Species Diversity, Biogeography, and the Evolution of Biotas1

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AMER. ZOOL., 34:33-47 (1994)

Specie

s Diversity , Biogeography , and the

Evolutio

n of Biotas 1 JOE

L CRACRAFT

Department

of Ornithology, American Museum of Natural History,Central Park West at 79th St., New York, New York 10024

SYNOPSIS

. A central scientific problem for ecologists and systematistshas been t o explain spatiotemporal patterns of species diversity. One aspec t of this question is how to understand the taxonomic assembly of biota s and their included ecosystems and communities. Four processesadd o r subtract species from a region: speciation, extinction, biotic dis- persion , and long-distance dispersal. Speciation and biotic dispersion are postulate d t o resul

t in historically structured (hierarchical) species assem-blages, whereas long-distance dispersal results

i n assemblages that wouldbe expected t o be historically unstructured (nonhierarchical). Continentalbiotas, a s exemplified by the Australian avifauna, are historically struc-tured: they ar e segregated into areas of endemism having hierarchicalrelationships that presumably arose as a result of their history being dom-inated b

y cycles of biotic dispersion and vicariance. It is also proposedthat these latter two processes are necessary,

an d i n many cases probablysufficient, t

o explain the taxonomic composition of communities withinthese areas of endemism. Long-distance dispersal appears

t o play a muchmore minor role i n th

e assembly of either continental biotas or theircommunities than current ecological theory would predict.

INTRODUCTIO

N

Biologist

s hav e describe d abou t 1.4 mil- lio n species , but virtually all would agreethat there ar e certainly no fewer than 10 millio n an d perhap s a s man y a s 50-10
0 mil -lion ye t t o b e discovered and named.Numerical claims such as this are essentiallyad hoc a t this stage of our knowledge, for w e reall y hav e comparativel y littl e empir -ical data t o make an accurate estimate of specie s diversity . Yet we do know that the figure o f 1.4 million species is based pri- maril y on descriptions of the largest, and therefor e mos t obvious , organism s an d tha t th e biologica l worl d o f the small has barelybeen subject t o study. Unfortunately, thatworld i s likely to remain relatively unex-plored unless additional systematic spe - cialist s of viruses, bacteria, fungi, nema- todes , and small arthropods are trained and 1 Fro m the symposium Science as a Way of Know-ing - Biodiversity presented a t th e Annual Meeting of th e America n Societ y of Zoologists, 27-30 December1992, a t Vancouver, Canada. se t to work. Three observations argue strongl y i n favor of there being tens of mil- lion s o f specie s her e o n Earth : (1 ) mos t kind s o f organism s ar e ver y small , (2 ) mos t specie s ar e narrowl y endemic , an d (3 ) al l species, eve n mos t o f th e smal l ones , harbo r smalle r parasiti c o r endosymbiotic species, which ar e themselve s narrowl y endemi c (i.e., i n term s of their host specificity). Fe w subject s ca n excite the intellect as muc h a s th e stud y o f biologica l diversit y for i t confront s us with some of the most fun- damenta l question s in evolutionary biol- ogy . Wha t ar e species ? Ho w man y ar e there ? Ho w d o species arise? How do we explain variatio n i n diversit y acros s spac e an d time ? Ho w ar e biotas assembled taxonomically ove r tim e t o form the diversely complex ecosystem s w e se e today ? Althoug h w e have som e answer s t o these questions, most cur- ren t hypothese s ar e based on the study of vertebrat e groups . The intellectual chal- leng e fo r those who study vertebrates is whethe r thes e conjecture s abou t specie s an d th e evolutio n o f diversit y wil l b e applicabl e t o th e world of the small - that is, whether 3

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34JOEL CRACRAFT

th e sam e pattern s an d processe s wil l b

eobservable but at smaller spatiotemporalscales - or whether the patterns and pro-cesses themselves will be fundamentally dif-ferent. As we study the world of the small,however, we will be required to expendmuch more effort in order to draw similarlyaccurate conclusions. It is, for example,much easier to determine the distributionsand relative abundances of

larg

e organismsthan of small ones. This is one reason whyinferences about biotic evolution are likelyto be based on the study of

larg e organismsfor some time to come. Fo r th e pas t severa l decade s a centra l sci

-entific question being asked about biologicaldiversity has been how we might explainspatial variation in species numbers. Theproblem is often discussed with reference toa particular spatial scale, for example, howwe might account for variation in diversityamong habitats or communities, amongecosystems, or among larger entities such asbiotas. The literature on this subject is toovoluminous to be reviewed here (see thesummaries of MacArthur, 1965; Pianka,

