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Inferring Evolutionary Process from Phylogenetic Tree Shape VOLUME 72, No. 1 THE QUARTERLY REVIEW OF BIOLOGY MARCH 1997 INFERRING EVOLUTIONARY PROCESS FROM PHYLOGENETIC TREE SHAPE ARNE 0. MOOERS Departmentof Zoology, Universityof British Columbia 6270 UniversityBoulevard, Vancouver, V6T 1Z4 Canada STEPHEN B. HEARD Departmentof Biological Sciences, Universityof Iowa 138 BiologyBuilding, Iowa City,Iowa 52242-1324 USA ABSTRACT Inferencesabout macroevolutionary processes have traditionallydepended solely on thefossil record,but such inferencescan bestrengthened by also consideringthe shapes of the phylogenetic treesthat link extant taxa. The realizationthat phylogenies reflect macroevolutionary processes has led to a growingliterature of theoretical and comparativestudies of tree shape. Two aspects oftree shape are particularly important: tree balance and thedistribution of branch lengths. We examineand evaluaterecent developments in and connectionsbetween these two aspects, and suggestdirections forfuture research. Studiesof tree shape promise useful and powerfultests of macroevolutionary hypotheses. With appropriatefurther research, tree shape may help us detectmass extinctions and adaptiveradia- tions,measure continuous variation in speciationand extinctionrates, and associatechanges in theserates with ecological or biogeographical causes. The usefulnessof treeshape extendswell beyondthe study of macroevolution.We discuss applicationsto otherareas ofbiology, including coevolution, phylogenetic inference, population biology,and developmentalbiology. INTRODUCTION Why are there so many species of passerine HERE IS probably no greater nor more birds (e.g., Raikow1986; Kochmer and Wagner T 1988) or beetles (e.g., Evans 1975)?J B S Hal- fundamental problem in ecology and dane's famous quip about God's inordinate evolutionarybiology than explaining the di- fondnessfor beetles can onlybe partof the story. versityof life on Earth. A comprehensive ex- Questions about the relativediversity of evo- planation of this diversityshould account for lutionarygroups are often posed in termsof not onlythe regulation of diversity in extanteco- taxonomies (see literaturereviewed byAnder- logical communities (Ricklefs and Schluter son 1974). Ultimately, however, these are 1993), but also provide an understanding of questions aboutvariationin speciation and ex- what factorshave determined historical pat- tinction rates, and their signatures in the terns of diversificationamong evolutionary shapes of phylogenetic trees. The recent ex- groups. This latterquestion is a familiarone: plosion in the availabilityof phylogeneticdata The QuarterlyReview of Bzology, March 1997, Vol. 72, No. 1 Copyright? 1997 by The Universityof Chicago. All rightsreserved. 0033-5770/97/7201-0002$1 .00 31 This content downloaded from 131.202.42.30 on Fri, 26 Apr 2013 09:59:50 AM All use subject to JSTOR Terms and Conditions 32 THE QUARTERLY REVIEW OF BIOLOGY VOLUME 72 (fueled in part by the molecular revolution) 450' has given us a new and powerfulapproach for Leaf beetles studyingtempo and mode in evolution:phylo- 350 - genetic trees as historiesof the diversification of clades. The availabilityof these data has in turn led to dramatic growthin the study of , 250- phylogenetictree shape, whichprovides a pos- sible new approach to the studyof evolution and diversification(there are other applica- g 150- tions of tree shape, but we touch on them only z briefly).The time is ripe for an appraisal of the power and prospects of the studyof tree 50 shape. Here we outline the historybehind the use of tree shape, provide some necessary 00 10 20 30 40 50 60 70 80 90 100 technical background, and consider current Numberof speciesper genus descriptionsof and proposed explanations for FIGURE 1. A HOLLOW CURVE DISTRIBUTION OF the shapes we observe.We then conclude with THE NUMBER OF GENERA CONTAINING suggestionsfor future study. VARIOUS NUMBERS OF SPECIES OF LEAF BEETLE (CHRYSOMELIDAE). HISTORICAL SKETCH Data fromWilliams (1964, Table 50). Branching diagrams were considered fit- ting representations of organismal affinities long before Darwin (Panchen 1992). How- (Fisher et al. 1943), or log-normalseries (Wil- ever, Darwin is usually recognized as the first liams 1964), and saw in them the stamp of de- to consider trees explicitlyas representations terministicprocesses (Williams 1964; Ander- ofactual genealogies ofspecies, and his theory son 1974). Walters (1961; see also Cronk of natural selection gave him definiteideas as 1989) considered them merelypsychohistori- to theirshape (Darwin 1859:110). Faced with cal artifacts.Yule (1924) and Wright (1941) limited data, however, Darwin turned to pointed to a stochasticor probabilisticexpla- taxonomies instead of phylogenies for evi- nation for the hollow curve