Selection on morphological, behavioral, and life history variation in a population of Darwin's medium ground finches (Geospiza fortis).

Abstract

In this four-part study, I use selection analyses to examine the evolutionary significance of morphological, behavioral, and life history variation in a population of Darwin's Medium Ground Finches (Geospiza fortis). The first section describes the natural selection on adult body size in G. fortis that occurred following the extremely wet conditions present during the El Nino event of 1983. High adult mortality was correlated with directional selection for small body size. This contrasts with the strong directional selection for large size observed following droughts, and implies that selection on body size in these birds may fluctuate regularly. In the second part, I focus on the heritability of and selection on clutch size in these birds. Quantitative genetic analyses indicate that, in contrast to all other species so far, there is no evidence for heritable variation in clutch size among females. Selection analyses suggest this may be due to a history of sustained directional selection favouring large clutches in this population. In part three, I use a pedigree analysis to estimate levels of inbreeding in the G. fortis population. Levels of inbreeding are comparable to those observed in other passerine species. Degree of pair-relatedness did not have any effect on the reproductive success of individuals and simulation analysis indicate that mate choice in these birds occurs at randomly with respect to relatedness. Kin avoidance does not appear to be an important determinant of levels of inbreeding in this population. Finally, in the last section, I examine the patterns and causes of changes in the frequency of different male song types. Over a six-year period the song type that was most common became significantly rarer while the other three less common song types all increased in frequency. This pattern of frequency-dependent change occurred because males with uncommon songs survived better and produced more offspring. Thus, song type variation appears to be adaptive in this population and biological and cultural evolution are more closely coupled processes in these birds than in other species.