1966

; Whittaker, 1972, 1977), but it is accu-rate to say that the emphasis of these papershas been on examining potential ecologicalprocesses that are postulated to "maintain"spatial variation in diversity. Although con-sideration is sometimes given to processesof speciation and extinction, especially whendiscussing gradients at large spatial scalessuch as among biotas, it is commonlyassumed that the patterns we observe arelargely a function of mechanisms operatingover relatively short temporal scales, that

is , over so-called "ecological time," and thathistory can be ignored (e.g., MacArthur, 1972

; Cody, 1975). This view is beginningto change, however, as ecologists increas-ingly take processes such as speciation intoaccount (Rosenzweig, 1975; Fowler andMacMahon, 1982; Brown and Maurer,

1987
, 1989; Ricklefs, 1987; Wiens, 1989). Th e goa l o f thi s pape r i s t o examin e ho

wspecies assemblages evolve (see also Cra-craft, 1985, 1992). Two perspectives will betaken. The first considers the alternativemodels that might be used to investigate theproblem of species assembly and thusfocuses on methods of historical analysis. Itis proposed that methods of historical bio-

geograph y mus t pla y a centra l rol e i n inves

-tigating the causal mechanisms underlyingpatterns of species diversity. The secondperspective attempts to challenge the pre-vailing causal worldview, which states thatwhile we might explain large-scale patternsby processes such as speciation and extinc-tion, observed patterns at the smaller scale,say among local communities, are bestexplained by population-level ("ecologi-cal") processes or "assembly rules." I willpropose that the processes operating at thehigher

scale s are necessary and probably suf-ficient to explain much, but not necessarily all , of the variation observed at lower scales.The implication is that if w

e do not under-stand the larger scale patterns and their cau-sation, our observations and explanationsat the smaller scale will be incomplete.

MODEL S O F SPECIE S ASSEMBL Y

Spatiotemporal scale, causation,

and language On e facto r contributin g t o th e lac k o f

arapprochement among investigators tryingto understand how species assemblagesevolve is that of the language being appliedto phenomena and causal mechanisms thatare manifested at different spatiotemporalscales. The same word applied to differentphenomena often obstructs our understand-ing of the processes responsible for thoseobserved patterns. The notion of "speciesassembly" itself carries different causalimplications. To a systematist, the buildupof species over time is seen in terms ofcoarse-grain changes in species diversity,with the processes of speciation and extinc-tion being central to the analysis. To manyecologists, in contrast, "species assembly"is the specific temporal sequence of speciesgains or losses to a local community

(e.g.,

Diamond

, 1975
; Nee , 1990
; Drake , 1990a
, b, 1991)
. Bot h thes e group s o f investigator

salso use the word "historical" to describethis temporal change in diversity, thusascribing different causal processes to thisword as well.

Ambiguitie

s i n languag e exten d t o th

eentities whose patterns and causal dynamicswe are trying to decipher. Knowledge aboutDownloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

EVOLUTION OF BIOTAS35

"specie s assembly, " fo r example , depend

son our perception of what "species" are. Atthe level of local communities, it will rarelymatter how species are defined, but at largerregional levels it may matter very muchbecause there we deal with isolated or sym-patric populations that exhibit varyingdegrees of differentiation, and the nature ofthat differentiation is the primary evidenceused to establish species limits. Ecology hasits language difficulties

also

. It is not straight-forward to talk about species assemblywithin communities if one cannot delineateboundaries of communities with somedegree of precision. As Drake (1990a) notes,ecologists frequently adopt a taxonomicperspective when circumscribing a partic-ular set of species as constituting a "com-munity"

{e.g., a "bird" community or an"insect" community), yet relative to energyflows, food webs, or biotic interactions, ataxonomic conception of a "community"may be misleading when asking why andhow these species associations were assem-bled.

Th e vie w o f "specie s assembly " adopte

dhere is "historical" in the sense of searchingfor those processes that, over time, have ledto the ensemble of species that is present ina given geographic region. I will not addressthe question of the precise temporalsequence of species addition into theseregions as this seems to be beyond our ana-lytical capabilities.

Processes of species assembly

Fou r primar y processe s ad d o r subtrac

tspecies from preexisting species assem-blages: speciation, extinction, biotic disper-sion, and long-distance dispersal. As theseprocesses are generally familiar to organis-mic biologists, only brief comments will beprovided here. At the same time, it isimportant that several meanings of theseprocesses be made explicit, especially as theymight apply to patterns of different spatio-temporal scale. The next section considerssome methodologies that can be used toassess the relative importance of each forexplaining the assembly of a given ensembleof species.

