Evolution__3rd_Edition

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Figure 21.2
Possible phylogeny of
Darwin’s finches, according to
Lack. The dashed lines indicate
uncertainty. Other phylogenies
have been suggested too. Was
there time enough for the
evolution of 14 species, by
selection within a population,
since the Galápagos were
colonized by the ancestral
finch maybe 570,000 years ago?
Redrawn, by permission of the
publisher, from Lack (1947).
The Grants observed rates of
change in finches ...
Cactospiza pallida
Cactospiza heliobates
Camarhynchus psittacula
Camarhynchus pauper
Camarhynchus parvulus
Platyspiza crassirostris
Vegetarian
Insectivorous
Geospiza magnirostris
Geospiza fortis
Geospiza fuliginosa
CHAPTER 21 / Rates of Evolution 595
Geospiza scandens
Granivorous Cactus-feeding
Geospiza difficilis
Tree finches Ground finches Warbler-like
Ancestral
finch species
Geospiza
conirostris
21.1.2 Rates of evolution observed in the short term can explain
speciation over longer time periods in Darwin’s finches
Certhidea olivacae
Pinaroloxias
inornata
The same point a that rates of evolution over different time periods are consistent a
can be made by another argument. We saw (Section 9.1, p. 223) how natural selection
operates on the beaks of Darwin’s finches. The evidence there was for natural selection
within a species. It demonstrated that individuals with larger beaks are favored when
the seeds and fruit that they eat are large, whereas smaller beaks are favored when the
food size is smaller. In 1976–77 (and subsequently), the Grants measured the strength
of selection on the finches’ beaks, and its evolutionary results (Figure 9.9, p. 241).
Today, 14 different finch species occupy the Galápagos (Figure 21.2). The species
mainly differ in their beak and body proportions. What we can do is calculate whether
the kind of selection observed in the short term would be enough to account for the
origin of all the finches in the Galápagos in the time available.
How long would it take for the process studied by the Grants in 1976–77 to convert
one species of finch into another? During the 1977 drought, the beak size of Geospiza
fortis on the island of Daphne Major increased by 4%. G. magnirostris is a close relative
of G. fortis; the two species differ mainly in body and beak proportions and they coexist
on many islands of the Galápagos. From the average difference in beak size between
G. fortis and G. magnirostris on Daphne Major, Grant (1986) estimated that 23 bouts of
evolution of the 1977 type would be enough to turn G. fortis into G. magnirostris. On
other islands G. fortis is larger than on Daphne Major and, using one of the larger
G. fortis populations as the starting point, only 12–15 such events would be needed.