Abstract
Ecological speciation seems to occur readily but is clearly not ubiquitous – and the relative contributions of different reproductive barriers remain unclear in most systems. We here investigate the potential importance of selection against migrants in lake/stream stickleback (Gasterosteus aculeatus) from the Misty Lake system, Canada. This system is of particular interest because one population contrast (Lake vs. Outlet stream) shows very low genetic and morphological divergence, whereas another population contrast (Lake vs. Inlet stream) shows dramatic genetic and morphological divergence apparently without strong and symmetric reproductive barriers. To test whether selection against migrants might solve this “conundrum of missing reproductive isolation”, we performed a fully factorial reciprocal transplant experiment using 225 individually marked stickleback collected from the wild. Relative fitness of the different ecotypes (Lake, Inlet, and Outlet) was assessed based on survival and mass change in experimental enclosures. We found that Inlet fish performed poorly in the lake (selection against migrants in that direction), whereas Lake fish outperformed Inlet fish in all environments (no selection against migrants in the opposite direction). As predicted from their phenotypic and genetic similarity, Outlet and Lake fish performed similarly in all environments. These results suggest that selection against migrants is asymmetric and, together with previous work, indicates that multiple reproductive barriers contribute to reproductive isolation. Similar mosaic patterns of reproductive isolation are likely in other natural systems.
Highlights
Ecological speciation occurs when reproductive isolation arises due to adaptive divergence between populations inhabiting ecologically different environments (Schluter 2000; Nosil 2012)
We ask whether selection against migrants is evident: that is, Lake fish should perform better than Inlet fish in the lake whereas Inlet fish should perform better than Lake fish in the inlet
In the Lake-Inlet contrast, survival of both ecotypes was lower in the lake than in the inlet, and survival of Inlet fish was lower than survival of Lake fish in both environments, and in this model there was no evidence for an ecotype 9 environment interaction (Table 1, Fig. 2A)
Summary
Ecological speciation occurs when reproductive isolation arises due to adaptive divergence between populations inhabiting ecologically different environments (Schluter 2000; Nosil 2012). A growing number of studies report speciation in the apparent absence of ecological differences (Rundell and Price 2009; Svensson 2012) These variable results highlight the value of considering the relative contributions of multiple reproductive barriers in taxa that vary in their progress toward speciation – whether ecological or otherwise. Such analyses should prove useful in attempting to delineate the conditions that do and do not promote ecological speciation – and the combinations of reproductive barriers that are most important
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