Possible human impacts on adaptive radiation: beak size bimodality in Darwin's finches

Abstract

Adaptive radiation is facilitated by a rugged adaptive landscape, where fitness peaks correspond to trait values that enhance the use of distinct resources. Different species are thought to occupy the different peaks, with hybrids falling into low-fitness valleys between them. We hypothesize that human activities can smooth adaptive landscapes, increase hybrid fitness and hamper evolutionary diversification. We investigated this possibility by analysing beak size data for 1755 Geospiza fortis measured between 1964 and 2005 on the island of Santa Cruz, Galápagos. Some populations of this species can display a resource-based bimodality in beak size, which mirrors the greater beak size differences among species. We first show that an historically bimodal population at one site, Academy Bay, has lost this property in concert with a marked increase in local human population density. We next show that a nearby site with lower human impacts, El Garrapatero, currently manifests strong bimodality. This comparison suggests that bimodality can persist when human densities are low (Academy Bay in the past, El Garrapatero in the present), but not when they are high (Academy Bay in the present). Human activities may negatively impact diversification in 'young' adaptive radiations, perhaps by altering adaptive landscapes.

Figure 1

Beak size variation in Academy Bay G. fortis. Both birds are mature males caught at the same time in the same mist net. Photograph by Andrew P. Hendry.

Figure 2

A map of Santa Cruz Island, Galápagos, showing paved roads (solid lines), gravel roads and major trails (broken lines), major towns (Puerto Ayora and Bellavista) and sites where G. fortis were sampled (Academy Bay, El Garrapatero and Borrero Bay).

Figure 3

Beak size variation in samples of more than 100 G. fortis. Shown are frequency histograms of PC1 (larger values correspond to larger beaks), with the black portions of bars representing mature males and the grey portions representing all other birds. Panels are labelled according to sampling site (Academy Bay, AB; El Garrapatero, EG; Borrero Bay, BB) and year. The 1973 Borrero Bay histogram is not shown here because of limited space, but is provided in electronic supplementary material D. Arrows show discontinuities between the small beak and large beak modes in the samples statistically confirmed to have strong bimodality (see figure 4). x-axes are not comparable across panels because PC1 was calculated within each sample.

Figure 4

Normal probability plots showing observed cumulative proportions for PC1 (points) versus expected cumulative proportions under a single normal distribution (line). Also shown are the results of Bayesian mixture models: Δ represents AIC_c for a single normal distribution minus AIC_c for the fitted mixture of two normal distributions (i.e. ΔAIC), and w represents the likelihood that a mixture of two normal distributions fits the data better than a single normal distribution (i.e. AIC_w). Also shown are our conclusions regarding bimodality: ONE represents strong support for a single normal distribution, TWO represents strong support for a mixture of two normal distributions, ONE≥TWO represents moderately more support for a single normal distribution, TWO≥ONE represents moderately more support for a mixture of two normal distributions, and ONE=TWO represents roughly equivalent support for a single normal distribution or a mixture of two normal distributions.

Figure 5

Variation in beak length and beak depth for samples of G. fortis collected by the same investigators between January and March 2004. Black points show mature males and grey points show all other birds. This figure illustrates the partial gap in beak size between modes at El Garrapatero, the filling of this gap at Academy Bay and the relative scarcity of the largest-beaked birds at Borrero Bay.