Biology Reference
In-Depth Information
Fig. 3.7
forms in current populations, and indicates that extinction is an inherent part of the
process of evolution. The same argument applies to the lack of intermediate species
in the fossil record. Thus the genetic composition changes with the generations, as
predicted by evolutionary theory, but the vast majority of species that have been
created by evolution are now extinct.
Direct Observation of Natural Selection in Finches
In 1973 Rosemary and Peter Grant started a study of natural selection among the
finch populations that inhabit the Galapagos Islands in the central Pacific Ocean, a
study that continues to this day. These birds invaded the islands from America some
two million years ago, and today are divided into 13 distinct species. Comparisons
of their DNA sequences show that they are all closely related. They are all similar
in body size and colour, but differ in the size and shape of their beaks because they
differ in what they eat. One small island called Daphne Major makes a good natural
laboratory because it is small and isolated, and contains only around 1200 birds, all
of which have been tagged by the Grants and their colleagues, and their beak depths
and other bodily features measured. The depth of an individual's beak is important
because it is correlated with the size of the seeds that the bird can crack - birds with
bigger beaks eat bigger seeds.
To test Postulate 1, every finch was measured for the size of its beak. Figure 3.8a
shows a graph of beak depth plotted against the number of finches, and you can see
that this character is indeed variable. Is this variation due to inherited differences
