Evolution__3rd_Edition

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Frequency
Frequency
Generation 1
Generation 2
Character
xp
Character
We construct a quantitative genetic
model of directional selection
R
S
x s
distinguished; disruptive selection is the third category, which we shall not discuss
further here. This section will be concerned with directional selection, which has particularly
been studied through artificial selection experiments. Artificial selection is
important in applied genetics, as it provides the means of improving agricultural stock
and crops.
If we wish to increase the value of a character by artificial selection, we can use any of
a variety of selection regimes. One simple form is truncation selection: the selector
picks out all individuals whose value of the character under selection is greater than a
threshold value, and uses them to breed the next generation (Figure 9.6). What will be
the value of the character in the offspring generation? First, we can define S as the mean
deviation of the selected parents from the mean for the parental population; S is also
called the selection differential. The response to selection (R) is the difference between
the offspring population mean and the parental population mean. In this case, calculating
the response to selection is found by regressing the character value in the offspring
on that in the parents, where the parents are the individuals that were selected to breed:
we plot the offspring’s against the parental deviation from the population average to
produce a graph like Figure 9.1. The slope of the graph for parents and offspring is
symbolized by b OP and we saw in the previous section that for any character b OP = h 2 ;
the parent–offspring regressional slope equals the heritability. Therefore:
R = bOPS or
R = h2S CHAPTER 9 / Quantitative Genetics 237
Figure 9.6
Truncation selection: the next generation is bred from those
individuals (shaded area) with a character value exceeding a
threshold value. The selection differential (S) is the difference
between the whole population mean (x p ) and the selected
subpopulation’s mean (x s ); S = x s − x p . Because R is about 0.4
of S we could deduce in this case that heritability, h 2 , ≈ 0.4.
This is an important result. The response to selection is equal to the amount by which
the parents of the offspring generation deviate from the mean for their population