Travel Reference
In-Depth Information
has the luxury of being able to specialize on one particular size of seed. As a
result, several seed-eating species can survive and thrive. But smaller islands
are so small that the amount of food they produce is only enough to support
one fi nch species. If two or more species of fi nch lived on these little islands,
each species would have such a small population size that it would be threat-
ened with extinction when times became hard. Thus, before long, only one
species would survive.
The plants on the small islands produce a range of seed sizes, just as they
do on the large islands, but there are simply not enough seeds of each size class
to maintain a specialist fi nch population. The fi nch species that can survive on
the smaller islands are those that are capable of eating a wide variety of seeds.
Members of the seed-eating fi nch species
Geospiza fortis
that live on a big
island such as Isabela are able to specialize on the island's abundant large
seeds. They have evolved large powerful beaks. But members of the same
species living on the tiny island of Daphne Major have a middle-sized beak,
suitable for cracking a wider range of dif erent seeds. Daphne Major is only
one-third of a square kilometer in area, and it is this generalist version of
G.
fortis
that makes up most of the fi nches on the island.
The species
Geospiza fuliginosa
on large islands has specialized on small
seeds, and has a small beak as a result. On the little cluster of three islands
of the coast of Isabela known as Los Hermanos, each of which is about the
size of Daphne Major,
G. fuliginosa
is the predominant fi nch species. But on at
least one of these little islands
G. fuliginosa
has evolved a more middle-sized,
generalist beak, like
G. fortis
on Daphne Major.
Dif erences in foraging behavior have evolved as well, so that the birds
on a large island behave dif erently from birds of the same species on a small
island. The result is that big islands, with their abundant resources, can sup-
port more species than smaller ones. On the larger islands it is possible for
species to be narrow specialists. On the smaller islands, where such narrow
specialists would be unable to obtain enough food, the species that are the
most resourceful and best able to adapt to a broad rather than a narrow eco-
logical niche are the ones that have won out.
Peter and Rosemary Grant were able to examine the individual factors
that have shaped the evolution of the relatively small number of fi nch species
