Morphological evolution of an anadromous threespine stickleback population within one generation after reintroduction to Cheney Lake, Alaska

Abstract

Background: In 2009, anadromous threespine stickleback (Gasterosteus aculeatus) were stocked in Cheney Lake, Alaska, following treatment with the ichthyocide rotenone to remove invasive northern pike (Esox lucius), but which also extirpated native species. This introduction provides an opportunity to directly observe the evolutionary transition of oceanic stickleback to a freshwater-adapted population.

Objective: Rear offspring of F1 Cheney Lake stickleback under two common garden treatments alongside fish from Rabbit Slough, their anadromous source population, to test for rapid morphological evolution and contributions of phenotypic plasticity to morphological divergence.

Methods: We reared 198 Cheney Lake and 209 Rabbit Slough stickleback under two conditions designed to simulate the habitat structure and diets of complex, shallow lake and simple, open-water habitats. We preserved stickleback at four time intervals to account for allometric growth effects. We made linear measurements of several traits and used geometric morphometrics to characterize variation in body shape.

Results: Both phenotypic plasticity between experimental treatments and evolutionary divergence between populations accounted for variation in the sample, and these effects were independent of one another. Morphological differences between treatments interacted with stickleback age group, indicating an effect of rearing condition on allometric growth. Thus stickleback morphology in Cheney Lake has evolved in a single generation, but phenotypic plasticity is likely to strongly influence morphological variation.