Evolution

What Begets Fitness?

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Science  18 Feb 2000:
Vol. 287, Issue 5456, pp. 1169
DOI: 10.1126/science.287.5456.1169g

A measure of the fitness of an individual organism, in the terminology of evolutionary biology, is the number of recruits it contributes to the next generation. In wild populations fitness can vary greatly between individuals, and the causes of the variation have proved hard to tease apart.

Since 1976, Rosemary and Peter Grant have been tracking populations of two species of Darwin's finch, Geospiza fortis and G. scandens, on the Galapagos island of Daphne major. They found that fitness, despite great variation from year to year, was closely correlated with longevity (these finches can live as long as 16 years) and reproductive output. Long life was associated with a number of more contingent factors, such as hatching early in the season (when food supply is most abundant), or large body size, or beak shape suited to the changing composition of the food supply. Piecing together their observations of different cohorts of the two finch species over several generations, they conclude that the heritable components of fitness can be selected in different directions and in different combinations according to the fluctuating environmental conditions, explaining why fitness variation was not heritable or random. The large variation in fitness in these finches also implies that the effective population size, in genetic terms, is a lot smaller than the actual population size.—AMS

Proc. R. Soc. London267, 131 (2000).

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