40 Years of Evolution by Grant Peter R. Grant B. Rosemary

Author:Grant, Peter R., Grant, B. Rosemary
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2014-03-14T16:00:00+00:00

THE POSSIBLE ROLE OF SELECTION ON G. MAGNIROSTRIS

Trends in beak size and shape were more the result of gradual accumulated change over several years than the result of occasionally strong change from year to year, as happened in fortis. We draw this conclusion from analyses of year-to-year selection, recognizing that sample sizes of adults are generally not large, and so the absence of demonstrable selection may simply mean we have failed to demonstrate it. For example, the 7 individuals that survived the drought of 2004 and bred in 2005 were large in beak and body size, and had robust beaks, compared with the 71 that died, but the selection coefficients were not significant (fig. 11.10). In two years the evidence for selection on beak size is strong. Birds with large beaks survived better than those with small beaks in 1993 and again in 1994. In three other years, 2006–8, the evidence is statistically weaker but points in the same direction. In two different years, 1992 and 2011, the evidence for selection on beak shape is strong (fig. 11.10). Generally small fluctuations that are random in direction predominate in other years.

The samples in 1993 and 1994 were large because they comprised several new immigrants together with the products of local breeding (next section). Thus the selective disadvantage of individuals with relatively small beaks should be understood to mean disappearance through death or emigration. In contrast, we believe there were relatively few immigrants on the island in 2006–8, when weaker beak-size selection occurred, and therefore birds that disappeared probably died on Daphne.

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