Abstract
Real-time observation of adaptive evolution in the wild is rare and limited to cases of marked, often anthropogenic, environmental change. Here we present the case of a small population of reed warblers (Acrocephalus scirpaceus) over a period of 19 years (1996–2014) after colonizing a restored wetland habitat in Malta. Our data show a population decrease in body mass, following a trajectory consistent with a population ascending an adaptive peak, a so-called Ornstein–Uhlenbeck process. We corroborate these findings with genetic and ecological data, revealing that individual survival is correlated with body mass, and more than half of the variation in mean population fitness is explained by variation in body mass. Despite a small effective population size, an adaptive response has taken place within a decade. A founder event from a large, genetically variable source population to the southern range margin of the reed warbler distribution likely facilitated this process.
Highlights
Real-time observation of adaptive evolution in the wild is rare and limited to cases of marked, often anthropogenic, environmental change
Through the course of 19 years, the population has gone through a decrease in body mass consistent with a model of adaptive evolution (Fig. 1c)
The trait fluctuates around the optimum due to genetic drift, plasticity and unmeasured direct and indirect selective forces acting on the trait
Summary
Real-time observation of adaptive evolution in the wild is rare and limited to cases of marked, often anthropogenic, environmental change. We present the case of a small population of reed warblers (Acrocephalus scirpaceus) over a period of 19 years (1996–2014) after colonizing a restored wetland habitat in Malta. Evolutionary changes are often associated with shifts in the environment, as changes in adaptive optima cause new selective pressures to operate, resulting in phenotypic evolution[5,6]. Is-Simar currently contains the highest number of Phragmites reed beds of the Maltese islands, in addition to tamarisk groves (Tamarix sp.), which are both suitable breeding habitats for reed warblers (Fig. 1b) It was rapidly visited by numerous reed warbler migrants, and within 2 years, they had established a small breeding population. Our results have important implications for conservation biology and evolution, especially regarding the potential success of habitat restoration in relation to a species’ ability to rapidly adapt to a new environment
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