Biology Reference
In-Depth Information
or to environmental effects or to both? The researchers measured the average beak
depth of families of birds after they had reached adult size, and compared it to the
average beak depth of their mother and father. Figure 3.8b shows that parents with
shallow beaks tend to have young with shallow beaks, and parents with deep beaks
tend to have young with deep beaks. Thus there is a large genetic component to the
determination of beak depth, supporting Postulate 2.
Fig. 3.8
The Galapagos Islands have a variable climate, and experience both droughts and
periods of excessive rainfall that affect the food supply of the finches. In 1977 the
rainfall was only about 20% of the normal rainfall so that the plants, whose seeds
are the main food of the birds, were much sparser than usual. As a result, 84% of
the birds died, as shown by the upper graph in Fig. 3.9a. It is the few survivors
that allowed the population to recover to normal levels after the drought had ended,
supporting Darwin's third postulate.
To test Postulate 4, we now need to ask whether those finches that survived the
drought were a random selection of all the finches or whether they were they a sub-
set, selected because they possessed some advantage. Small, soft seeds are preferred
to large, hard seeds because the finches find them easier to crack. But the types
of seeds available as food changed as a result of the drought. The lower graph in
Fig. 3.9a shows that the proportion of large, hard seeds increased and that of small,
soft seeds declined. But only birds with deep beaks can crack the larger seeds. So as
the drought progressed, more of the birds with deep beaks survived than birds with
