Testing limits to adaptation along altitudinal gradients in rainforest Drosophila

Abstract

Given that evolution can generate rapid and dramatic shifts in the ecological tolerance of a species, what prevents populations adapting to expand into new habitat at the edge of their distributions? Recent population genetic models have focused on the relative costs and benefits of migration between populations. On the one hand, migration may limit adaptive divergence by preventing local populations from matching their local selective optima. On the other hand, migration may also contribute to the genetic variance necessary to allow populations to track these changing optima. Empirical evidence for these contrasting effects of gene flow in natural situations are lacking, largely because it remains difficult to acquire. Here, we develop a way to explore theoretical models by estimating genetic divergence in traits that confer stress resistance along similar ecological gradients in rainforest Drosophila. This approach allows testing for the coupling of clinal divergence with local density, and the effects of genetic variance and the rate of change of the optimum on the response to selection. In support of a swamping effect of migration on phenotypic divergence, our data show no evidence for a cline in stress-related traits where the altitudinal gradient is steep, but significant clinal divergence where it is shallow. However, where clinal divergence is detected, sites showing trait means closer to the presumed local optimum have more genetic variation than sites with trait means distant from their local optimum. This pattern suggests that gene flow also aids a sustained response to selection.

Species' Range: Adaptation in Space and Time

“…A recent preliminary study by Bridle et al (2009) reveals results that are reasonably consistent with some of the predictions of Kirkpatrick and Barton (1997), but as far as we are aware, no study provides the data needed to test any of the predictions above. What could be done in principle?…”

A framework to diagnose barriers to climate change adaptation

“…Here, as is consistent with the IPCC (1), we refer to limits as obstacles that tend to be absolute in a real sense: they constitute thresholds beyond which existing activities, land uses, ecosystems, species, sustenance, or system states cannot be maintained, not even in a modified fashion (19)(20)(21). Beyond such limits looms irreversible loss (and the adjustment to living with that loss) and/or radical system shifts, including innovation and novelty (22,23).…”

Limits to adaptation along environmental gradients

“…However, until now, no predictive theory has included the fundamental stochastic processes of genetic drift and demographic stochasticity. Genetic variation is often measurably lower in peripheral populations (4,14) and experimental evidence suggests that low genetic variance coupled with high gene flow can prevent adaptation at the edge of a species' range in nature (15)(16)(17). Thus, there is clear evidence that low genetic variation may limit adaptive range expansion.…”