Speciatio

n i s generall y viewe d a s a pro -cess of large spatiotemporal scale. Popula-tions become isolated regionally and then differentiate . Th e absolut e scal e o f specia

-tion is dependent upon the organisms inquestion, and many small-sized organismsapparently speciate at very small scales (atleast from human perspectives). Theextreme case of this would probably beinstances of sympatric speciation (to theextent that this occurs). We will considerspeciation as contributing to an increase inthe species number of an area if that area ispart of the region in which the species hasdifferentiated {i.e., if that area is a part ofthe species' area of endemism).

Specie

s extinctio n i s alway s a loca l phe

-nomenon in the sense that extinction is, bydefinition, a statistical summation of indi-viduals failing to replace themselves repro-ductively over the range of the

species

. Thus,species themselves do not have to becomeextinct before that process can be said to beimportant at the level of local speciesassemblages. The distinction between pop-ulation- and species-extinction is thereforea function of the spatiotemporal scale of theproblem under study. Without some his-torical documentation of a species' formerpresence, the effects of extinction on changesin diversity within an assemblage are diffi-cult to assess.

A clea r distinctio n mus t b e mad e betwee

ndispersion and dispersal (Platnick, 1976),inasmuch as these terms identify two dis-tinct process-level phenomena, each imply-ing different predictions about species' his-tories (see next section). Dispersion is apopulation-level phenomenon and describesa change in range of the population of aspecies, or of the populations of numerousspecies jointly (biotic dispersion), throughspace. Dispersion involves expansion ofdistributions over distances that would nor-mally be no greater than the home range ofindividuals of the population. Long-dis-tance dispersal, in contrast, is a process ofsmall numbers of individuals rather thanpopulations, and involves relatively rapidcolonization across a barrier beyond thenormal home-range distance (dispersionusually consists of slow range expansionacross a landscape). From a systematic per-spective, long-distance dispersal may leadto differentiation following isolation, or ifDownloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

36JOEL CRACRAFT

TAXONOMI

C ASSEMBL Y O F REGIONA L BIOTA S

Processe

s Leadin g t o Historicall yStructured (Hierarchical) Assemblages 1 . Speclatlon via vlcarlance.Local biota under study

Ancestra

l biot a 2 . Blotic dispersion leoaingto cosmopolitanism followingbreakdown of barrier

Proces

s Leadin g t o HIstorlcaH y

Unstructure

d (Nonhlerarchlca

OAssemDIages

1 . Long-distance dispersalacross barriers FIG

. 1. Basic models oftaxonomic assembly of biotasof large and small spatiotemporal scales. Two pro-cesses, speciation and dispersion, produce historically(hierarchically) structured biotas. Subdivision (vicari-ance) of an ancestral biota results in two smallerdescendant biotas. Species differentiating in each ofthese are more closely related to each other than tospecies in a third area. Speciation thus contributes spe-cies to local biotas. Biotic dispersion also contributesspecies to local biotas. Dispersion is highly constrainedand reflects historical patterns of areas of endemism.Long-distance dispersal produces biotas that are gen-erally not hierarchically structured. See text.

differentiatio n doe s no t tak e place , the n dis

-persal produces widespread, patchy distri-butions. It is worth noting that "speciesassembly" within the ecological literaturealmost always pertains to long-distance dis-persal into a region from some distant "spe-cies pool"

(e.g., Diamond, 1975; Nee, 1990;Patterson, 1990; Drake, 1990a, b, 1991).

Historically

structured species assemblages: expectations of pattern given process Th e argumen t t o b e develope d here , an

despecially in the next section discussingmethods of historical analysis, is that twoprocesses - speciation involving vicarianceof widespread ancestral populations, and theprocess of dispersion - can be expected to

produc e historically structured specie s assemblages . Long-distanc e dispersal , o n th eother hand, can be expected to result in his-torically unstructured species assemblages. I t i s importan t t o emphasiz e tha t th e con - cep t o f "historically " structure d o r unstruc - ture d refer s t o th e typ e o f phylogeneti c pat - tern s w e expec t t o observ e fo r constituen t species , no t t o th e expecte d tempora l order - in g o f th e taxonomi c buildu p o f a n assem - blage . I f w e envisio n a n ancestra l biot a occu - pyin g a relativel y larg e area , an d i f a barrie r arise s withi n tha t biot a t o subdivid e it , the n th e population s o f th e specie s occupyin g th e communitie s an d ecosystem s o f thes e tw o subdivision s wil l becom e isolate d fro m eac h othe r (Fig . 1) . Ove r tim e thes e population s wil l differentiat e an d som e proportio n wil l eventuall y becom e diagnosabl y distinc t taxa . I n thi s case , speciatio n vi a vicarianc e ha s resulte d i n component s o f thes e subdivide d biota s sharin g a commo n historica l pattern , fo r th e tw o descendan t specie s o f eac h o f th e clade s ar e mor e closel y relate d t o eac h othe r tha n t o thei r sister-specie s i n som e thir d area . Speciatio n ha s thu s produce d descendan t biota s tha t ar e historicall y struc - tured .

