Species Collapse via Hybridization in Darwin’s Tree Finches

Abstract

Species hybridization can lead to fitness costs, species collapse, and novel evolutionary trajectories in changing environments. Hybridization is predicted to be more common when environmental conditions change rapidly. Here, we test patterns of hybridization in three sympatric tree finch species (small tree finch Camarhynchus parvulus, medium tree finch Camarhynchus pauper, and large tree finch: Camarhynchus psittacula) that are currently recognized on Floreana Island, Galápagos Archipelago. Genetic analysis of microsatellite data from contemporary samples showed two genetic populations and one hybrid cluster in both 2005 and 2010; hybrid individuals were derived from genetic population 1 (small morph) and genetic population 2 (large morph). Females of the large and rare species were more likely to pair with males of the small common species. Finch populations differed in morphology in 1852-1906 compared with 2005/2010. An unsupervised clustering method showed (a) support for three morphological clusters in the historical tree finch sample (1852-1906), which is consistent with current species recognition; (b) support for two or three morphological clusters in 2005 with some (19%) hybridization; and (c) support for just two morphological clusters in 2010 with frequent (41%) hybridization. We discuss these findings in relation to species demarcations of Camarhynchus tree finches on Floreana Island.

Divergent foraging behavior in a hybrid zone: Darwin’s tree finches (Camarhynchus spp.) on Floreana Island

“…Birds were mist-netted and observed in the Scalesia forest at the base of Cerro Pajas volcano (1°17′S, 90°27′W, elevation 250-350 m) (O'Connor et al, 2010a All guidebooks refer to three Darwin's tree finch species on Floreana Island: small tree finch, medium tree finch, and large tree finch (C. parvulus, C. pauper and C. psittacula, respectively). But a recent study by Kleindorfer et al (2014a) found only two genetic groups and one hybrid cluster of tree finches on Floreana Island. The hybrid offspring were the result of pairings between females of the critically endangered medium tree finch and males of the common small tree finch (Kleindorfer et al, 2014a).…”

“…But a recent study by Kleindorfer et al (2014a) found only two genetic groups and one hybrid cluster of tree finches on Floreana Island. The hybrid offspring were the result of pairings between females of the critically endangered medium tree finch and males of the common small tree finch (Kleindorfer et al, 2014a). At present we do not know whether hybridization extends beyond the F1 generation.…”

“…From 2005 to 2010, Darwin's tree finches on Floreana Island increased the proportion of hybrids from 19% to 41% (Kleindorfer et al, 2014a). During that same time period, hybrid body size increased by 5% while body size in both parental species remained similar (Kleindorfer et al, 2014a).…”

“…During that same time period, hybrid body size increased by 5% while body size in both parental species remained similar (Kleindorfer et al, 2014a). Given the increase in relative abundance of hybrids and their body size, we ask if hybrid and parental tree finches occupy novel ecological space in sympatry.…”

Hybridization masks speciation in the evolutionary history of the Galápagos marine iguana

“…On Daphne Island, it was the introgressive hybridization between Geospiza fortis and G. scandens-rather than conspecific gene flow with immigrants from other islands-that increased the genetic and morphological variation of resident populations and enhanced their evolutionary potential, enabling the species to more rapidly react to environmental changes [14]. In other cases, hybridization resulted in despeciation of sister species [15], or the complete disappearance of a species, as was the case for the large tree finch on Floreana island [16]. Further examples are available from the wellstudied Galápagos giant tortoises, where lineage fusion through introgressive hybridization was recently revealed on the largest island of the archipelago, Isabela.…”