Abstract
Identifying the processes by which new phenotypes and species emerge has been a long‐standing effort in evolutionary biology. Young adaptive radiations provide a model to study patterns of morphological and ecological diversification in environmental context. Here, we use the recent radiation (ca. 12k years old) of the freshwater fish Arctic charr (Salvelinus alpinus) to identify abiotic and biotic environmental factors associated with adaptive morphological variation. Arctic charr are exceptionally diverse, and in postglacial lakes there is strong evidence of repeated parallel evolution of similar morphologies associated with foraging. We measured head depth (a trait reflecting general eco‐morphology and foraging ecology) of 1,091 individuals across 30 lake populations to test whether fish morphological variation was associated with lake bathymetry and/or ecological parameters. Across populations, we found a significant relationship between the variation in head depth of the charr and abiotic environmental characteristics: positively with ecosystem size (i.e., lake volume, surface area, depth) and negatively with the amount of littoral zone. In addition, extremely robust‐headed phenotypes tended to be associated with larger and deeper lakes. We identified no influence of co‐existing biotic community on Arctic charr trophic morphology. This study evidences the role of the extrinsic environment as a facilitator of rapid eco‐morphological diversification.
Highlights
Identifying the environmental agents of natural selection has proven difficult because organisms live in environments that are profoundly complex, with multiple and potentially conflicting selection pressures
We found that the degree of head depth variation in an Arctic charr population was significantly predicted by ecosystem size
Our study suggests that interspecific competition might not always limit the ability of a species to diversify, even in low productivity postglacial lakes; rather, the trophic morphology of an Arctic charr population depends primarily on the abiotic environmental characteristic of ecosystem size
Summary
Identifying the environmental agents of natural selection has proven difficult because organisms live in environments that are profoundly complex, with multiple and potentially conflicting selection pressures. It is increasingly recognized that phenotypic change can arise surprisingly fast. This has been proven experimentally (Blount, Borland, & Lenski, 2008; Kawecki et al, 2012), through artificial selection such as crop modification and animal breeding (Conner, 2003; Meyer, DuVal, & Jensen, 2012; Neff & Rine, 2006), and shown in some naturally occurring populations as a response to diversifying selection (Elmer, Lehtonen, Kautt, Harrod, & Meyer, 2010; Franks, Sim, & Weis, 2007; Hendry, Nosil, & Rieseberg, 2007). Rapid adaptive radiations across isolated islands and lakes are well recognized as important models for disentangling how diversity arises in nature, as they provide relatively simple replicated environments. Some of the best known examples include Darwin’s finches on the Galapagos Islands (Grant & Grant, 2011), Hawaiian silverswords (Baldwin, 1997), Anolis lizards on Caribbean islands (Losos, 2009), cichlid fishes inhabiting the Great African Rift (Turner, 2007) and Central American crater lakes (Recknagel, Elmer, & Meyer, 2014), and some northern postglacial fishes (Schluter, 2000)
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