Demography and the Genetically Effective sizes of Two Populations of Darwin's Finches

Abstract

The purpose of this study was to identify and quantify the demographic and genetic factors that favor long—term persistence of resident bird populations on small islands. Two species of Darwin's Ground Finches, Geospiza scandens (Cactus Finch) and G. fortis (Medium Ground Finch), were studied on Isla Daphne Major, Galapagos, from 1975 to 1991. Four cohorts born in the years 1975—1978 were followed to the point where almost every individual had died. Life tables were constructed from survival and reproductive data, and used to calculate genetically effective population sizes. Annual rainfall was highly variable and erratic. Extremes were 0 and 1359 mm. As a consequence the finch populations fluctuated in all demographic parameters. In years of little or no rainfall breeding did not occur, in years of abundant rainfall as many as eight breeding attempts were made by individual pairs. Maximum ages were 15 yr for G. scandens and 14 yr for G. fortis. Males of both species tended to live longer than females and to breed later; most females bred for the first time at ages 1—3 yr, whereas most males bred for the first time at ages 2—6 yr. An unusual feature of the survival and reproductive schedules is an increase in reproductive value sometimes occurring moderately late in life more than once, associated with occasional extremely favorable conditions for reproduction. Harmonic mean breeding population Sizes (N) were 94 G. scandens and 197 G. fortis. Effective population sizes (Ne) were much lower, principally as a result of a large variance in the production of recruits per parent, especially by G. fortis. Average effective sizes were 38 G. scandens and 60 G. fortis by one method of calculation, and slightly larger by another. The proportional rate of loss of selectivity neutral heterozygosity or additive genetic variance in quantitative traits potentially caused by random genetic drift (1/2Ne) in these populations is °0.003—0.005, or 0.3—0.5%/yr, and between 0.8 and 1.4% per generation. We suggest that a general estimate for the effective size of terrestrial bird populations is about one—quarter of average breeding numbers. These results are discussed in relation to the long—term viability of the populations and the maintenance of genetic variation. The demographic features that enable the finch species to persist in the face of extreme environmental stochasticity are a high maximum life—span, a generally high adult survival under the stressful conditions of drought, a flexible period of maturity, and a high reproductive rate. Despite their relatively small effective population sizes and the likelihood of genetic impoverishment through random drift they remain genetically variable through gene flow, principally hybridization. This study highlights fluctuating (unstable) age structures as a methodological constraint on some of the theoretical calculations. An expanded demographic and population—genetic theory is needed to overcome this constraint.