Lake-stream divergence in stickleback life history: a plastic response to trophic niche differentiation?

Speciation can be promoted by phenotypic plasticity if plasticity causes populations in ecologically different habitats to diverge in traits mediating reproductive isolation. Although this pathway can establish reproductive barriers immediately and without genetic divergence, it remains poorly investigated. In threespine stickleback fish, divergence in body size between populations represents a potent source of reproductive isolation because body size often influences reproductive behavior. However, the relative contribution of phenotypic plasticity and genetically based divergence to stickleback body size evolution has not been explored. We here do so by using populations residing contiguously in Lake Constance (Central Europe) and its tributaries, a system where lake fish exhibit strikingly larger size and greater age at maturity than stream fish. Laboratory experiments reveal the absence of substantial genetic divergence in intrinsic growth rates and maturation size thresholds between lake and stream fish. A field transplant experiment further demonstrates that lake fish display the life history typical of stream fish when exposed to stream habitats for one year, confirming that life history divergence in this system is mainly plastic. This plasticity appears to be driven by restricted food availability in the lake relative to the stream habitat. We thus propose that in this stickleback system, the exploitation of different trophic niches immediately promotes reproductive isolation via resource-based plasticity in life history.