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BIOLOGICAL SCIENCES / EVOLUTION
Trait changes in a harvested population are driven by a dynamic tug-of-war between natural and harvest selection

Eric Edeline^*, Stephanie M. Carlson^*,, Leif C. Stige^*, Ian J. Winfield, Janice M. Fletcher, J. Ben James, Thrond O. Haugen^*,, L. Asbjørn Vøllestad^*, and Nils C. Stenseth^*,¶

*Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, P.O. Box 1066 Blindern, 0316 Oslo, Norway; School of Aquatic and Fishery Sciences, University of Washington, Box 355020, Seattle, WA 98195; Centre for Ecology and Hydrology, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster, Lancashire LA1 4AP, United Kingdom; and Norwegian Institute for Water Research, Gaustadalléen 21, 0349 Oslo, Norway

Edited by Simon A. Levin, Princeton University, Princeton, NJ, and approved August 23, 2007 (received for review June 23, 2007)

Selective harvest of large individuals should alter natural adaptive landscapes and drive evolution toward reduced somatic growth and increased reproductive investment. However, few studies have simultaneously considered the relative importance of artificial and natural selection in driving trait changes in wild populations. Using 50 years of individual-based data on Windermere pike (Esox lucius), we show that trait changes tracked the adaptive peak, which moved in the direction imposed by the dominating selective force. Individual lifetime somatic growth decreased at the start of the time series because harvest selection was strong and natural selection was too weak to override the strength of harvest selection. However, natural selection favoring fast somatic growth strengthened across the time series in parallel with the increase in pike abundance and, presumably, cannibalism. Harvest selection was overridden by natural selection when the fishing effort dwindled, triggering a rapid increase in pike somatic growth. The two selective forces appear to have acted in concert during only one short period of prey collapse that favored slow-growing pike. Moreover, increased somatic growth occurred concurrently with a reduction in reproductive investment in young and small female pike, indicating a tradeoff between growth and reproduction. The age-specific amplitude of this change paralleled the age-specific strength of harvest pressure, suggesting that reduced investment was also a response to increased life expectancy. This is the first study to demonstrate that a consideration of both natural selection and artificial selection is needed to fully explain time-varying trait dynamics in harvested populations.

adaptive landscapes | conservation | contemporary evolution | fisheries | top predators

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

^¶To whom correspondence should be addressed. E-mail: n.c.stenseth{at}bio.uio.no

© 2007 by The National Academy of Sciences of the USA

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PNAS 2007 104: 15585-15586. [Full Text]