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.
Highlights
Across three generations of our molecular pedigree, we could not demonstrate a clear pattern of change in body size for female Atlantic salmon at the phenotypic level, congruent with both the observed stasis in breeding values and the predicted evolutionary stasis according to the secondary theorem of selection (STS)
We used the Δβ test to infer that missing traits correlated with female body size were likely present, and that using the estimated phenotypic selection gradient in the univariate Breeder's Equation (BE) will likely lead to a biased microevolutionary prediction—that is that larger body size should evolve, when no evolutionary trend was observed, nor was any predicted by the STS
Our results caution against naïve expectations of directional evolution, even when the key “ingredients” of directional selection and heritability are present, especially in studies where power may be low
Summary
The Burrishoole catchment in the West of Ireland (Figure S1) drains an area of approximately 100 km of varying topography and land use (de Eyto et al, 2016) and consists of three major lakes: brackish Lough Furnace, connected to the sea by the Burrishoole River, and the larger, freshwater Lough Feeagh and Bunaveela, with Atlantic salmon spawning in a series of afferent rivers. The probability that the observed change in EBVs was different from a scenario of genetic drift was calculated by simulating random breeding values for Size’ down the pedigree using the rbv() function in MCMCglmm (Hadfield, 2010) for each of the 2,000 posterior samples of the univariate animal model for Size’ based on the estimated VA. The STS, on the other hand, predicted a rate of evolutionary change in female body size of −0.004 Haldanes, with credible intervals broadly overlapping zero (95% HPD: −0.21, 0.10; Figure 3); that is, it predicted a lack of any consistent response to selection, which was concordant with the observed lack of temporal trend in estimated breeding values or mean phenotype. Credible intervals overlapped zero (95% HPD: −0.21, 1.7: Figure 4) with 94.2% of the slope estimates greater than zero
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