Likewise

, whe n barrier s ar e los t o r thei r effect s mitigated , population s o f specie s withi n th e previousl y isolate d biota s wil l hav e a n opportunit y t o mov e acros s th e landscape , wit h th e amoun t o f dispersio n bein g determine d b y geography, climate, and th e autecologica l requirement s o f th e con - stituen t species . A majo r effec t o f bioti c dis - persio n i s th e intermixin g ("hybridization" ) o f onc e separate d biota s (Fig . 1) . Becaus e th e specie s withi n a biot a exhibi t som e degre e o f similarit y i n thei r ecologica l response s t o th e environmenta l condition s unde r whic h the y hav e evolved , w e woul d predic t tha t the y woul d shar e similaritie s i n thei r subsequen t histor y o f dispersion . Spe - cies , after all, are ecologically bonded to othe r specie s i n thei r communities , thu s ensurin g som e degre e o f correlate d chang e i n distribution . Becaus e o f thei r share d respons e t o changin g geographi c an d eco - logica l conditions , specie s i n a biot a wil l ofte n ten d t o b e limite d i n thei r dispersio n b y th e sam e barrier s (e.g., a dr y savanna h actin g a s a barrie r t o a mois t fores t biota) . Eve n thoug h dispersio n promote s cosmo - politanism - th e tendenc y fo r specie s t oDownloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

EVOLUTION OF BIOTAS37

becom e mor e widespread - i t doe s no t pro

-mote randomization of the component spe-cies of the biota. The process of dispersion,therefore, can also be expected to producedistribution patterns that have a high degreeof historical structure because it will be theserelatively cosmopolitan biotas that will besubdivided by subsequent vicariance.

Som e fe w specie s withi n biota s ar e pre

-disposed ecologically and behaviorally forlong-distance dispersal across barriers thatotherwise constrain the distributions of mostof the species of the biota (Fig. 1). Long-distance dispersal, by its very nature, resultsin species having widespread, but patchy,distributions and promotes a tendencytoward less congruent patterns of area rela-tionships. Thus, individuals of a long-dis-tance dispersing species will be successful atcolonizing only when they find an area char-acterized by suitable ecological conditions.Because of the idiographic and contingentnature of dispersal and colonization, wewould not expect hierarchical distributionpatterns to be shared among those specieswithin a biota that have a predispositiontoward dispersal.

BLOGEOGRAPHI

C ANALYSIS !

IDENTIFYIN

G HISTORICA L STRUCTUR E I N

EVOLVIN

G BIOTA S Th e precedin g sectio n sketche d som e o

fthe historical patterns that component spe-cies of biotas might be expected to showgiven certain assumptions about processesthought to be involved in species assembly.This section makes those predictions of pat-terns more explicit by outlining some of themethods used for historical analysis. Thekey element for developing an understand-ing of the history of biotas is the applicationof the methods of vicariance biogeography(Nelson and Platnick,

1981

; Humphries andParenti 1986), which seeks to discover con-gruence in the spatial histories of cladeswhose constituent taxa share common pat-terns of endemism. Once phylogenetichypotheses for these different clades havebeen generated by cladistic analysis (El-dredge and Cracraft, 1980; Nelson and Plat-nick, 1981), hypotheses about the historiesof the areas of endemism themselves canthen be constructed.Congruence in patterns of speciation:

Reconstructing

the history of areas of endemism Th e majorit y o f specie s ar e nonrandoml

ydistributed and clustered into areas ofendemism. General knowledge about areasof endemism has been available for centu-ries, but interest in them has recentlyincreased because of their importance inexamining the evolutionary history of spe-cies and biotas, and because the identifi-cation of these areas plays a central role inefforts to conserve biodiversity.

Conside

r fou r mois t fores t area s o

fendemism in eastern Australia (Fig. 2). Wewish to investigate the history of these areas:How might they have arisen? Do they havea historical relationship with each other orwith some other area? Answers to thesequestions are sought using the methods ofvicariance biogeography. Clades are chosenthat have endemic species in three or moreof the areas, and the cladistic relationshipsof each group of species are then investi-gated. In Figure 2 the phylogenetic rela-tionships of four species in each of two cladesof birds (parrots and finches) reveal precisecongruence in their area-relationships. It istherefore most parsimonious to infer ashared history of the species and their areasof endemism. In this

case

, a common ances-tral biota is hypothesized to have been sub-divided by a barrier into two areas at timeT, (Cape York, on the one hand, and Ath-erton Plateau + Eastern Queensland +Southeastern Forest, on the other). This isfollowed at time T

2 b y a secon d

vicarianceevent that subdivides the latter area into theAtherton Plateau and Eastern Queensland+ Southeastern Forest components, andthen at time T

3 th e latte r are a i s furthe

rsubdivided by the origin of a third barrier.Barriers can be physiographic, ecological, orsome combination.

