Re: Paper: Environmental Coupling of Selection and Heritability Limits Evolution


"Robert Karl Stonjek" <rstonjek@xxxxxxxxxxxxxx> wrote in message news:e6phq6$lti$1@xxxxxxxxxxxxxxxxxxxxxx
Environmental Coupling of Selection and Heritability Limits Evolution
A. J. Wilson, J. M. Pemberton, J. G. Pilkington, D. W. Coltman, D. V.
Mifsud, T. H. Clutton-Brock, L. E. B. Kruuk

There has recently been great interest in applying theoretical quantitative
genetic models to empirical studies of evolution in wild populations.
However, while classical models assume environmental constancy, most natural
populations exist in variable environments. Here, we applied a novel
analytical technique to a long-term study of birthweight in wild sheep and
examined, for the first time, how variation in environmental quality
simultaneously influences the strength of natural selection and the genetic
basis of trait variability. In addition to demonstrating that selection and
genetic variance vary dramatically across environments, our results show
that environmental heterogeneity induces a negative correlation between
these two parameters. Harsh environmental conditions were associated with
strong selection for increased birthweight but low genetic variance, and
vice versa. Consequently, the potential for microevolution in this
population is constrained by either a lack of heritable variation (in poor
environments) or by a reduced strength of selection (in good environments).
More generally, environmental dependence of this nature may act to limit
rates of evolution, maintain genetic variance, and favour phenotypic stasis
in many natural systems. Assumptions of environmental constancy are likely
to be violated in natural systems, and failure to acknowledge this may
generate highly misleading expectations for phenotypic microevolution.

Source PLoS (Free)
http://biology.plosjournals.org/perlserv/
?request=get-document&doi=10%2E1371%2Fjournal%2Epbio%2E0040216

Thx, RKS. This study is interesting to me for a number of reasons.
One is that it touches on issues related to maternal effects and
Edser's eccentric definition of fitness. Birth weight depends more
on the mother's genetics than the lamb's, and it has a big impact on
whether the lamb survives to maturity. So this is a case where Edser's
view of fitness may actually work better than the conventional view.

But it also touches on one of my recent themes - the heritability of
fitness and of fitness components. The apparent paradox is that
birthweight seems to be heritable, and is positively correlated with
fitness of lambs, but sheep are not evolving toward higher weights.
Why not?

The answer seems to be that there is a balance when averaged over
a varying environment. In bad years, it is best to be a mother producing
high-birth-weight lambs, which can survive the harsh conditions. But in
good years, it is better for the mother to produce low-birth-weight
twins, who have good survival chances in those years. Amusingly, Edser's
viewpoint sees this as natural, while the more conventional approach
needs considerable analytical machinery (negative covariances, etc.) in
order to provide the explanation. And it is fairly unintuitive to
claim that one feature of the lamb's phenotype that must be fed into
the analysis is whether or not the lamb has a twin sibling.


.

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