Evolution__3rd_Edition

248 PART 2 / Evolutionary Genetics
Some force of natural selection is
needed to explain observed levels
of genetic variation
Figure 9.15
(a) Natural selection favors
collared flycatchers (Ficedula
albicollis) with longer legs.
The results are expressed as a
selection differential. The
average selection differential
over this 20-year period was
just below +0.2 (indicated by
the dotted line). (b) Observed
changes in tarsal length. No net
change is statistically detectable
over the 20-year period. Year 0
is 1980, year 20 is 2000. From
Kruuk et al. (2001).
We do not have space to go far into either theoretical system, or their relative merits.
However, the research so far suggests that selection–mutation balance can explain some
heritable genetic variation, but not the high levels typically seen in nature (Figure 9.14).
Something else is probably at work too.
Natural selection can favor the maintenance of genetic variation. We look at the
evidence in more detail in a later chapter about biological species (Sections 13.6–13.7,
pp. 366–73). A simple version of the argument is as follows. Suppose, for example, that
the members of a finch population have a range of beak sizes. The beaks may be
adapted to a range of seed sizes: finches with larger beaks are better at eating larger
seeds, and finches with smaller beaks are better at eating smaller seeds. (In terms of
Chapter 5, this corresponds to multiple niche polymorphism: Section 5.13, p. 128.) If
the seeds in the local environment are all the same size, then natural selection will produce
a bird population with beaks of one size. If the seeds are a range of sizes, natural
selection will favor a range of beak sizes in the birds. The actual seed size distribution
available to the bird population will depend on whether any competitors are present, as
well as what seeds are produced by the local plants. However, although this “ecological”
kind of selection can theoretically maintain genetic variation, we do not know whether
it is in fact causing the genetic variation observed in the heritability measurements of
Figure 9.14.
Recent research has uncovered another puzzle in the observations we have on heritabilities.
A number of species have been the subjects of long-term studies in nature.
Each generation, the action of natural selection is measured, as a selection differential.
For example, in European collared flycatchers, the tarsus lengths of reproductively successful
birds and of average birds have been measured from 1980 to 2000 (Figure 9.15a).
(The tarsus is part of the bird’s leg.) In most years the selection differential is positive:
the average number is about 0.2. (This is a “standardized selection differential,” and
means that the successful birds have tarsus lengths 0.2 of a standard deviation longer
than average birds.) The character is also heritable, with a heritability of about 0.35.
Natural selection favors longer tarsuses in collared flycatchers, and collared flycatchers
have genetic variation for tarsal size.
Selection differential on tarsus length
(a) (b)
0.8
19.7
0.7
19.6
0.6
19.5
0.5
19.4
0.4
0.3
0.2
19.3
19.2
0.1
19.1
0.0
19.0
–0.1
18.9
–0.2
18.8
–0.3
0 2 4 6 8 10 12 14 16 18 20
18.7
0 2 4 6 8 10 12 14 16 18 20
Year
Year
Mean tarsus length (mm) ± s.e.
..