Evolution__3rd_Edition

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Behavior
Physiology
Life history
Morphology
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Heritability
Genetic variation has been studied
by two theoretical approaches
CHAPTER 9 / Quantitative Genetics 247
Figure 9.14
Heritabilities of quantitative characters in Drosophila. Hoffmann
(2000) compiled field and laboratory estimates of heritabilities
for many characters in fruitflies. He divided the characters into
four categories. Each datapoint in the figure is an average for a
number of estimates for one character. For instance, heritabilities
have been estimated for four life history characters in fruitflies.
Any one of those characters may have been studied more
than once, and the results of the different studies produce a
range of estimates, but only the average is shown here. The
29 morphological characters are things like “wing length” and
“tibia width.” The heritabilities of morphological and physiological
characters tend to be higher than those for behavioral and life
history characters, but it is questionable whether the difference is
biologically meaningful. Drawn from data in Hoffmann (2000).
ground. We have been looking at widely accepted theories and results. The results in
Figure 9.14 are also widely accepted, as is the reasoning in Section 9.10. But when we
move on to think about what maintains the genetic variation we are moving on to a
frontier research problem. The question does not yet have a generally accepted answer.
9.12 Levels of genetic variation in natural populations are
imperfectly understood
For characters subject to stabilizing selection, two processes can explain the existence of
heritable genetic variation. One is mutation–selection balance. The character may have
some optimum value, and natural selection eliminates genes that cause deviations from
that optimum. But mutations will continually arise, causing no deviations from the
optimum. The result is an equilibrium, at which some genetic variation exists because
selection cannot clear out mutations instantly with 100% efficiency.
For any one locus, the amount of variation maintained by this selection–mutation
balance is low because mutation rates are low (Section 5.11, p. 122); but for a polygenic
character, mutation rates should be approximately multiplied by the number of loci
influencing the character. A character controlled by 500 loci will have 500 times the
mutation rate of a one-locus character. The amount of variability that can be maintained
is proportionally increased.
How much genetic variation will exist? The question has been thought about in two
main ways. One, revived and developed by Lande (1976), considers stabilizing selection
on a continuous character (such as body size) controlled by many loci. Mutations at
any of the loci can influence the character; because a genotype may be above or below
the optimal value for the trait, a small random mutation has a 50% chance of being an
improvement. The other, revived and developed by Kondrashov & Turelli (1992), does
not consider stabilizing selection on a phenotypic character, but supposes mutations
are occurring at many loci and the great majority (many more than 50%) are deleterious.
The result is a balance between selection and deleterious mutation at many loci.