Evolution__3rd_Edition

238 PART 2 / Evolutionary Genetics
Sustained directional selection
reduces heritability
Figure 9.7
Response of corn (Zea mays)
artificially selected for high or
low oil content. The experiment
began in 1896 when, from a
population of 163 corn ears, the
high line was formed from the
24 ears highest in oil content
and the low line from the 12
ears with lowest oil content.
The low line was discontinued
after generation 87. Modified,
by permission of the publisher,
from Dudley & Lambert (1992).
multiplied by the character’s heritability. (The response to selection or the parent–
offspring regression can be used to estimate the heritability of a character; for a selected
population, they are two ways of looking at the same set of measurements.)
A real example of directional selection may not have the form of truncation selection.
In truncation selection, all individuals above a certain value for the character
breed and all individuals below do not breed. All the selected individuals contribute
equally to the next generation. It could be instead that there is no sharp cut off, but that
individuals with higher values of the character contribute increasing numbers of offspring
to the next generation. However, the same formula for evolutionary response
works for all forms of directional selection. The difference between the mean character
value in the whole population and in those individuals that actually contribute to the
next generation (if necessary, weighted by the number of offspring they contribute) is
the “selection differential” and can be plugged into the formula to find the expected
value of the character in the next generation.
A population can only respond to artificial selection for as long as the genetic variation
lasts. Consider, for example, the longest running controlled artificial selection
experiment. Since 1896, corn has been selected, at the State Agricultural Laboratory in
Illinois, for (among other things) either high or low oil content. As Figure 9.7 shows,
even after 90 generations the response to selection for high oil content has not been
exhausted. However, the oil content finally became negligibly low in the line selected
for low oil content. The seeds had become difficult to maintain and the “low oil” experiment
was discontinued after 87 generation.
The “high oil” experiment continues, but it too will eventually come to a stop. As the
corn is selected for increased oil content, the genotypes encoding for high oil content
will increase in frequency and be substituted for genotypes for lower oil content. This
process can only proceed so far. Eventually all the individuals in the population will
come to have the same genotype for oil content. At the loci controlling oil content, no
Oil (%)
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Generation
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