| "When we made the comparison
between the size of the offspring generation and the population before selection, we found
a measured, evolutionary response had taken place and it was almost identical to what we
had predicted."
-Peter Grant
Darwin thought that evolution took place over hundreds or thousands of
years and was impossible to witness in a human lifetime. Peter and Rosemary Grant have
seen evolution happen over the course of just two years.
The Grants study the evolution of Darwin's finches on the Galapagos
Islands. The birds have been named for Darwin, in part, because he later theorized that
the 13 distinct species were all descendants of a common ancestor. Each species eats a
different type of food and has unique characteristics developed through evolution. For
example, the cactus finch has a long beak that reaches into blossoms, the ground finch has
a short beak adapted for eating seeds buried under the soil, and the tree finch has a
parrot-shaped beak suited for stripping bark to find insects.
The Grants have focused their research on the medium ground finch, Geospiza
fortis, on the small island of Daphne Major. Daphne Major serves as an ideal site for
research because the finches have few predators or competitors. (The only other finch on
the island is the cactus finch.) The major factor influencing survival of the medium
ground finch is the weather, and thus the availability of food. The medium ground finch
has a stubby beak and eats mostly seeds. Medium ground finches are variable in size and
shape, which makes them a good subject for a study of evolution.
The first event that the Grants saw affect the food supply was a
drought that occurred in 1977. For 551 days the islands received no rain. Plants withered
and finches grew hungry. The tiny seeds the medium ground finches were accustomed to
eating grew scarce. Medium ground finches with larger beaks could take advantage of
alternate food sources because they could crack open larger seeds. The smaller-beaked
birds couldn't do this, so they died of starvation.
In 1978 the Grants returned to Daphne Major to document the effect of
the drought on the next generation of medium ground finches. They measured the offspring
and compared their beak size to that of the previous (pre-drought) generations. They found
the offsprings' beaks to be 3 to 4% larger than their grandparents'. The Grants had
documented natural selection in action.
While beak size is clearly related to feeding strategies, it is also
related to reproduction. Female finches tend to mate with males that have the same size
beaks. These factors together can add to the development of new species.
The Grants return each year to Daphne Major to observe and measure
finches. They have been collecting data on the finches for over 25 years and have
witnessed natural selection operating in different ways under different circumstances. |
