Unidirectional response to bidirectional selection on body size II Quantitative genetics

Abstract

Anticipating the genetic and phenotypic changes induced by natural or artificial selection requires reliable estimates of trait evolvabilities (genetic variances and covariances). However, whether or not multivariate quantitative genetics models are able to predict precisely the evolution of traits of interest, especially fitness-related, life-history traits, remains an open empirical question. Here, we assessed to what extent the response to bivariate artificial selection on both body size and maturity in the medaka Oryzias latipes, a model fish species, fits the theoretical predictions. Three populations were selected for divergent body size while maintaining a constant selection pressure against late maturity. The observed evolutionary trends did not match the predictions from a bivariate quantitative genetics “animal” model. The most parsimonious model identified environmental, but not genetic, covariances between both traits, which cannot explain why body size did not evolve in the line selected for a smaller body length. We investigated alternative mechanisms (including genetic drift, inbreeding depression, natural selection, scaling or genetic asymmetry issues, and undetected genetic correlations) but could not attribute the deviation from theory to any single explanation. Overall, these results question the ability of multivariate quantitative models to provide valid and operational predictions of the evolutionary response to multivariate selection on complex traits.

Unidirectional response to bidirectional selection on body size. I. Phenotypic, life‐history, and endocrine responses

“…(2015) involved mating the fish 14 days after that 50% of the population reached maturity, a delay that was possibly not long enough to allow for 100% of the fish to reach maturity, in which case selection was applied both on body size and for maturity (similar to our own design). As discussed by Le Rouzic et al (in press), available evidence suggests that response to such bivariate selection on correlated traits is often erratic.…”

“…Therefore, we chose to derive three large‐population lines (25 < N e < 30 in each) rather than replicating small‐population treatments. A pedigree‐based quantitative genetic analysis suggests that medaka trait dynamics in the Large line were not compatible with random drift, and instead reflected deterministic evolutionary processes (Le Rouzic et al, in press).…”

“…Inability of medaka to respond to selection for a smaller body size is a warning signal that calls for increasing research efforts to assess life‐history evolvability in wild populations. A crucial line of work in achieving this goal will consist in accurately measuring the multivariate components of selection that act on correlated life‐history traits such as body size and maturity (Lande & Arnold, 1983; Le Rouzic et al, in press), both in the wild and in laboratory experiments. The other key element of this effort will rely on developing diagnosis tools to evaluate potential for (and signature of) adaptive response to size‐dependent, anthropogenic selection (Therkildsen et al., 2019).…”

“…We proceeded with selection on the F 1 – F 7 generations from February 2014 to August 2016 (30 months, Appendix ). A size‐dependent selection differential was applied both on families at 60 dph and on mature individuals at 75 dph, an age at which 86% of the fish were mature on average (for dynamics of maturity in each line, see Le Rouzic et al, in press).…”

Size-dependent eco-evolutionary feedbacks in fisheries

“…The outcome of evolution also depends on whether food availability is sufficient to support fast somatic growth (Chase 1999). To our knowledge, the available empirical and experimental evidence more often supports delayed maturation at a larger body size when predation targets small-sized individuals (Edley and Law 1988, Wellborn 1994, Beckerman et al 2010, Le Rouzic et al 2020).…”