Evolution__3rd_Edition

242 PART 2 / Evolutionary Genetics
Look at the bimodal response in the
figure opposite!
Relations between genotype and
phenotype can be linear or nonlinear
measure of the amount of variation that is independent of the units used to do the
measuring. The equation R = h 2 S works with standardized selection differentials too,
and gives the response as a fraction of the standard deviation of the population.)
Figure 9.10 shows that standardized selection differentials are mainly in the range
–0.25 to +0.25. Kingsolver et al. (2001) and Hoekstra et al. (2001) use the results of
their survey to make some tentative further deductions, but for our purposes their
survey shows that we have a large amount of evidence, in which quantitative genetic
techniques have been used to study directional selection in nature. It also shows the
general range of results of those studies.
9.9 Relations between genotype and phenotype may
be non-linear, producing remarkable responses
to selection
Figure 9.11 illustrates a remarkable artificial selection experiment. Scharloo selected
a population of fruitflies for increased relative length of the fourth wing vein
(Figure 9.11a). The figure shows the frequency distribution of vein lengths in the population
for 10 generations. A length of 60–80 has been reached by about generation 5.
At this stage the frequency distribution (amid the scatter that is often seen in real
experiments) starts to show a consistent bimodality: it is clearest in generations 5–7,
with only the high peak being maintained in generations 8–10. The experiment
suggests that more complicated things can occur in artificial selection experiments
than we have seen so far. What is going on?
The key to understanding the shape of the response is the relation between genotype
and phenotype. A simple response, such as that for oil content in Figure 9.7 or bristle
number in Figure 9.8, results when there is an approximately linear relation between
genotype and phenotype (Figure 9.12a). Genotype here is expressed as a metrical variable.
The easiest way to think of this is to imagine that the character is controlled by
many loci; at each, some alleles (+) cause the phenotypic character to increase, and
others (−) to decrease. The more positive genes an individual has, the higher its genotypic
value (Figure 9.12a). Then when we select for an increase in the character, we pick
the individuals with more positive genes, and the value of the character will increase
smoothly between generations in the manner of the Illinois corn oil experiment.
The approximately linear form of Figure 9.12a is not the only possible relation
between genotype and phenotype (cf. Figure 9.12b and c). The bimodal response in
Figure 9.11 is thought to result from a threshold relation between genotype and phenotype
(Figures 9.12c and 9.13). In Figure 9.13, the graph has been rotated through 180°
relative to the form in Figure 9.12; the x-axis (genotype) is drawn down the page on the
left. The genotype is thought to control the amount of some vein-inducing substance.
Vein length is shown across the top of the graph. The relation between substance and
vein length is hypothesized to contain a jump at vein length 60–80 (where the artificial
selection response goes bimodal).
Imagine the course of selection for longer wing veins with this threshold relation
between genotype and phenotype. The population starts at the top left of the graph with
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