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Recent Studies of Darwin’s Finches
Generations of biologists since Charles Darwin have studied the Galápagos finches and
asked questions about them. Are the species really related and descended from a
single common ancestral species that colonized the islands long ago? Or are they
unrelated and the result of multiple colonizations of the islands by different migrants?
Where did the migrants come from and how many were there? Which bird species is
the closest living relative of Darwin’s finches? How does natural selection affect the
finches?
In the past, studies attempting to answer such questions relied solely on anatomical and
behavioral data. These data were generally informative but unable to provide convincing
answers. Today, molecular techniques allow comparisons of DNA among species, and
these powerful new methods are providing answers to many questions about Darwin’s
finches. In the process, they confirm and extend many of Darwin’s evolutionary
principles. This is a clear example of how revisiting old questions using new
technologies advances scientific knowledge.
Comparison of DNA from Darwin’s finches with that of several extant mainland Central
and South American birds reveal that Darwin’s finches form a single cluster of 14 closely
related species. Members of this group are more closely related to each other than to
any other living birds.
DNA sequence data confirm that the different species of Darwin’s finches are all
descended from a single ancestral species that somehow reached one of the Galápagos
Islands from the mainland around 2.3 million years ago. There is enough genetic
diversity today among Darwin’s finches to indicate that it was not a breeding pair or
pregnant female that reached the islands but likely a small flock of at least 40
individuals.
Across time, different subpopulations branched from this ancestral population and
spread among the islands. Each local population then began adapting to the particular
environment of its new home. Eventually, these subpopulations diversified into the
different species of Darwin’s finches that we find today. The process of diversification
into several related species by adaptation to new environments is adaptive radiation.
As the single species of finch to arrive on the
Galápagos evolved into many (seen here), its
beak followed suit, resulting in a variety of beak
shapes and sizes perfectly suited to each bird's
environment and lifestyle.
Researches have also compared the DNA of several bird species that are not part of the
Darwin’s finch group. The comparison identified one of these species, the dull-colored
grassquit (Tiaris obscura), as the closest living relative of the Darwin’s finch group. This
mainland bird is widely distributed today across Central and South America. It feeds on
seeds and builds dome-shaped nests, just as Darwin’s finches do. The dull-colored
grassquit itself was probably not the ancestor of Darwin’s finches. Much more likely, the
grassquit group and the finch group each branched from a still unknown common
ancestor on the mainland around 2.3 million years ago.
The ancestral species itself no longer exists and the finch and grassquit lineages that
branched from it diverged into two separate lineages: (1) In the Galápagos, the ancestral
species diversified into today’s 14 finch species, plus an unknown number of extinct
species; (2) On the mainland, the ancestral species diversified across time into a
number of other finch-like species, including the dull-colored grassquit.
The most extensive field study of the Galápagos finches ever conducted is the ongoing
investigation by Peter and Rosemary Grant. The Grants have lived on the islands for
extended periods of time each year since 1973. Over the years, they have trapped,
measured, weighed, and released more than 20 generations of finches. The Grants’
data—behavioral, anatomical, and molecular—provide extensive evidence in support of
Darwin’s ideas of natural selection and of species divergence.