Single‐Generation Estimates of Individual Fitness as Proxies for Long‐Term Genetic Contribution

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Abstract
Individual fitness is a central evolutionary concept, but the question of how it should be defined in empirical studies of natural selection remains contentious. Using founding cohorts from long-term population studies of two species of individually marked birds (collared flycatcher Ficedula albicollis and Ural owl Strix ura- lensis), we compared a rate-sensitive (lind) and a rate-insensitive (life- time reproductive success (LRS)) estimate of individual fitness with an estimate of long-term genetic fitness. The latter was calculated as the number of gene copies present in the population after more than two generations, as estimated by tracing genetic lineages and ac- counting for the fact that populations were not completely closed. When counting fledglings, rate-insensitive estimates of individual fitness correlated better than rate-sensitive estimates with estimated long-term genetic contribution. When counting recruits, both classes of estimates performed equally well. The results support the con- tention that simple, rate-insensitive measures of fitness, such as LRS, provide a valid and good estimate of fitness in evolutionary studies of natural populations.