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

Anthropogenic perturbations such as harvesting often select against a
large body size and are predicted to induce rapid evolution towards
smaller body sizes and earlier maturation. However, body-size evolvability
and, hence, adaptability to anthropogenic perturbations remain seldom
evaluated in wild populations. Here, we use a laboratory experiment over 6
generations to measure the ability of wild-caught medaka fish (Oryzias
latipes) to evolve in response to bidirectional size-dependent selection
mimicking opposite harvest regimes. Specifically, we imposed selection
against a small body size (Large line), against a large body size (Small
line) or random selection (Control line), and measured correlated
responses across multiple phenotypic, life-history and endocrine traits.
As expected, the Large line evolved faster somatic growth and delayed
maturation, but also evolved smaller body sizes at hatch, with no change
in average levels of pituitary gene expressions of luteinizing,
follicle-stimulating or growth (GH) hormones. In contrast, the Small
medaka line was unable to evolve smaller body sizes or earlier maturation,
but evolved smaller body sizes at hatch and showed marginally-significant
signs of increased reproductive investment, including larger egg sizes and
elevated pituitary GH production. Natural selection on medaka body size
was too weak to significantly hinder the effect of artificial selection,
indicating that the asymmetric body-size response to size-dependent
selection reflected an asymmetry in body-size evolvability. Our results
show that trait evolvability may be contingent upon the direction of
selection, and that a detailed knowledge of trait evolutionary potential
is needed to forecast population response to anthropogenic change.