Fission and fusion of Darwin's finches populations

Abstract

This study addresses the causes and evolutionary consequences of introgressive hybridization in the sympatric species of Darwin's ground finches (Geospiza) on the small island of Daphne Major in the Galápagos archipelago. Hybridization occurs rarely (less than 2% of breeding pairs) but persistently across years, usually as a result of imprinting on the song of another species. Hybrids survive well under some ecological conditions, but not others. Hybrids mate according to song type. The resulting introgression increases phenotypic and genetic variation in the backcrossed populations. Effects of introgression on beak shape are determined by the underlying developmental genetic pathways. Introgressive hybridization has been widespread throughout the archipelago in the recent past, and may have been a persistent feature throughout the early history of the radiation, episodically affecting both the speed and direction of evolution. We discuss how fission through selection and fusion through introgression in contemporary Darwin's finch populations may be a reflection of processes occurring in other young radiations. We propose that introgression has the largest effect on the evolution of interbreeding species after they have diverged in morphology, but before the point is reached when genetic incompatibilities incur a severe fitness cost.

Figure 1

Survival on a natural log scale of three cohorts (a, 1983; b, 1987; c, 1991) of hybrids and backcrosses (H; solid squares) in relation to the parental species, G. fortis (F; open circles) and G. scandens (S; open squares), over their lifetime on Daphne Major Island. Initial sample sizes are: (a) F=1019, S=553, H=12; (b) F=955, S=164, H=7; (c) F=581, S=108, H=19. Adapted from Grant & Grant (2008).

Figure 2

Mating patterns of hybrids and backcrosses. Both sons and daughters imprint on their father's song and mate according to species song type. Hybrids do not sing intermediate songs. As a result of introgressive hybridization genes flow from one species to another, but the two populations are kept apart by song. FS refers to a hybrid, the product of a G. fortis father imprinted on a G. scandens song mated to a G. scandens female. FSS refers to a first-generation backcross, FSSS refers to a second-generation backcross, etc. Adapted from Grant & Grant (2008).

Figure 3

Morphological convergence on Daphne Major Island as a result of introgressive hybridization. Polygons enclose members of each population identified by song (males) or the song of mates (females). (a) 1978–1982 and (b) 1990–2003.

Figure 4

Character displacement on Daphne Major Island as a result of differential survival during the drought of 2003–2004. White bars are non-survivors and black bars are survivors. The species differed more in mean beak depth (a) after the drought than (b) before the drought.

Figure 5

Increase in beak shape variance in G. scandens on Daphne Major Island as a result of introgressive hybridization with G. fortis. Adapted from Grant et al. (2004).

Figure 6

Hybridization in the recent past is indicated by the greater genetic similarity of sympatric populations of species than allopatric populations of the same species. Different symbols indicate different combinations of species of (a) ground finches (Geospiza spp.) and (b) tree finches (Camarhynchus spp.) From Grant et al. (2005). D indicates genetic distance.

Figure 7

Morphological and genetic convergence of G. fortis and G. scandens on Daphne Major Island. Standardization was achieved by giving a value of 1.0 to the difference between the species, in 1982, in beak shape and in Nei's D calculated from alleles at 16 microsatellite loci. Adapted from Grant et al. (2004). Squares, genetic distance; triangles, beak shape.

Figure 8

Schematic of speciation. Two populations diverge and eventually reach a stage at which they are incapable of exchanging genes. Darwin's finches are at an early stage in this process and Ficedula flycatchers are at a late stage.