Hybridization increases population variation during adaptive radiation

Abstract

Adaptive radiations are prominent components of the world's biodiversity. They comprise many species derived from one or a small number of ancestral species in a geologically short time that have diversified into a variety of ecological niches. Several authors have proposed that introgressive hybridization has been important in the generation of new morphologies and even new species, but how that happens throughout evolutionary history is not known. Interspecific gene exchange is expected to have greatest impact on variation if it occurs after species have diverged genetically and phenotypically but before genetic incompatibilities arise. We use a dated phylogeny to infer that populations of Darwin's finches in the Galápagos became more variable in morphological traits through time, consistent with the hybridization hypothesis, and then declined in variation after reaching a peak. Some species vary substantially more than others. Phylogenetic inferences of hybridization are supported by field observations of contemporary hybridization. Morphological effects of hybridization have been investigated on the small island of Daphne Major by documenting changes in hybridizing populations of Geospiza fortis and Geospiza scandens over a 30-y period. G. scandens showed more evidence of admixture than G. fortis Beaks of G. scandens became progressively blunter, and while variation in length increased, variation in depth decreased. These changes imply independent effects of introgression on 2, genetically correlated, beak dimensions. Our study shows how introgressive hybridization can alter ecologically important traits, increase morphological variation as a radiation proceeds, and enhance the potential for future evolution in changing environments.

Keywords: Darwin’s finches; beaks; hybridization; introgression; speciation.

Fig. 1.

Proportions of admixed individuals in samples of G. fortis (FS) and G. scandens (SF).

Fig. 2.

Annual mean beak dimensions with 95% confidence limits. Mean beak length of G. fortis is almost identical to mean beak depth as a consequence of a strong phenotypic correlation (5). G. scandens samples vary from 15 (1974) to 364 (1983) with a mean of 135, and G. fortis samples vary from 31 (1974) to 933 (1976) with a mean of 311.

Fig. 3.

Annual fluctuations in CVs.

Fig. 4.

Average CVs of beak length for male specimens of each species of Darwin’s finches in relation to their ages. Points fitted by nonlinear, polynomial regression: peak, Gaussian, and 4 parameters. The adjusted R² value is 0.23. Included are 2 pairs of populations, Certhidea fusca from Española and San Cristóbal and Geospiza septentrionalis from Wolf and Darwin. These are the only pairs of conspecific populations with estimated ages of independence (40). Morphological data are from ref. . The curve for beak depth variation is almost identical.

Fig. 5.

CVs for beak dimensions of adult males of 75 populations of Darwin’s finches (41).