Evolution__3rd_Edition

250 PART 2 / Evolutionary Genetics
Summary
1 Quantitative genetics, which is concerned with
characters controlled by many genes, considers the
changes in phenotypic and genotypic frequency distributions
between generations, rather than following
the fate of individual genes.
2 The phenotypic variance of a character in a
population can be divided into components due
to genetic and environmental differences between
individuals.
3 Some of the genetic effects on an individual’s
phenotype are inherited by its offspring; others are
not. The former are called additive genetic effects and
the latter are due to such factors as dominance and
epistatic interaction between genes.
4 The heritability of a character is the proportion of
its total phenotypic variance in a population that is
additive.
5 The heritability of a character determines its evolutionary
response to selection.
6 The additive genetic variance can be measured by
the correlation between relatives, or by artificial selection
experiments.
response to selection can be analyzed by means of the heritability of a character, which
is the fraction of its variation due to additive genetic effects. However, even with simple
directional selection, the exact response depends on the underlying genetic control. For
example, the possible threshold relation between the genotype and phenotype for the
wing veins of the fruitfly generates an interesting bimodal response to selection. Here,
the heritability of the character would show strange changes as the character evolved.
Directional selection unambiguously should continue to alter a character until its heritability
is reduced to zero. With stabilizing selection, it might be thought that many
genotypes could be maintained if they all produce the same intermediate phenotype.
However, even here it can be argued that all but one of the genotypes should eventually
be eliminated by selection. The argument appears to be contradicted by the facts, and
biologists do not yet fully understand the observed values of heritabilities in natural
populations.
7 The strength of selection can be estimated as a
selection differential, either directly by measuring
reproductively successful individuals and average
individuals in a population, or indirectly, using measurements
of heritability and of observed evolutionary
change in a population. Selection differentials often
have values in the range −0.2 to +0.2 in populations
that are experiencing directional selection.
8 The response of a population to artificial selection
depends on the amount of additive genetic variability
and on the relation between genotype and phenotype.
If the relation is non-linear, strange bimodal responses
can arise.
9 Stabilizing selection acts to reduce the amount of
genetic variability in a population. However, polygenic
characters show non-trivial values for heritability.
10 Biologists do not completely understand the obseved
levels of heritability for characters in natural populations.
Genetic variation is maintained by some mix of
mutation and selection. Some characters do evolve by
the amount predicted from their heritability and the
strength of selection; others, however, seem not to.
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