Th e mor e clade s exhibitin g congruenc

ewith the finches and parrots, the more sup-port there would be for the hypothesis thata series of vicariance events has influencedthe patterns of differentiation of differentgroups similarly and that the areas ofendemism have had a specific historicalrelationship to one another. This procedureconstitutes a powerful method with whichDownloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

38 JOEL CRACRAFT

VICARIANC

E MODE L S E P I F l E Q P 2 F 2 AT H P 3 F 3 C Y P 4 F 4

Area-cladogram

s fo r parrot s an d finche s C Y AT H E Q S E C Y AT H E Q S E C Y AT H E Q S E METHO D 1 . Delineate areas of endemism 2 . Construct phylogenetic hypotheses for clades3. Search for congruence FIG

. 2. Historical structure among areas is identified using methods of vicariance biogeography. Areas ofendemism are delimited, and phylogenetic hypotheses are constructed for clades (here, parrots and finches)having endemics in the

areas

. The resulting cladograms are then compared for congruence. The area-relationshipsof these two clades are precisely congruent, and we therefore infer a common history of vicariance (right). SeeFigure 5 for a map of the areas of endemism of Australia. Abbreviations for areas: SE, Southeastern Forest;

EQ , Eastern Queensland; ATH, Atherton Plateau; and CY, Cape York Peninsula. See text. t o searc h fo r historica l structure , tha t is , t

oask whether the areas, and the taxa endemicin them, have a common shared history. Itshould be noted that applications of thismethod to the real-world are invariablymore complex than this hypothetical exam-ple might suggest. Many analyses are com-plicated by the presence of widespread taxa,which may have obtained their distribu-tions in multiple areas via long-distance dis-persal rather than vicariance. This being the

case

, they may introduce "noise" into ananalysis in that the underlying vicariancepattern, to the extent that it does exist, ismore difficult to resolve. Likewise, compli-cations arise when the areas in question are"composites" or "hybrids," that is, whenthe taxa endemic in an area are products ofmultiple vicariance events or instances of

bioti c dispersio n an d hav e biogeographi c relationship s t o mor e tha n on e area . Thi

sdiscussion does not begin to describe themethodological complexities contained invicariance biogeography, but a clear intro-duction to these can be obtained by con-sulting Humphries and Parenti (1986; seealso Nelson, 1984; Nelson and Platnick,

1978
; Platnick, 1981; Platnick and Nelson, 1984
; Cracraft, 1988a; Page, 1988, 1989, 1990)
.

Reconstructing

the historical constraints on biotic dispersion I t wa s propose d earlie r tha t bioti c dis - persion - th e wholesal e expansio n o f pop - ulation s fro m on e regio n t o another - woul dDownloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

EVOLUTION OF BIOTAS39

giv e ris e t o nonrando m distributio n pat - tern s becaus e th e exten t o f dispersio n o f var - iou s specie s withi n th e biot a woul d likel ybe controlled by similar geographic or cli-matic/ecological conditions or barriers

. Thatbeing the prediction, how might we inves-tigate whether this is the case or not? Ananswer to this problem is more subtle thanthe search for vicariance patterns amongendemic taxa. For one thing, two areas ofendemism might share a widespread taxon,but this fact does not provide positive evi-dence for a vicariant relationship betweenthese areas because (1) the distributions mayhave arisen as a result of long-distance dis-persal, or (2) the species may have beenpresent in a larger ancestral biota but failedto differentiate once the areas became seg-regated. In each case, the two areas sharingthe widespread species could have their truearea-relationships with other areas ofendemism.

Bu t ther e i s anothe r possibility : th e pres -ence of a species in two areas, while notpositive evidence for their close relation- ship

, is nevertheless not inconsistent withthis hypothesis. Thus, an ancestral areamight be partitioned into two by a newlyarisen barrier, and some of the widespreadtaxa could fail to differentiate. Indeed, itmight be expected that areas of endemismwhich have close historical connections willshare a greater number of widespread spe-cies than will those areas of endemism hav-ing more distant vicariant relationships. Thehypothesis of vicariance depends uponancestral cosmopolitanism prior to isola-tion of the areas, and it is not an unreason-able assumption to suggest there will be ageneral inverse relationship between therecency of common ancestry among theareas and the proportion of species that areundifferentiated. Thus, more closely relatedareas might be expected to share relativelymore widespread (i.e., undifferentiated)species.

