Lesser Antilles have only a small radiation consisting of four species of thrashers (Hunt et al 2001). Based on this comparison, Ricklefs & Bermingham (2007a) speculated on the causes of the difference in achieving secondary sympatry among these island groups.…”

“…(b) Pathogens and secondary sympatry To explain the low rate of secondary sympatry in birds of the Lesser Antilles, Ricklefs & Bermingham (2007a) suggested that incompatibility among sister populations might result from parasites that coevolve low virulence with host populations locally, but are pathogenic in sister populations lacking recent exposure to the allopatric parasite. The mechanism is thus 'apparent competition' (Holt 1977;Holt & Lawton 1994), in which potentially shared parasites cause incompatibility between two host populations in sympatry.…”