Evolution__3rd_Edition

246 PART 2 / Evolutionary Genetics
. . . will tend to become uniform,
with homozygotes at all loci, ...
. . . and yet real characters show
much genetic variation
have the optimal phenotype, whereas some of the offspring of genotypes (1) and (2) do
not. In a population made up of these three genotypes, selection slightly favors genotype
(3). If the environment were constant for a long time, always favoring the same
phenotype, selection should eventually produce a uniform population with a genotype
like (3).
We can take the argument a stage further. Genotype (3) is not the only true-breeding
homozygote that can produce the intermediate optimal form. All the following do too:
+–+–+–+–+– ++––++––+– +++–––++––
+–+–+–+–+– ++––++––+– +++–––++––
(4) (5) (6)
Suppose there was a population made up of genotypes (3) to (6), and selection still
favors the intermediate phenotype. What will happen now? Evolution will again tend
toward a population with only one genotype, and that genotype should be a multiple
homozygote.
The reason is that any one of the homozygotes that happens to have a slightly higher
frequency than the others has an advantage. Suppose, for example, that genotype (4)
had a higher frequency than (3), (5), and (6). All the genotypes will now be most likely
to mate with genotype (4). When genotype (4) mates with genotype (4), all their offspring
have the favored phenotype, identical to their parents. But when genotype (3),
(5), or (6) mates with genotype (4), the offspring contain potentially disadvantageous
genotypes. The offspring of a mating between genotype (3) and genotype (4) will be
+–+–+–+–+–/++––++––+– and has the favored intermediate phenotype. However,
its offspring will contain disadvantageous recombinants. The end result is for
selection to produce a uniform population, in which minority genotypes are selected
against because they do not fit in with the majority form. Selection eventually reduces
the genetic variability to zero, even with stabilizing selection.
In conclusion, whether a character is subject to directional or stabilizing selection,
the effect of selection is to reduce the amount of genetic variation, and the heritability.
If selection were the only factor at work, and it worked steadily for a period of time,
heritability would be reduced to zero.
9.11 Characters in natural populations subject to stabilizing
selection show genetic variation
The conclusion of the previous section is contradicted by observable facts. Heritabilities
can be measured for real characters, and many show significant genetic variation.
Figure 9.14 summarizes some measurements for Drosophila. It suggests that typical
values for heritability are in the range 20–50%. Heritabilities have been measured in
other species too, such as the Galápagos finch, and the results fit the same pattern. Real
characters have heritabilities of more than zero.
If selection, whether directional or stabilizing, eliminates genetic variation, why does
all this genetic variation exist? Until now, in this chapter, we have been on fairly solid
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