When thinking about ecological speciation, we often assume that habitat-related local adaptation drives genetically based population divergence, and that the resulting phenotypic divergence promotes reproductive isolation. However, strong phenotypic divergence and associated reproductive isolation might also arise directly via phenotypic plasticity. In a paper that just went online, we start exploring the latter pathway to speciation in lake and stream stickleback populations in the Lake Constance basin (Central Europe). This system is cool because the stream populations consistently show the relatively small body size at reproduction typical of stream stickleback worldwide (Moser et a. 2012, PLoS One), whereas the lake fish reproduce at much larger body size (the photo below shows a representative reproductive male from Lake Constance and a tributary). Given that experimental work in many stickleback systems has shown that body size differences between populations represent a major sexual reproductive barrier, we wanted to understand how the body size differences among lake and stream fish arise. A combination of laboratory and field transplant work (see photos below) revealed that the divergence seen in the wild is largely plastic, most likely driven by differences in resource availability between the two habitat types: lake fish use a relatively poor limnetic food base, grow slowly, fail to reach reproductive size after one year, and hence reproduce only after two years at large size. Stream fish, by contrast, exploit a rich benthic food base, reach critical size after one year, reproduce directly at relatively small size, and die. In the laboratory and in field transplants, these life history differences disappear. We thus hypothesize that in this system, sexual reproductive barriers might have established immediately due to plastic divergence in life history. Evaluating this hypothesis experimentally is the next step. Also, it would be great to know if in other lake-stream stickleback systems, body size differences do have a stronger genetic component than in the Lake Constance basin.
If interested in the findings, see Moser et al. (2015), Evolutionary Biology: Lake-stream divergence in stickleback life history: a plastic response to trophic niche differentiation? http://link.springer.com/article/10.1007/s11692-015-9327-6