Balancing selection on size: effects on the ...

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Title Balancing selection on size: effects on the incidence of an alternative reproductive tactic
Author(s) N. Aubin-Horth, D. A. J. Ryan, S. P. Good, J. J. Dodson
Journal Evolutionary Ecology Research
Date 2005
Volume 7
Issue 8
Start page 1171
End page 1182
Abstract Question: Does fluctuating natural selection on body size of fish among years (balancing selection) influence the frequency of an alternative male reproductive tactic? Hypothesis: When the surviving juveniles of a cohort are larger because of selection, a higher proportion of the population will develop as mature 'sneaker' males than expected in the absence of selection. In the case where selection favours smaller individuals, a lower proportion will develop as mature 'sneaker' males. Organisms: Juvenile Atlantic salmon (Salmo salar) from a naturally sustained population in the Ste-Marguerite River, Centre Interuniversitaire de Recherche sur le Saumon Atlantique, Province of Quebec, Canada. Methods: The presence of balancing size-selection was examined by measuring the proportional shift of the mean size and variance of juvenile salmon sampled in the autumn and following spring at multiple sites for each of three annual cohorts. The proportional shift in mean size of individuals over winter was then correlated with the incidence of early male maturity (sneakers) observed at the same sites for each cohort the following fall. Conclusions: Winter mortality decreased the size of surviving fish in one cohort by 7.8% on average, increased size by 2.7% on average in another and had little effect on the third. Proportionally more juvenile males adopted the sneaker tactic when juveniles surviving winter were larger, whereas fewer juvenile males adopted the sneaker tactic when surviving juveniles were smaller. The fluctuating nature of selection on body size indirectly maintains life-cycle divergence through a direct effect on size frequencies within a cohort.

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