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Darwin’s Finches
The variety of finches of the Galapagos Islands is a result of natural selection and speciation.
Compared to the giant tortoises, strange flightless birds, and sea iguanas living there, Darwin’s finches are not
particularly interesting-that is, not until the saga of their evolution is revealed.
The finches are all 10 to 20 cm long, and both sexes are drab-colored browns and grays. The finches
are split between ground species, each feeding on different, seeds or cactus, while the rest are tree finches. In
each species of bird the beak has become modified for its specific diet.
Why have these little birds become so important to our understanding of speciation? Darwin concluded
(and his conclusions are backed by years of careful research by others since) that the different species of
finches were all descended from the same common ancestor. Long ago, about 10,000 years ago, the volcanic
islands were colonized by South American finches that probably were blown out to sea by a storm.
Apparently, conditions on the islands were favorable, and the finches flourished. Their descendants eventually
populated all the islands by occasional island hopping. However, the island hopping was rare enough to
ensure the isolation of each population. What followed was speciation. In this example of speciation many
species emerged from a common ancestor that was introduced to a new environment with new opportunities
and new problems for the species to survive.
According to our scenario, the original ground finches from South America had the islands to
themselves, as far as they were concerned. There was a great variety of food. They were already well
adapted for foraging for small seeds on the ground, but there were other plentiful untapped food resources –
food not ordinarily eaten by finches. Soon enough the expanding populations were depleting the available
supply of small seeds. Thus, natural selection began to favor birds that could also cope with larger seeds and
with other food resources. In time, the bird’s bill sizes changed through the process of natural selection as
each population began to adapt more closely to the different kinds of food found on each island.
As differences in lifestyles and specialization became magnified among diverging populations,
competition for food would have been reduced. After thousands of years of divergence on the Galapagos
Islands the different finch species were unable to interbreed. Therefore, the birds from the different islands
were so different genetically that any island hoppers would find themselves to be reproductively incompatible
with the residents of other islands. Clearly, speciation (the formation of new species) had occurred.
Family Tree of Darwin’s Finches
Common Ancestor
(from South America)
Darwin’s Finches Honors
Name: __________________________________
1. To demonstrate your understanding of the reading on Darwin’s Finches and our notes on natural
selection and speciation, circle the correct effect that the environment of the Galapagos Islands would
have on the finch population:
A. The need to survive would cause each finch to change their own beak to better feed themselves.
B. Only those finches with specialized beaks would find food and live to reproduce.
C. The finches would decide to mutate to grow specialized beaks.
Through the completion of the following questions, you will explain how speciation and natural
selection created the different species of finches on the Galapagos Islands, which are now recognized
as “Darwin’s Finches”.
2. State the first step of speciation, and explain how the common ground finch from South America went
through the first step of speciation.
3. The second step of speciation is how different populations of a species adapt to their environment through
natural selection. In the numbered boxes below state the FIVE steps of natural selection and explain in
detail how Darwin’s Finches went through the FIVE steps of natural selection to obtain their different size and
shape beaks.
Step 1:
Step 2:
Step 3:
Step 4:
Step 5:
4. The third step of speciation is divergence. What proof is mentioned in the reading that demonstrates the
divergence of Darwin’s Finches into the different species of finches?
5. How does the “Family Tree of Darwin’s Finches” (shown below the reading) provide a visual model for
speciation? Explain your answer.