Thi s lead s u s t o predic t tha t bioti c dis

-persion will be constrained historically. Ifthis is indeed the case, we would expect thatdistributions of species, including those thatare widespread, will exhibit hierarchicalcongruence when examined cladistically. A

HISTORICA

L ANALYSI S O F BIOTI C DISPERSIO N Area s o f endemism : Metho d o f coding :

Species

' distribution s rt - 1 4 1 1 1 1 - ou t are a r 5 roo tA B C Area s A B C D outare a 1 1 1 0 0 02 1 1 1 0 03 1 1 1 1 0 4 1 1 0 0 05 1 0 0 1 0 FIG . 3. A method for discovering historical (hierar- chical ) structur e amon g area s o f endemis m tha t i s pro - duce d largel y b y bioti c dispersion . Se e text . metho d fo r investigatin g thi s conjectur e i

sshown in Figure 3 (see Cracraft, 1991, fordetails). Areas of endemism are first delin-eated, then species distributions are codedfor their presence (1) or absence (0) in these

areas

. In a subsequent cladistic analysis ofthis data matrix, areas would be comparableto "taxa" and the species' distributionscomparable to "characters." An outgrouparea that is coded all 0s can be added toroot the area-cladogram. For those specieswith subspecies-level taxa, some historicalinformation might be included by first cod-ing the distributions of the subspecies thenof the entire species. In this type of proce-

dure , only taxa having distributions acrosstwo or more areas will provide data unitingareas together. Onc e a n area-cladogra m i s obtaine d base

don raw distributions, it can be compared toone derived from a vicariance analysis ofclades having endemics in the same areas.Because the cladogram of raw distributionshas a minimum of historical relationshipsbuilt into the data, the resulting hierarchicalpattern can be interpreted as reflecting con-siderable historical constraint in raw distri-butions (i.e., primarily widespread species)if that pattern is largely congruent with onederived from a vicariance analysis, whichis specifically designed to recover history.Little or no congruence between these twoanalyses would indicate, in contrast, thatDownloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

40JOEL CRACRAFT

DETECTIN

G TH E ABSENC E O F HISTORICA L STRUCTUR

E IN AREA-RELATIONSHIPS

S E P I F l PI L F 5EQ P 2 F 2 S W P 5ATH P 3 F 3 C Y P 4 F 4

Areocladogram

s of parrots an d finches 1 . Undertake vicariance analysis 2 . Identify species in areas that are par t of the general vicariance pattern FIG

. 4. A method for detecting the absence of historical (hierarchical) structure as evidence for long-distancedispersal using areas of endemism in Australia as an example. Abbreviations for areas: SE, Southeastern Forest;PIL, Pilbara; SW, Southwestern Forest; EQ, Eastern Queensland; ATH, Atherton Plateau; and CY, Cape YorkPeninsula (see Fig. 5 for a map of the areas). See text.

cosmopolita n distribution s lac k significan t historica l constraint .

Long-distance

dispersal: Recognizing the absence of historical structure A s note d above , long-distanc e dispersa

lwould be expected to produce distributionsthat lack historical structure. If a populationbecomes established via long-distance dis-persal, the distribution pattern is often char-acterized as being "spurious" or an "out-lier." This assessment is most often madein the absence of phylogenetic considera-

tions

, especially if the colonizing populationis not perceived to have differentiated andis "widespread." The question arises, how-ever, as to how we might objectively iden-tify such distributions. Vicariance bioge-ography provides a methodology to do this(Fig. 4).

I f congruen t vicariance pattern s ca n b emost parsimoniously interpreted as evi- denc e fo r a share d histor y o f th e area s o f endemis m an d thei r taxa , the n observe d lac

kof congruence suggests the absence of gen-erality in the species' distributions as wellas the areas they occupy (Nelson and Plat-nick, 1981). One species in each of the twoclades of Figure 4 is consistent with thisinterpretation. If upon analysis of otherclades, the area-relationships (((SE + EQ)+ ATH) + CY) continue to be corroboratedand areas PIL and SW are not found to bepart of this general pattern, then the twospecies in those areas are most reasonablyinterpreted as outliers and the result of long-distance dispersal. Hence, the importanceof long-distance dispersal for the build-upof species diversity within a biota can beassessed by asking to what extent theincluded species are part of

a general vicar-iance pattern.Downloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

EVOLUTION OF BIOTAS41

SPECIE

S ASSEMBLAGE S O F LARG E SCALE : HISTORICA L CONSTRAIN T

PREDOMINATE

S Thi s sectio n summarize s som e o f th e evi

-dence pertaining to the evolution of large-scale species assemblages, in this case thehistory of areas of endemism within con-tinental

biotas . Aspects of the history of spe- cie s assemblage s o f smal l scale , a t th e eco -system/community level, will be taken upbelow. I t mus t b e stresse d tha t ou r understandin

gof the empirical patterns is still very poorlydeveloped. We lack a great deal of relevantphylogenetic information for the taxa ofcontinental and marine biotas. Moreimportantly, very few studies haveattempted to identify areas of endemism andthen investigate the relationships of taxaendemic to those areas. As a consequence,there is a scarcity of the base-line phylo-genetic data that can be used to discoverbiogeographic congruence.

