TREE vol. 13, no. 1 January 1998 31Key innovations are aspects of organis-mal phenotype important to the ori-gin or subsequent success of a taxonomicgroup. This concept is controversial, how-ever, because it is difficult to test hy-pothesized key innovations1and becauseresearchers understand the concept indifferent ways (see Box 1). Nevertheless,the various definitions of key innovationshare the basic idea that some attributes oforganisms have been important over evolu-tionary time. The concept links autecologyand macroevolution, or more specifically,the summed performance of individualsand the performance of a taxonomic groupto which the individuals belong. Whenproperly investigated (Boxes 2 and 3), keyinnovations can potentially link evolu-tionary processes acting on different hier-archical levels. Nevertheless, key inno-vation hypotheses are not attempts toreduce the causes of biological expansiondown to a single factor.Historically, researchers have meas-ured evolutionary ‘success’ by the appear-ance of higher taxa, the proliferation ofspecies or the generation of new mor-phologies. Each measures a different as-pect of expansion in the use and control ofenergy2, and key innovations may promotethis expansion. Recent investigators havetended to focus on taxonomic diversifi-cation, usually the number of species in agroup, whereas older literature was moreconcerned with major adaptive shifts rec-ognized by the appearance of higher taxa.Here, I outline how the key innovation con-cept has itself evolved, explain what can belearned from older approaches, indicateproblems in current analytical methods,and offer some alternatives. Key innovations and higher taxaThe key innovation concept has al-ways been linked to the origin of highertaxa3,4, specifically to explain how highertaxa arise in terms of population level pro-cesses. Miller3used the origin of ground-foraging thrashers from among tree-foraging mockingbirds as a case study of ataxon (genus Toxostoma) originating fromwithin another (genus Mimus) and de-scribed the differentiation in digging abilityamong thrashers as a possible ‘genus inthe making’. The acquisition of simple, butfunctionally important, changes in beakform makes possible the appearance ofsets of birds (thrashers and mockingbirds)with different functional abilities and eco-logical tendencies. Traditional systemati-cists would recognize the adaptive andecological distinction between these birdsby placing them in different taxa. These taxa are, of course, those ofevolutionary systematics. The differencesbetween such taxa can be large, whereasMiller saw natural selection as capable ofonly incremental change and felt that ex-tinction is insufficient to account for thedistinctiveness of major groups3. Instead,small changes in form can have a large func-tional significance (i.e. key innovations)and bring a lineage into a new ecologicalsphere where it can diverge free from com-petition with related incumbent species3. This connection between key inno-vations and the origin of higher taxa madebiological sense to early workers. Invasionof new adaptive zones (i.e. a set of relatedecological niches) was seen to precede theorigin and diversification of (and within)higher taxa, and key innovations wereseen to facilitate a transition into a newadaptive zone. Historically, studies of keyinnovations have focused on charactersthat diagnose higher taxa and set themapart adaptively from their close rela-tives5as opposed to characters that pro-mote diversification per se. For example,among mammalian orders, the Tubuliden-tata (aardvarks) are not and probablynever have been particularly diverseeither morphologically or taxonomically;nevertheless, aardvarks possess key char-acters that have essentially committedthem to eating colonial insects, a way oflife quite different from other ‘subungu-lates’5. Traditional (‘evolutionary’) sys-tematists would recognize the distinctive-ness of aardvarks by placing them in theirown order. If macroevolution is con-cerned with such ‘character-state transi-tions that diagnose evolutionary differ-ences of major taxonomic rank’6(e.g.between the order Tubulidentata andother mammalian orders), then key inno-vations should occupy a central place inthe study of macroevolution.Historically, the rug might have beenpulled out from under the key innovationconcept with the decline of evolutionarysystematics and the rise of phylogeneticsystematics. When systematists began togroup species into holophyletic (mono-phyletic sensu stricto) clades rather thanhigher taxa, investigators of macroevolu-tion shifted their focus from the biologicaldifferences between groups to the differ-ential performance of clades, most easilymeasured as numbers of species. Becausemany traditional higher taxa are paraphy-letic, unacceptable within phylogenetic sys-tematics, many macroevolutionists viewedthe biological distinctiveness of higher taxaas a partial result of an arbitrary classifi-cation7. On the other hand, differences inthe number of species between cladescould be investigated as the result ofCopyright © 1998, Elsevier Science Ltd. All rights reserved. 0169-5347/98/$19.00PERSPECTIVESPII: S0169-5347(97)01273-1Key innovations and the ecology ofmacroevolutionJohn P. HunterThe origin or evolutionary ‘success’ of taxa is often attributed to key innovations –aspects of organismal phenotype that promote diversification. Different ways ofdelimiting taxa and measuring ‘success’ (i.e. number or longevity of species,morphological variety or differential control of energy) give rise to different ideas ofhow key innovations might operate. Key innovations may enhance competitiveability, relax adaptive trade-offs or permit exploitation of a new productive resourcebase. Recent key innovation studies comparing species richness in extant sisterclades may miss important observations possible only with consideration of the fossilrecord, 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.Box 1. Contrasting definitions ofkey innovationMiller (1949): ‘… key adjustments in the mor-phological and physiological mechanism whichare essential to the origin of new major groups’3.Van Valen (1971): ‘A key character, in the adap-tive sense, is a structure or element of physiologythat makes a taxon more or less committed toa way of life different from, or
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