Evolution__3rd_Edition

596 PART 5 / Macroevolution
. . . and those rates are more than
adequate to explain the radiation
on the Galápagos
Observed rates depend on the
measurement interval ...
And how much time is available? The Galápagos archipelago is made up of volcanic
islands. The current islands probably first emerged from the ocean about 4 million
years ago, and all the islands had appeared by 1–0.5 million years ago (see Sequeira et al.
2000 for differing views on the dates). The common ancestor of Darwin’s finches has
been estimated to have arrived from South America about 570,000 years ago, so the
radiation of the 14 finch species has occurred in about 0.5 million years. Using either
the low estimate of 12–15 events (where an “event” is a bout of evolution such as
occurred in 1977 on Daphne Major) or the higher estimate of 25 events, we can see that
even if only one such event took place per century, the evolutionary divergence between
the two species could still have been accomplished well within the time available. In
fact, the real evolutionary transition probably did not happen that way. The 1982–83 El
Niño event reversed the evolution of 1977 and there was probably not a steady transition
from one population into another, but frequent reversals of accumulated small
changes. In any case, it is unlikely that G. fortis simply changed into G. magnirostris (or
vice versa); they probably both diverged from another common ancestor.
The rough calculation is not intended to represent the exact history of the birds.
Instead, it illustrates how we can extrapolate from the rate of evolution observed over a
few years within a species to explain the diversification of the finches from a single common
ancestor about 570,000 years ago to the present 14 species. If the extrapolation is
correct, the reason for the speciation in the finches was the same process as has been
observed in the present a natural selection for changes in beak shape, which were
probably in turn due to changes in food types through time and between islands.
Although the finches have speciated rapidly, no peculiar mechanism of evolution is
needed to account for it. Arguments of this general kind are common in the theory of
evolution. We met a similar argument in Section 18.6.2 (p. 542), where natural selection
over long periods was used to explain the major evolutionary transition from the
mammals to the reptiles.
21.2 Why do evolutionary rates vary?
Paleobiologists have studied a number of generalizations about evolutionary rates. For
example, it has been suggested that species usually change more rapidly during, rather
than between, speciation events; that structurally more complex forms evolve faster
than simpler forms; and that some taxonomic groups evolve more rapidly than others,
that mammals, for instance, evolve faster than mollusks (this is an old idea a it was one
of Lyell’s favorite generalizations). We shall examine the first of these issues in more
detail later. But before doing so, let us return to Gingerich’s (1983) compilation of
evolutionary rates and consider a general point about their study.
Gingerich observed, in his compilation of evolutionary rates, an inverse relation
between the rate and the time interval over which it was measured. The observed cases
of rapid evolution have tended to be for shorter intervals than the cases of slower evolution
(Figure 21.3). The relation is unlikely to be due to any strong force in the evolutionary
process itself. Nothing in evolutionary theory constrains rapid evolution, at
these speeds, to take place in short intervals and slower evolution in longer intervals. At
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