Empirical evidence suggests that fluctuating selection is a major evolutionary mechanism. The most straightforward consequence of rapid changes of the fitness function is the induced response of the mean phenotype in the population. Yet, repeated back-and-forth evolutionary trajectories are also suspected to affect the genetic architecture underlying the phenotypic characters subject to continuous adaptation. In order to better understand the long-term consequences of fluctuating selection, we modeled the response of complex, multilocus genetic architectures to various natural selection regimes -- stabilizing, directional, and fluctuating. This model accounts for gene-gene interactions (through multilinear epistasis), and thus allows to investigate the dynamics of evolutionary potential at two distinct levels: (i) the standing genetic variation, i.e. the capacity for the population to respond immediately to directional selection, and (ii) the level of canalization (measured as the average effect of new mutations),which reflects the capacity for the population to replenish genetic variation. Both analytical results and individual-based simulations show that fast fluctuations (white noise change in the phenotypic optimum every generation) are essentially similar to stabilizing selection, promoting a degree of genetic canalization and low evolvability. In contrast, when large fluctuations of the phenotypic optimum (beyond the phenotypic range of the population) occur every 10 to 100 generations, equilibrium mutational effects and genetic variance are higher and the population is more evolvable. However, there was no evidence that decanalization and increased evolvability were adaptive, and fluctuating selection remains intrinsically more constraining than genetic drift.
Author(s): Voje, KL, Mazzarella AB, Hansen TF, Østbye K, Klepaker T, Bass A, Herland A, Bærum KM, Gregersen F, Vøllestad A
The morphological differences between the marine ancestor and the descendant freshwater populations of threespine sticklebacks constitute a well-studied example of a phenotypic radiation. However the exact selective agents that drive these changes are not yet fully understood. We present a comparative study across 74 freshwater populations of threespine sticklebacks in Norway to test whether evolutionary changes in stickleback morphology can be explained as adaptations to lake characteristics thought to reflect different habitats and feeding niches. Only weak indications of adaptation were found, and the rates of adaptation varied from immediate to more constrained evolution among traits. Instead, populations have diversified in phenotypic directions predictable from allometric scaling relationships. This indicates that evolutionary constraints may have played a role in structuring phenotypic variation across freshwater populations of stickleback.