Evol Ecol Res 13: 387-399 (2011)     Full PDF if your library subscribes.

The evolution of sex-change timing under environmental uncertainty: a test by simulation

Lock Rogers1 and Alan Koch2

1Department of Biology, Agnes Scott College, Decatur, Georgia, USA and 2Department of Mathematics, Agnes Scott College, Decatur, Georgia, USA

Correspondence: L. Rogers, Department of Biology, Agnes Scott College, 141 E. College Avenue, Decatur, GA 30030-3770, USA.
e-mail: lrogers@agnesscott.edu


Background: Life-history theory predicts that selection for changing from an initial sex to a second sex exists when the size- or age-specific reproductive-rate curves for males and females cross. The optimal timing of sex change will be the size or age at which the curves cross. But wild populations of many sex-changing fishes do not follow this prediction; individuals generally change sex at a size or age at which the sex that they become cannot yet reproduce. This phenomenon is termed ‘early sex change’.

Question: Does uncertainty in the advent of successful reproduction in the second sex favour the evolution of early sex change?

Method: Genetically explicit simulation to track the evolution of sex change timing.

Key assumption: Reproduction in the initial sex (female) is assured but of low value, whereas reproduction in the second sex (male) is uncertain but of high value.

Organisms: Our model is primarily based on the natural history of the bluehead wrasse, Thalassoma bifasciatum, but is designed to apply to many other species of sex-changing fishes.

Results: Early sex change rapidly evolved under the conditions of the model. Moreover, the timing of sex change became highly variable within a population. The mean age of switching from female to male depended on the ratio of male to female reproductive rates: the higher the ratio, the earlier the mean age at which females switched. A second mechanism for early sex change arose from the asymmetrical fitness surface of females: those that switch earlier than the optimum time pay a smaller fitness penalty than those that switch later.

Keywords: asymmetric fitness surface, early sex change, genetic algorithm, probabilistic size-advantage

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