Changing times, spaces, and faces: tests and implications of adaptive morphological plasticity in the fishes of northern postglacial lakes

TLDR

Freshwater fishes in postglacial lakes exhibit high phenotypic variation and rapid divergence, yet the contribution of phenotypic plasticity to this diversity has been largely unexamined. The authors synthesize existing literature and perform a meta‑analysis to test whether morphological plasticity in these fishes is adaptive. A meta‑analysis of published data across multiple fish families was conducted to evaluate adaptive responses to littoral and pelagic lake environments. Morphological plasticity is common, often induced by habitat conditions, typically adaptive, heritable, and appears to promote rather than constrain rapid adaptive divergence in postglacial lake fishes.

Abstract

The phenotypic diversity exhibited within and among populations of freshwater fishes in postglacial lakes has intrigued biologists for two reasons: (i) their high phenotypic variation and (ii) the apparently recent and rapid divergence of forms. Genetic and ecological studies of these taxa are shedding new light on mechanisms of divergence and species formation. Surprisingly, the roles of phenotypic plasticity in the origins, maintenance, and generation of phenotypic diversity in this system are rarely directly addressed. We synthesize the available literature on morphological plasticity in these fishes and, using a meta-analysis, test for adaptive plasticity. We conclude that (i) morphological plasticity is common in at least six families of northern freshwater fishes, (ii) plastic responses can often be induced by conditions related to littoral and pelagic lake environments, (iii) plasticity often represents adaptive responses to conditions in these habitats, and (iv) that although rarely tested, heritable variation in morphological plasticity is present. The rich amount of phenotypic plasticity has not constrained recent adaptive divergence and species formation in postglacial fishes, and instead plasticity may play a role in the notably high rates of divergence observed in these and other fishes currently undergoing adaptive radiation.