Abstract
BackgoundIt is a common observation in evolutionary studies that larger, more ornamented or earlier breeding individuals have higher fitness, but that body size, ornamentation or breeding time does not change despite of sometimes substantial heritability for these traits. A possible explanation for this is that these traits do not causally affect fitness, but rather happen to be indirectly correlated with fitness via unmeasured non-heritable aspects of condition (e.g. undernourished offspring grow small and have low fitness as adults due to poor health). Whether this explanation applies to a specific case can be examined by decomposing the covariance between trait and fitness into its genetic and environmental components using pedigree-based animal models. We here examine different methods of doing this for a captive zebra finch population where male fitness was measured in communal aviaries in relation to three phenotypic traits (tarsus length, beak colour and song rate).ResultsOur case study illustrates how methods that regress fitness over breeding values for phenotypic traits yield biased estimates as well as anti-conservative standard errors. Hence, it is necessary to estimate the genetic and environmental covariances between trait and fitness directly from a bivariate model. This method, however, is very demanding in terms of sample sizes. In our study parameter estimates of selection gradients for tarsus were consistent with the hypothesis of environmentally induced bias (βA = 0.035 ± 0.25 (SE), βE = 0.57 ± 0.28 (SE)), yet this differences between genetic and environmental selection gradients falls short of statistical significance.ConclusionsTo examine the generality of the idea that phenotypic selection gradients for certain traits (like size) are consistently upwardly biased by environmental covariance a meta-analysis across study systems will be needed.
Highlights
Selection acts on the phenotypes of individuals, while an evolutionary response to selection requires genetic transmission to following generations [1]
In our study parameter estimates of selection gradients for tarsus were consistent with the hypothesis of environmentally induced bias (bA = 0.035 ± 0.25 (SE), bE = 0.57 ± 0.28 (SE)), yet this differences between genetic and environmental selection gradients falls short of statistical significance
The second aim of the current study is to relate song rate, beak colour and tarsus length to an important fitness component: success at fertilizing eggs under freeflying aviary conditions, and decompose the selection acting at the phenotypic level into genetic and environmental selection gradients to see if the observed selection can lead to an evolutionary response
Summary
Selection acts on the phenotypes of individuals, while an evolutionary response to selection requires genetic transmission to following generations [1]. One might observe a positive correlation between a phenotypic trait like body size and fitness in a population of birds (Figure 1). It may be that a large body size reflects good early growth conditions and that these good conditions constitute the causal link to fitness (by affecting other aspects of the phenotype e.g. health). In this example a large body size per se does not cause higher fitness, and the environmental components of body size are not inherited to the generation. Numerous studies on wild animal populations have found consistent positive selection for body size, condition or laying date without finding any indication of a resulting
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