Evolutionary stasis of a heritable morphological trait in a wild fish population despite apparent directional selection

Abstract

Comparing observed versus theoretically expected evolutionary responses is important for our understanding of the evolutionary process, and for assessing how species may cope with anthropogenic change. Here, we document directional selection for larger female size in Atlantic salmon, using pedigree‐derived estimates of lifetime reproductive success as a fitness measure. We show the trait is heritable and, thus, capable of responding to selection. The Breeder's Equation, which predicts microevolution as the product of phenotypic selection and heritability, predicted evolution of larger size. This was at odds, however, with the observed lack of either phenotypic or genetic temporal trends in body size, a so‐called “paradox of stasis.” To investigate this paradox, we estimated the additive genetic covariance between trait and fitness, which provides a prediction of evolutionary change according to Robertson's secondary theorem of selection (STS) that is unbiased by missing variables. The STS prediction was consistent with the observed stasis. Decomposition of phenotypic selection gradients into genetic and environmental components revealed a potential upward bias, implying unmeasured factors that covary with trait and fitness. These results showcase the power of pedigreed, wild population studies—which have largely been limited to birds and mammals—to study evolutionary processes on contemporary timescales.

Captive-bred Atlantic salmon released into the wild have fewer offspring than wild-bred fish and decrease population productivity

“…This trapping regime has allowed for the collection of annual census data based on total counts, phenotypic and genetic sampling of the potential anadromous wild-spawning population (electronic supplementary material, table S2) which, in combination with molecular parentage assignment, has facilitated the estimation of lifetime fitness for individual fish using a molecular pedigree. See [46,47] for details of tissue sampling procedures, identity and parentage analysis, sex determination and pedigree reconstruction. Pedigree data available at [48].…”

Maturation in Atlantic salmon (Salmo salar, Salmonidae): a synthesis of ecological, genetic, and molecular processes

“…How maturation at different life history stages influences fitness is not well understood. This can be solved by long-term pedigree studies, breeding experiments and modeling approaches aimed at understanding the ecological factors and underlying genetics of reproductive fitness (e.g., McGinnity et al 2003;Mobley et al 2020;O'Sullivan et al 2019). For example, noting the strong effect of incubation temperature on the growth and timing of the return migration, how incubation temperature affects the reproduction and survivorship of the F1 generation is still unknown.…”

Reproductive success is influenced by early development: the covariation of natal body size and reproductive performance in grey seals (Halichoerus grypus)

“…If this trait is at stasis, genetic constraints may also prevent the evolution of body size (e.g. a lack of heritable variation, genetic correlations between traits under selection); or, genetic evolution of larger size is occurring but these changes are masked at the phenotypic level by changes in environmental conditions (Pujol et al 2018, O'Sullivan et al 2019.…”