‘IRE vol. 5, no. 1, January 1990

- Reply from B. Charlesworth

Michod states that my phrase ‘repair provides a sufficient explanation for sex and recombination in all species” misrepresents his views. My wording was intended to convey the sub- stance of statements published by rim and his colleagues, which seemed to me to indicate that they t-rink that recombinational repair is lie primary factor responsible for the maintenance of sexual reproduction. Maynard Smith, in his critique of tqeir ideas, appears to interpret them similarly*. The crucial question is: what prevents the spread of an asexual variant in a diploid, outcross- iiig population?

The analysis by Bernstein et aL3 of tie equilibrium mean fitnesses of asexual versus random mating popu- lations subject to mutation-selection balance led them to the conclusion tiat there is no mean fitness advan- tage to a random mating population. (Note that this conclusion is depen- dent on their assumption of multi- plicative fitnesses, and that there can indeed be such an advantage to sexual reproduction when there are

synergistic effects of mutations at different loci1,4,5.) In view of this ap- parent lackofanyotherthan transient effects of breeding system on popu- lation fitness, they suggested (Ref. 3, p. 1279) that ‘In apomixis, meiosis is suppressed, and there is a single mitotic maturation division. This largely abandons recombinational repair of double-strand damage and is thus a costly strategy. . . . In con- clusion, shifting from outcrossing to any of the reproductive systems considered reduces fitness. This reduction is transient for selfing or automixis and permanent for apomixis and endomitosis, since the latter have reduced capacity for recombinational repair.’

In the paper that I cited (Ref. 6, pp. 141-2), they made the slightly differ- ent statement that ‘Our basic view concerning the evolution of outcross- ing in diploids is as follows. Physical recombination produces allelic re- combination and, hence, produces homozygosity at heterozygous loci. Thus, recombinational repair, which requires physical recombination,

tends to express deleterious recess- ive alleles at heterozygous loci. Out- crossing functions to mask the expression of these deleterious recessives.’

Brian Charlesworth

Dept of Ecology and Evolution, University of Chicago, 1103 E. 57th St, Chicago, IL 60637, USA

References 1 Charlesworth, B. (1989) Trends Ecol. Evol. 4,264-267 2 Maynard Smith, J. (1988) in The Evolution of Sex (Michod, R.E. and Levin, B.R., eds), pp. 106125, Sinauer 3 Bernstein, H., Byerly, H.C., Hopf, F.A. and Michod, R.E. (1985) Science 229, 1277-1281 4 Kimura, M. and Maruyama, T. (1966) Genetics 54,1337-l 351 5 Kondrashov, AS. (1988) Nature 336, 435-440 6 Bernstein, H., Hopf, F.A. and Michod, R.E. (1988) in The Evolution of Sex (Michod, R.E. and Levin, B.R., eds), pp. 13!3-160, Sinauer

-. The Study of Instinct

Tim Roper’s review of The Study of textbook’ appear nowhere in the Instinctby Nikolaas Tinbergen’ was a blurb, and we naturally assume most interesting assessment of the nothing of the kind. What we said is: b,ookand its author. I cannot imagine, ‘Tinbergen’s insights undoubtedly however, how Roper came to be paved the way for significant obser- ‘alarmed that the publishers (accord- vational, experimental, and theoreti- ing to the blurb on the back cover) cal advances in behavioural ecology consider the book still to have a role and evolution over the past two dec- as an introductory textbook for ades. This book is reissued to make it students.’ The words ‘introductory available to a new generation of re-

searchers and students.’ Could any- one seriously quarrel with that?

Judith May

Reference

Oxford University Press, Walton Street, Oxford OX2 6DP, UK

1 Roper, T. (1989) Trends Ecol. Evol. 4, 318-319