Evolution__3rd_Edition

224 PART 2 / Evolutionary Genetics
Figure 9.1
Parents with larger than average
beaks produce offspring with
larger than average beaks in
Geospiza fortis on Daphne
Major, showing that beak size
is inherited. Results are shown
here for 2 years in the 1970s.
Grant & Grant (2000) show
that the result persisted in
future years. (0.4 in ≈ 10 mm.)
Redrawn, by permission of the
publisher, from Grant (1986).
. . . influences the food supply ...
. . . leading to evolution in Darwin’s
finches
Offspring bill depth (mm)
11.0
10.0
9.0
8.0
G. fortis
1976
1978
8.0 9.0 10.0 11.0
Midparent bill depth (mm)
about 180 individuals, with females being particularly hard hit; the sex ratio at the end
of 1977 was about five males per female. As the sex difference shows, not all finches suffered
equally a smaller birds died at a higher rate. The reason, again, lies in the food
supply. At the beginning of the drought, the various types of seeds were present in their
normal proportions. G fortis of all sizes take small seeds, and as the drought persisted,
these smaller seeds were relatively reduced in numbers. The average available seed size
became larger with time (Figure 9.2). Now the larger finches were favored, because they
eat the larger, harder seeds more efficiently. The average finch size increased as the
smaller birds died off. (Females died at a higher rate than males because females are on
average smaller.) Size, as we have seen, is inherited. The differential mortality in the
drought therefore caused an increase in the average size of finches born in the next
generation: G. fortis born in 1978 were about 4% larger on average than those born
before the drought.
Four years later, in November 1982, the weather reversed. The rainfall of 1983 was
exceptionally heavy and the dry volcanic landscape was covered with green in the periodic
disturbance called El Niño (see Plate 4c and d, between pp. 68 and 69). Seed
production was enormous. The theory developed for 1976–78 could now be tested.
The conditions had reversed: the direction of evolution should go into reverse too.
In the year after the 1983 El Niño event, there were more small seeds. If the smaller
finches can in fact exploit small seeds more efficiently, the smaller finches should
survive relatively better. The Grants again measured the sizes of G. fortis on Daphne
..