pattern. Ecologi- dence to support his hypotheses (Darwin cal theory-e.g., MacArthur's(1957) "broken- 1859:110). The use of taxonomies as reflec- stick" model of ecological diversity,which is tions of phylogeneticpattern continues up to also probabilistic-was adapted to explain the present. hollow curves,most prominentlybyAnderson Willis (1922) noted the concave shape of (1974). Recent investigatorshave even de- many frequency distributionsof taxonomic scribed the hollow curves in terms of fractal units containingvarious numbers of subunits geometry(Burlando 1990; Minelli et al. 1991; (e.g., species per genus), when units are or- see also Nee et al. 1992). Information-rich,rig- dered fromthose withthe greatestnumber to orous phylogenies have now largelyreplaced those withfewest subunits, and called this the the use of taxonomic listsin the studyof mac- "hollow curve"distribution. For example, Fig- roevolution (but see exceptions in Dial and ure 1 shows the sizes of chrysomelidbeetle Marzluff1989; Ricklefsand Renner 1994; Tiff- genera (from Williams 1964; based on data ney and Mazer 1995). collected by Willis). It plots the number of The modern study of phylogenetic tree chrysomelidbeetle genera withvarying num- shape and macroevolution can be traced to a bers of species. The hollow curve phenome- set of meetingsof paleontologistsand popula- non is familiarto anyone who has noticed the tion biologists at Woods Hole in the early conspicuous abundance of,for example, bee- 1970s (Raup et al. 1973; Gould et al. 1977; tles among insects,passerines among birds,or Schopf 1979), and to a biogeographic study teleostsamong fish. (Rosen 1978; Simberloffet al. 1981; Simber- Earlyauthors described these hollow curves loff1987). Both setsof investigatorswere seek- as hyperbolic(Chamberlin 1924), logarithmic ing falsifiablehypotheses of phylogenetictree This content downloaded from 131.202.42.30 on Fri, 26 Apr 2013 09:59:50 AM All use subject to JSTOR Terms and Conditions MARCH 1997 PHYLOGENETIC TREE SHAPE 33 shape. The "Woods Hole group" (coined by and unsupported null expectation that all la- Slowinski and Guyer 1989) was concerned beled cladograms were equally likely,and pre- thatpaleontologists took an overlydescriptive sented two other null distributions(see the and deterministicapproach when consider- section below on null models fortree balance, ing the patternsof fossildiversity. The group p 35). Simberloffet al. (1981) highlightedthe felt that some deductive methodology was necessity of using clear null models when needed, with a view to formulatinggeneral studyingcongruence among phylogenies,and evolutionarylaws (Raup et al. 1973). Taking showed how certain shapes of trees mightbe their cue from ecology (Simberloff 1972), expected to arise in nature. Together, the theycreated a simple stochasticmodel of mac- Woods Hole group's agenda of a quantitative roevolution (with lineages splittingand be- approach to paleontological studies of tempo coming extinct) and used it to simulate phy- and mode, and the response to the use of null logenies evolving through geological time. models in testsof biogeography,set the stage Because the distribution of shapes of real forthe modern use ofphylogenetic tree shape clades fromthe literatureresembled the simu- in the studyof macroevolution. The two ap- lated random clades, Raup et al. (1973; Gould proaches also highlight two differentbut et al. 1977) held that deterministicexplana- closely related aspects of tree shape: changes tions for clade shape were unnecessary.How- in diversificationrates through time,and dif- ever, Stanley et al. (1981) found that with ferences in diversificationpatterns among more realistic parameter values the histories contemporaryclades. These mightbe thought of clades fromthe fossil record could not be of as two differentaxes of variation.We con- reproduced with simple stochastic models. sider the latter aspect first,beginning with a The Woods Hole studiesare partof a richliter- briefsummary of terminologyand the statisti- ature concerning the course of diversityin the cal approaches used. fossil record, but we will not review it here. Currentstudies of phylogenetic tree shape are TERMINOLOGY FOR PHYLOGENETIC generally intellectual descendants of the TREE SHAPE Woods Hole group and itsuse of a simulation- Most biological taxa have arisenby a branch- driven,stochastic model of tree production. ing evolutionaryprocess of descent withmodi- The biogeographic approach owes much to fication (we are stilla long wayfrom a proper Hennig (1966), who postulated a series of bio- account of reticulation in phylogenetic the- geographic "rules" (summarized by Ashlock ory). While taxonomies generally reflect (to 1974) by which cladograms could be used to various degrees) historical relationships be-
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