Evolution__3rd_Edition

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Beak size influences feeding
efficiency on different food types
Beak size is inherited
The weather in the Galápagos ...
CHAPTER 9 / Quantitative Genetics 223
9.1 Climatic changes have driven the evolution of beak size
in one of Darwin’s finches
Fourteen species of Darwin’s finches live in the Galápagos archipelago, and many of
them differ most obviously in the sizes and shapes of their beaks. A finch’s beak shape,
in turn, influences how efficiently it can feed on different types of food. Peter and
Rosemary Grant, together with a team of researchers, have been studying these finches
since 1973, and they have evidence that beak size influences feeding efficiency. It comes
from a comparison of two species (see Plate 4a, and b, between pp. 68 and 69), the
large-beaked Geospiza magnirostris and the smaller G. fortis, feeding on the same kind
of hard fruit.
The large-beaked G. magnirostris can crack the fruit (called the mericarp) of caltrop
(Tribulus cistoides) transversely, taking on average only 2 seconds and exerting an
average force of 26 kgf (255 N); it can then easily, in about 7 seconds, eat all the 4–6
seeds of the smashed fruit. The smaller G. fortis is not strong enough to crack Tribulus
mericarps and instead twist open the lower surface, applying a force of only 6 kgf and
taking 7 seconds on average to reach the seeds inside. Only one or two of the seeds
can be obtained in this way and it takes an average of 15 seconds to extract them.
G. magnirostris usually has an advantage with these large, hard types of food.
Smaller finches are probably more efficient with smaller types of food, but this is
more difficult to show. Both large and small finches on the Galápagos do in fact eat
small seeds, though there is an indirect reason (as we shall see) to believe that smaller
finches do so more efficiently. From the evidence we have met so far, we can predict
that natural selection would favor larger finches when large fruits and seeds are abundant.
The prediction should apply both within and between species. A G. magnirostris
finch looks like an enlarged G. fortis, and a larger individual G. fortis can probably deal
with a large food item more efficiently than can a smaller conspecific, much as an
average specimen of G. magnirostris is more efficient than an average G. fortis. When
large seeds are common, we might expect the average beak size in a population of G.
fortis to increase between generations, and to decrease when large seeds are rare a if
beak size is inherited.
If beak size is inherited ...but is it? Beak size is inherited if parental finches with
larger than average beaks produce offspring with larger than average beaks. The Grants
measured the sizes of parental and offspring finches in several families and plotted the
latter against the former (Grant 1986) (Figure 9.1). Large-beaked parental finches do
indeed produce large-beaked offspring: beak size is inherited. It therefore makes sense
to test the prediction that changes in beak size should follow changes in the size distribution
of food items. The test was carried out on the species G. fortis, on one of the
Galápagos islands, Daphne Major. Since the study began, this species has undergone
two major, but contrasting, evolutionary events.
In the Galápagos, the normal pattern of seasons is for a hot, wet season from about
January to May to be followed by a cooler, dryer season through the rest of the year. But
in early 1977, for some reason, the rain did not fall. Instead of the normal progression,
the dry season that began in mid 1976 continued until early 1978: one whole wet season
did not happen. The finch population of Daphne Major collapsed from about 1,200 to