Evolution__3rd_Edition

..
Some heritable characters do not
respond to selection ...
. . . and the reason is the subject of
unfinished research
In the same way as we did with salmon and finches (Sections 9.7 and 9.8), we can use
the standard equation R = h 2 S to predict how the flycatchers will evolve. The evolutionary
response should be a bit less than 0.35 × 0.2 per generation. This is a “standardized”
response, expressed as a fraction of 1 standard deviation. In absolute terms, it corresponds
to a predicted increase of about 0.04 mm per generation (+0.022 mm per year).
Kruuk et al. (2001) tested the prediction by measuring tarsus length over the 20-year
period in a large sample of birds. The result is shown in Figure 9.15b. Tarsus length has
been net constant. (In fact it shows a fractional decrease, though the decrease is statistically
and probably biologically insignificant.)
The collared flycatcher is not the only species in which a character is apparently
subject to directional selection, and apparently shows heritability, but is not showing
an evolutionary response. Biologists are puzzled by these results and have a number
of hypotheses about them. The measurements of heritability may be erroneous or
inappropriate. Migration, or hybridization with other species, may be confusing the
picture.
A biologically more interesting possibility is that the effect of selection, to increase
(for example) the tarsus length each generation, is balanced by some other force that
decreases tarsus length by approximately the same amount. For instance, the population
might be in selection–mutation equilibrium (Section 5.11, p. 122, but allowing
for multiple loci). If a large tarsus is advantageous, the effect of disadvantageous mutations
each generation will be to reduce average tarsus length. Then, at equilibrium, the
increase in tarsus length each generation by directional selection could be balanced by a
decrease due to mutation. The character would be both constant over time and heritable.
Mutation is not the only factor that could balance directional selection in this
way; intraspecific competition could too. However, these ideas are all hypotheses to be
tested. The lack of evolutionary response in heritable characters apparently subject to
directional selection is not understood. Quantitative genetics is one of the oldest topics
in evolutionary biology, and contains many solid findings. But it has its share of
unsolved problems for the future too. Non-evolution, in species that are predicted to
show evolutionary change, is one of them.
9.13 Conclusion
CHAPTER 9 / Quantitative Genetics 249
One- and two-locus population genetics are used for characters controlled by one or
two loci and whose genetics are known. Quantitative genetics provides the techniques
to understand evolution in characters that are influenced by a large number of genes,
and for which the exact genotype (or genotypes) producing any given phenotype are
unknown. It is possible that the majority of characters have this kind of genetics, in
which case quantitative genetics would be appropriate for understanding the majority
of evolution; at any rate, it is a highly important set of techniques. We have seen in this
chapter how quantitative genetics divides up the variation in a character, to recognize
the component a the additive genetic effect a that controls how offspring resemble
their parents. The additive genetic effect plays the same role in quantitative genetics as a
knowledge of Mendelian genetics in one- and two-locus population genetics. The