TREE vol. 13, no. 7 July 1998 281are obtaining extra-pair young’ and he suggeststhat these data disagree with predictions from thegood genes hypothesis. However, under the goodgenes hypothesis, we expect females to base theirchoice for extra-pair copulations on theirassessment of the relative quality of their ownmate versus the available extra-pair males (i.e. usually their neighbours). Thus, quality shouldnot be seen in an absolute sense, but relative toother options available for each female. Ideally,pairwise comparisons of the characteristics(quality) of within-pair and extra-pair males fromthe same nests should be made, such as those inthe recent study of great reed warblers(Acrocephalus arundinaceus)1. We suggested, under the genetic qualityhypothesis, that low levels of extra-pair paternitymight be expected in populations that hadundergone a bottleneck, because genetic qualitydifferences among males would be low. Corderosuggests there are alternative explanations.According to the ‘heterozygosity’ theory, femaleschoose mates to avoid the expression of lethal ordeleterious genes by producing heterozygousrather than homozygous offspring. This theorydoes make predictions about the frequency ofextra-pair copulations in relation to the overallgenetic diversity in populations, but in theopposite direction to that expected from geneticquality benefits. In populations with lower geneticdiversity, the risks of inbreeding depression arehigher and, therefore, females might be morelikely to seek extra-pair matings if the main benefitis the production of heterozygous offspring.We agree that avoiding inbreeding effects orgenetic incompatibility is important for a femaleand might explain some multiple mating patterns (such as when a female has several extra-pairpartners). However, it is not yet clear whether thesemechanisms, by themselves, can maintain the highvariance in male mating success, which is a featureof species with high levels of extra-pair paternity2.It is this nonrandom distribution of matings thatprovides the evolutionary force necessary toproduce the sexual ornaments characteristic ofpopulations with extra-pair paternity3and that ismore readily explained by genetic quality benefits.Marion PetrieEvolution and Behaviour Research Group, Dept of Psychology,University of Newcastle, Newcastle-upon-Tyne, UK NE1 7RU ([email protected])Bart KempenaersKonrad Lorenz Institute for Comparative Ethology (KLIVV), Austrian Academy of Sciences,Savoyenstrasse 1a, 1160 Vienna, Austria([email protected])References1 Hasselquist, D., Bensch, S. and von Shantz, T.(1996) Nature381, 229 –2322 Møller, A.P. (1998) Sperm Competition andSexual Selection(Birkhead, T.R. and Møller, A.P.,eds), pp. 53 –89, Academic Press3 Møller, A.P. and Birkhead, T.R. (1994) Evolution48, 1089 –1100Key innovations?Hunter’s TREEreview1of ‘key innovations’ raisesseveral concerns:(1) Most definitions of key innovations portraythem as playing a causal role in diversification.They are identified as adaptive features present inall members of a diverse clade and, by implication,in the clade’s ancestor. But how is a cladedelineated? A clade is diagnosed bysynapomorphies implicitly present in a commonancestor. So, the features used to define a cladeare also advanced as the explanation for itsexistence – thereby confusing correlation andcausation.(2) Hunter’s terminology ignores the 1980srevolution in macroevolutionary theory –particularly his reference to ‘successful’ cladesseizing ‘opportunities’ to diversify into new‘adaptive zones’. Hierarchy theorists laboured toexpunge such ideological thinking. Vrba’s ‘effect’hypothesis2argues that apparently ‘successful’clades are actually intolerant clades; theirmembers have narrowly defined resourcerequirements, and environmental change exposesthem to negative selection (mortality orreproductive failure). Can such a clade sensibly belabelled more successful than a more tolerantgroup (which can roll with the punches), which isconsequently species-poor and likely to persist inthe face of habitat deterioration?(3) This value-laden terminology points to aconfused model of speciation and/or radiation. Themost enduring conflict of evolution concerns therole of natural selection in the origin of species.Key innovations imply features that enhancesurvival or reproduction in the face of competition,predation or environmental challenge. If keyinnovations are a cause of radiation, it must be inone of these contexts. However, there is littleevidence to support such a claim3. Adherents ofboth the traditional synthesis4–6and the expandedmacroevolutionary synthesis7,8argue that radiationis more rapid and more extensive in the absenceof competitor and predator pressure, for exampleon oceanic islands or in the period immediatelyfollowing a mass extinction. Furthermore, Hunter’sreview indicates that the role of key innovations isto allow organisms to escape these very sourcesof selection, leaving environmental challenge asthe sole directional pressure driving diversification.If key innovations allow taxa to invade newadaptive zones, they must arise before suchinvasions and without the benefit of directselection by the environment to which they arefortuitously suited. They are true exaptations9. Wehave no difficulty believing the concept that anexaptation might allow a population to persistunder altered environmental conditions; this is theessence of Darwin’s model. What we fail to see iswhy an exaptation should ‘promote speciation’.Why should the possession of a fortuitouscharacter that has allowed a population to surviveone punishing round of selection, provoked byexposure to a novel environment, now inspire itsdescendants to invade new and differentenvironments with similarly destructiveconsequences? Isn’t it more likely that the realfactor promoting speciation is not the keyinnovation but the same kind of environmentalchange that destroyed the population’s habitat inthe first place, forcing it to adapt to a new niche?The concept of key innovations does more toconfuse macroevolution than to clarify it. Judith C. MastersDept of Zoology, University of Fort Hare,Private Bag X1314, Alice 5700, South Africa ([email protected])Richard J. RaynerFaculty of Science, University of SouthAfrica, PO Box 392, Pretoria 0001, South Africa ([email protected])References1 Hunter, J.P. (1998) Trends Ecol. Evol. 13, 31–362
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