Evolution__3rd_Edition

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Figure 14.3
Song form may be
pleiotropically correlated
with beak shape in Darwin’s
finches. The beaks and sound
spectrograms are shown for
eight species from one of the
Galápagos Islands. The
spectrogram has time on the
x-axis (the bar represents 0.5
seconds) and frequency on the
y-axis. Notice that the species
with larger beaks produce
slower trills (less of the
recognizable units per unit
time) and with a lower range of
frequencies. Statistical analysis
shows that the effect is not due
to phylogenetic relationship.
Redrawn, by permission of the
publisher, from Podos (2001).
. . . and hitch-hiking
Geospiza magnirostris
Geospiza fortis
Geospiza fuliginosa
Geospiza scandens
Cactospiza parvulus
Cactospiza psittacula
Cactospiza pallida
Cactospiza olivacea
CHAPTER 14 / Speciation 387
adaptation will also cause some reproductive isolation. The pleiotropy arises because
the same morphological character (the beak) influences both feeding and mating.
Hitch-hiking means that when natural selection favors a gene at one locus, genes at
linked loci may also increase in frequency (Section 8.9, p. 210). In Dodd’s experiment,
natural selection increased the frequency of genes coding for appropriate digestive
enzymes. Maybe a closely linked locus influences the fruitfly’s courtship dance. Then,
when a gene for an ecological adaptation (the digestive enzyme) increases in frequency,
it may drag along with it a linked gene for a new courtship dance step. Prezogyotic
isolation could again evolve as a by-product, but the genetic mechanism is hitch-hiking
rather than pleiotropy.
14.3.3 Reproductive isolation is often observed when members of
geographically distant populations are crossed
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The populations of a species in different geographic areas tend to evolve genetic differences
(Section 13.4, p. 359). We usually do not know whether the members of different
populations can interbreed because they live in separate places and the opportunity for
interbreeding does not arise. But in some cases, a biologist has brought samples from
distant populations into the lab, and measured the amount of reproductive isolation.
Figure 14.4 shows an example from the Californian flower Streptanthus glandulosus.
The flower lives on serpentine soils, which are found in discrete, local areas. The flower
therefore has a discontinuous distribution, with many small local populations. The