Th e studie s reviewe d belo w focu s o n con

-tinental vertebrate biotas, particularly thepatterns revealed by an analysis of the avi-faunas. It is not yet apparent how generalthese results might be, but some circum-stantial evidence will be cited to support theconjecture that species distributions withinbiotas are highly structured historically.Three lines of evidence will be summarized:areas of endemism, congruence in the his-tories of vicariance, and historical patternsof dispersion.

Areas of endemism are evidence for historical structure Th e fac t tha t th e distribution s o f organ

-isms are clustered into areas of endemismis strong evidence that biotas are histori-cally structured. The observation that closelyrelated forms (taxa) replace each other fromone region to the next

(i.e., are vicars of oneanother) was known long before Darwinwrote about it in 1859. Although areas ofendemism are better known for vertebratetaxa such as birds and mammals, speciesdistributions in those groups are still poorlyunderstood for many regions of the world.Because of the difficulties of determiningranges of small organisms, it is not inac-curate to say that areas of endemism for

species-leve l tax a ar e essentiall y unknow nfor virtually all the world's biota. I t i s difficul t t o imagin e ho w area s o fendemism within continental or marinebiotas could be explained as a

general phe-nomenon except as a result of vicariance - through the origin of physical or ecologicalbarriers - of once more cosmopolitan bio-

tas

. If that is true, then the species of thoseareas of endemism can be expected to exhibithierarchical relationships and the areasthemselves will eventually be shown toexhibit a historical pattern. It is possible, ofcourse, that some areas of endemism mightarise as a result of repeated colonizationcaused by long-distance dispersal - say, forexample, in some insular situations - butthis appears much less likely to occur oncontinents or within marine biotas. In any

case

, the methods of vicariance biogeogra-phy provide a direct way of testing thesealternatives (Figs. 2-4).

Congruence

in vicariance history is probably common I f th e specie s o f a grou p ar e distribute d i n area s o f endemis m withi n a regiona l biota , the n th e simples t explanatio n i s tha t th e grou p wil l b e par t o f a genera l vicarianc e histor y involvin g thos e areas . Conse - quently , give n th e ubiquit y o f well-marke d area s o f endemis m withi n continenta l ver - tebrat e faunas , w e ca n conclud e tha t ther e i s muc h mor e historica l structur e t o thos e biota s tha n previou s model s o f bioti c evo - lution - dominate d a s the y hav e bee n b y th e processe s o f dispersio n an d long-distanc e dispersal - hav e le d u s t o believe . Althoug h vicarianc e ha s undoubtedl y playe d a majo r rol e i n th e evolutio n o f biotas , ou r empirica l understandin g o f th e exten t o f it s impor - tanc e i s stil l inadequate .

Vicarianc

e pattern s hav e bee n recognize d fo r area s o f endemis m locate d withi n bot h continenta l an d insula r context s an d usin g a variet y o f taxonomi c groups , includin g spider s o f th e montan e fores t area s i n Afric a (Griswold , 1991)
, eucnemi d beetle s o f

Indomalaysi

a an d th e southwes t Pacifi c (Muona , 1991)
, butterflie s o f Wallace a (Vane-Wright , 1991)
, fresh-wate r fishes o f

Centra

l Americ a (Rosen , 1978
, 1979)
, an dDownloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

42JOEL CRACRAFT

Anthe m Lan dCape York Peninsula

Kimberle

y Platea u

Atherto

n Platea u outare a

Kimberte

y Platea u Arnhe m Lan d

CapeYor

k Peninsul a

Atherto

n Platea u

Easter

n Queenslan d

Southeaster

n Fores t

Pllbar

a

Wester

n Deser t

Easter

n Deser t

Norther

n Deser t

Tasmani

a

Southwester

n Fores t Eyr e Peninsul a

Adelaid

e FIG

. 5. Australian areas of endemism based on avian distributional patterns (from Cracraft, 1991). The clado-gram shows the hierarchical structure contained in the raw distributions of the species and subspecies of Australianbirds using the method described in Figure 3. Compare to Figure 6. See Cracraft (1991) and text for details.

