Evolution__3rd_Edition

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New variation can be introduced by
recombination or mutation
Figure 9.8
Artificial selection for increased
numbers of scutellar chaetae in
Drosophila melanogaster. The
response took place in two
rapid steps that coincided with
observable changes in the form
of chromosomes 2, and 2 and 3,
respectively; the changes are
thought to have been
recombinational events.
Redrawn, by permission of the
publisher, from Mather (1943).
CHAPTER 9 / Quantitative Genetics 239
Table 9.2
The heritability of oil content in corn populations after different numbers of generations of
artificial selection for high or low oil content, in the Illinois corn experiment (see Figure 9.7).
The heritability declines as selection is applied. From Dudley & Lambert (1992).
Heritability of oil content
Generation High line Low line
1–9 0.32 0.5
10–25 0.34 0.23
26–58 0.11 0.1
59–90 0.12 0.14
additive genetic variance will then be left; heritability will have been reduced to zero
and the response to artificial selection will come to a stop. In the Illinois corn experiment,
the process has not yet run its full course. The heritability of oil content in both
the high and low selected lines decreased early in the experiment (Table 9.2), but since
then it has been constant at about 10–15% for about 65 generations. The population
continues to respond to selection because heritability continues to be above zero.
In other artificial selection experiments, the full process has been recorded. Figure 9.8
shows the response of a population of fruitflies to consistent directional selection for
increased numbers of scutellar chaetae (i.e., bristles on a dorsal region of the thorax).
Initially the population responded; then, as the additive genetic variation was used up
(or, as its heritability declined), the rate of change slowed down to a stop in generations
4–14. It also appeared that, if selection was still continued after the response stopped,
Mean chaetae number
5.4
5.2
5.0
4.8
4.6
4.4
4.2
Chromosome
2
Response due to change in:
Chromosomes
2 and 3
4.0 0 5 10 15 20 25
Generations of selection