PERSPECTIVES Key innovations and the ecology of macroevolution John P Hunter The origin or evolutionary success of taxa is often attributed to key innovations aspects of organismal phenotype that promote diversification Different ways of delimiting taxa and measuring success i e number or longevity of species morphological variety or differential control of energy give rise to different ideas of how key innovations might operate Key innovations may enhance competitive ability relax adaptive trade offs or permit exploitation of a new productive resource base Recent key innovation studies comparing species richness in extant sister clades may miss important observations possible only with consideration of the fossil record traditional higher taxa and phenotypic diversity John P Hunter is at the Dept of Anatomy New York College of Osteopathic Medicine Old Westbury NY 11568 USA K ey innovations are aspects of organismal phenotype important to the origin or subsequent success of a taxonomic group This concept is controversial however because it is difficult to test hypothesized key innovations1 and because researchers understand the concept in different ways see Box 1 Nevertheless the various definitions of key innovation share the basic idea that some attributes of organisms have been important over evolutionary time The concept links autecology and macroevolution or more specifically the summed performance of individuals and the performance of a taxonomic group to which the individuals belong When properly investigated Boxes 2 and 3 key innovations can potentially link evolutionary processes acting on different hierarchical levels Nevertheless key innovation hypotheses are not attempts to reduce the causes of biological expansion down to a single factor Historically researchers have measured evolutionary success by the appearance of higher taxa the proliferation of species or the generation of new morphologies Each measures a different aspect of expansion in the use and control of energy 2 and key innovations may promote this expansion Recent investigators have tended to focus on taxonomic diversification usually the number of species in a group whereas older literature was more concerned with major adaptive shifts recognized by the appearance of higher taxa Here I outline how the key innovation concept has itself evolved explain what can be learned from older approaches indicate problems in current analytical methods and offer some alternatives Key innovations and higher taxa The key innovation concept has always been linked to the origin of higher TREE vol 13 no 1 January 1998 taxa3 4 specifically to explain how higher taxa arise in terms of population level processes Miller3 used the origin of groundforaging thrashers from among treeforaging mockingbirds as a case study of a taxon genus Toxostoma originating from within another genus Mimus and described the differentiation in digging ability among thrashers as a possible genus in the making The acquisition of simple but functionally important changes in beak form makes possible the appearance of sets of birds thrashers and mockingbirds with different functional abilities and ecological tendencies Traditional systematicists would recognize the adaptive and ecological distinction between these birds by placing them in different taxa These taxa are of course those of evolutionary systematics The differences between such taxa can be large whereas Miller saw natural selection as capable of only incremental change and felt that extinction is insufficient to account for the distinctiveness of major groups3 Instead small changes in form can have a large functional significance i e key innovations and bring a lineage into a new ecological sphere where it can diverge free from competition with related incumbent species3 This connection between key innovations and the origin of higher taxa made biological sense to early workers Invasion of new adaptive zones i e a set of related ecological niches was seen to precede the origin and diversification of and within higher taxa and key innovations were seen to facilitate a transition into a new adaptive zone Historically studies of key innovations have focused on characters that diagnose higher taxa and set them apart adaptively from their close relatives5 as opposed to characters that promote diversification per se For example among mammalian orders the Tubuliden tata aardvarks are not and probably never have been particularly diverse either morphologically or taxonomically nevertheless aardvarks possess key characters that have essentially committed them to eating colonial insects a way of life quite different from other subungulates 5 Traditional evolutionary systematists would recognize the distinctiveness of aardvarks by placing them in their own order If macroevolution is concerned with such character state transitions that diagnose evolutionary differences of major taxonomic rank 6 e g between the order Tubulidentata and other mammalian orders then key innovations should occupy a central place in the study of macroevolution Historically the rug might have been pulled out from under the key innovation concept with the decline of evolutionary systematics and the rise of phylogenetic systematics When systematists began to group species into holophyletic monophyletic sensu stricto clades rather than higher taxa investigators of macroevolution shifted their focus from the biological differences between groups to the differential performance of clades most easily measured as numbers of species Because many traditional higher taxa are paraphyletic unacceptable within phylogenetic systematics many macroevolutionists viewed the biological distinctiveness of higher taxa as a partial result of an arbitrary classification7 On the other hand differences in the number of species between clades could be investigated as the result of Box 1 Contrasting definitions of key innovation Miller 1949 key adjustments in the morphological and physiological mechanism which are essential to the origin of new major groups 3 Van Valen 1971 A key character in the adaptive sense is a structure or element of physiology that makes a taxon more or less committed to a way of life different from or appreciably more efficient than that of its ancestors 5 Levinton 1988 key innovation is necessary but not sufficient for a subsequent radiation 6 Baum Larson 1991 a trait that greatly modifies the selective regime of the lineage
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