CORRESPONDENCE are obtaining extra pair young and he suggests that these data disagree with predictions from the good genes hypothesis However under the good genes hypothesis we expect females to base their choice for extra pair copulations on their assessment of the relative quality of their own mate versus the available extra pair males i e usually their neighbours Thus quality should not be seen in an absolute sense but relative to other options available for each female Ideally pairwise comparisons of the characteristics quality of within pair and extra pair males from the same nests should be made such as those in the recent study of great reed warblers Acrocephalus arundinaceus 1 We suggested under the genetic quality hypothesis that low levels of extra pair paternity might be expected in populations that had undergone a bottleneck because genetic quality differences among males would be low Cordero suggests there are alternative explanations According to the heterozygosity theory females choose mates to avoid the expression of lethal or deleterious genes by producing heterozygous rather than homozygous offspring This theory does make predictions about the frequency of extra pair copulations in relation to the overall genetic diversity in populations but in the opposite direction to that expected from genetic quality benefits In populations with lower genetic diversity the risks of inbreeding depression are higher and therefore females might be more likely to seek extra pair matings if the main benefit is the production of heterozygous offspring We agree that avoiding inbreeding effects or genetic incompatibility is important for a female and might explain some multiple mating patterns such as when a female has several extra pair partners However it is not yet clear whether these mechanisms by themselves can maintain the high variance in male mating success which is a feature of species with high levels of extra pair paternity2 It is this nonrandom distribution of matings that provides the evolutionary force necessary to produce the sexual ornaments characteristic of populations with extra pair paternity 3 and that is more readily explained by genetic quality benefits Marion Petrie Evolution and Behaviour Research Group Dept of Psychology University of Newcastle Newcastle upon Tyne UK NE1 7RU marion petrie ncl ac uk Bart Kempenaers Konrad Lorenz Institute for Comparative Ethology KLIVV Austrian Academy of Sciences Savoyenstrasse 1a 1160 Vienna Austria b kempenaers klivv oeaw ac at References 1 Hasselquist D Bensch S and von Shantz T 1996 Nature 381 229 232 2 M ller A P 1998 Sperm Competition and Sexual Selection Birkhead T R and M ller A P eds pp 53 89 Academic Press 3 M ller A P and Birkhead T R 1994 Evolution 48 1089 1100 TREE vol 13 no 7 July 1998 Key innovations The concept of key innovations does more to confuse macroevolution than to clarify it Judith C Masters Hunter s TREE review1 of key innovations raises several concerns 1 Most definitions of key innovations portray them as playing a causal role in diversification They are identified as adaptive features present in all members of a diverse clade and by implication in the clade s ancestor But how is a clade delineated A clade is diagnosed by synapomorphies implicitly present in a common ancestor So the features used to define a clade are also advanced as the explanation for its existence thereby confusing correlation and causation 2 Hunter s terminology ignores the 1980s revolution in macroevolutionary theory particularly his reference to successful clades seizing opportunities to diversify into new adaptive zones Hierarchy theorists laboured to expunge such ideological thinking Vrba s effect hypothesis2 argues that apparently successful clades are actually intolerant clades their members have narrowly defined resource requirements and environmental change exposes them to negative selection mortality or reproductive failure Can such a clade sensibly be labelled more successful than a more tolerant group which can roll with the punches which is consequently species poor and likely to persist in the face of habitat deterioration 3 This value laden terminology points to a confused model of speciation and or radiation The most enduring conflict of evolution concerns the role of natural selection in the origin of species Key innovations imply features that enhance survival or reproduction in the face of competition predation or environmental challenge If key innovations are a cause of radiation it must be in one of these contexts However there is little evidence to support such a claim3 Adherents of both the traditional synthesis4 6 and the expanded macroevolutionary synthesis7 8 argue that radiation is more rapid and more extensive in the absence of competitor and predator pressure for example on oceanic islands or in the period immediately following a mass extinction Furthermore Hunter s review indicates that the role of key innovations is to allow organisms to escape these very sources of selection leaving environmental challenge as the sole directional pressure driving diversification If key innovations allow taxa to invade new adaptive zones they must arise before such invasions and without the benefit of direct selection by the environment to which they are fortuitously suited They are true exaptations9 We have no difficulty believing the concept that an exaptation might allow a population to persist under altered environmental conditions this is the essence of Darwin s model What we fail to see is why an exaptation should promote speciation Why should the possession of a fortuitous character that has allowed a population to survive one punishing round of selection provoked by exposure to a novel environment now inspire its descendants to invade new and different environments with similarly destructive consequences Isn t it more likely that the real factor promoting speciation is not the key innovation but the same kind of environmental change that destroyed the population s habitat in the first place forcing it to adapt to a new niche Dept of Zoology University of Fort Hare Private Bag X1314 Alice 5700 South Africa masters ufhcc ufh ac za Richard J Rayner Faculty of Science University of South Africa PO Box 392 Pretoria 0001 South Africa raynerj alpha unisa ac za References 1 Hunter J P 1998 Trends Ecol Evol 13 31 36 2 Vrba E S 1980 S Afr J Sci 76 61 84 3 Masters J C and Rayner R J 1993 Biol J Linn Soc 49 87 98
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