Evolution__3rd_Edition

..
Population size
Seed √depth × hardness index
1,600
1,400
1,200
1,000
800
600
400
200
G. fortis
0
J S N J MM J S N J MM J S N J MM J S N J
1975 1976 1977 1978
(c)
6.5
6.0
5.5
5.0
4.5
4.0
3.5
(a)
J S N J M M J S N J M M J S N J M M J S N J
1975 1976 1977 1978
Selection fluctuates over time
Seed abundance (g/m 2 )
12.0
10.0
8.0
6.0
4.0
2.0
0
(b)
CHAPTER 9 / Quantitative Genetics 225
J S N J M M J S N J M M J S N J M M J S N J
1975 1976 1977 1978
Figure 9.2
During a drought in 1976–77, (a) the population of Geospiza
fortis decreased on the island of Daphne Major in the Galápagos
archipelago, due to (b) the decline of the food supply. (c) The
average size of the seeds available as food increased during the
drought. Redrawn, by permission of the publisher, from Grant
(1986).
Major in 1984–85 and found that the smaller birds were indeed favored. Finches born
in 1985 had beaks about 2.5% smaller than those born before the El Niño downpours.
The theory, that seed sizes control beak size in these finches, was confirmed. Further
confirmation came at the next El Niño event in 1987. This time, the seed size distribution
was hardly changed at all, leading to the prediction that the beak sizes of the finches
would show no evolutionary change either; nor did they (Grant & Grant 1995).
The fluctuations in the direction of selection on beak shape a with beaks evolving up
in some years, down in other years, and staying constant in yet other years a probably
results in a kind of “stabilizing” selection over a long period of time such that the average
size of beak in the population is the size favored by long-term average weather.
(Later in the chapter, we shall see how the degree of selection can be expressed more
exactly; Figure 9.9 will show the results for 1976–77 and 1984–85.)