Southwester

n Fores tSoutheastern Fores t bird s o f Sout h Americ a (Cracraf t an d Prum , 1988
; Cracraft, 1988a), Australia (Cracraft, 1986

, 1991), and Wallacea (Cracraft, 19886).Vicariance patterns have also been postu-lated for some marine biotas, for examplein marine water striders (Andersen, 1991).These studies, along with the substantialdata-base that exists on areas of endemism,lead to the prediction that vicariance of

rel - ativel y cosmopolita n ancestra l biota s ha s bee n a majo r mechanis m producin g th e his - torica l pattern s w e observ e today .

Biotic

dispersion is historicallyconstrained A metho d t o investigat e whethe r bioti

cdispersion is historically constrained is dis-cussed above and summarized in Figure 3.When this method is applied to a real-world example, namely the avifauna of

Aus - tralia , i t ca n b e conclude d tha t historica

lpatterns of biotic dispersion are indeedstrongly influenced by the same history thatproduced the areas of endemism. Figure 5shows a map for 14 areas of endemism asdetermined for birds, as well as the mostparsimonious tree for raw distributions. Thelatter was generated by coding for the pres-ence or absence of all avian species- andsubspecies-level taxa in the sample dis-cussed in Cracraft

(1991) . In contrast to thatearlier analysis, the tree of Figure

5 does notinclude codings for genera, thus only wide-spread distributions at these lower taxo-nomic-levels are used in determining thearea-relationships. Nevertheless, both anal-yses find the same tree.

Th e questio n no w arise s a s t o ho w closel

ythis result matches area-cladograms basedon vicariance analysis. That is, to whatextent does the pattern of Figure

5 representan accurate estimate of the "true" area-rela-tionships rather than a spurious pattern thatreflects "false" relationships due to uncon-strained dispersion? Although we by nomeans have adequate phylogenetic data toconstruct a well corroborated area-clado-gram, available data exhibit moderatelystrong congruence to the tree of Figure 5(see results in Cracraft, 1986). Figure 6, forexample, shows phylogenetic hypotheses forfour clades having endemics in some of theareas of endemism in eastern Australia.Taking into account taxa missing from cer-tain areas in Figure 5 (especially NewGuinea; NG), and the ambiguities intro-duced by widespread taxa (see Nelson andPlatnick, 1981; Humphries and Parenti,

1986)

, a common pattern can be suggested(Fig. 6E) that unites the Eastern QueenslandDownloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

EVOLUTION OF BIOTAS43

(EQ ) an d Atherto n Platea u (ATH ) area s o

fendemism, which together are related to theSoutheastern Forest area (SE), and finallythose three are related to the Cape YorkPeninsula area (CY). This is precisely thepattern seen in the tree based on raw dis-tributions (Fig.

5) . Th e dat a use d i n thi s analysi s offe r addi

-tional evidence suggesting strong historicalconstraint (Cracraft, 1991). The 204 sub-species-taxa of birds were present, on aver-

age

, in only two areas of endemism, andfully 50.5% were present in only one area.Nearly 87% were endemic to three or fewer

areas

. The pattern for species-level taxa isthe same. For 300 species-level taxa, 23.0%are endemic in one area, 41.7% are endemicin two or fewer, and 70.0% are endemic infour or fewer

areas

. Even "widespread" taxaare therefore relatively narrowly endemic.Again, the simplest hypothesis to accountfor this observation is that dispersion itselfis nonrandom and limited geographically.

SPECIE

S ASSEMBLAGES OF SMALL-SCALE:

TH

E IMPRINT OF HISTORY MAY

B

E PERVASIVE

Alternative

conceptualizations of species assembly at small-scale A majo r are a fo r researc h withi n ecolog y ha s bee n th e origi n an d developmen t o f spe - cie s assemblage s a t smal l spatiotempora l scales , suc h a s thos e o f ecosystem s o r loca l communitie s {e.g., see Cody and Diamond, 1975
; Strong et ai, 1984; Diamond and Case , 1986
; an d Wiens , 1989
, fo r genera l summarie s o f th e literature) . Systematist s hav e scarcel y pai d attentio n t o th e questio n o f communit y o r ecosyste m assembly , pri - maril y becaus e th e domai n o f thei r inter - ests - species , speciation , an d historica l bio - geography - i s no t considere d t o b e applicabl e a t thi s smal l scale . Thi s sectio n wil l sugges t tha t systemati c dat a an d inter - pretation s d o hav e a contributio n t o mak e t o ou r understandin g o f ho w thes e small - scal e specie s assemblage s evolved . Ho w i s th e taxonomi c compositio n o f a n assemblag e acquire d ove r time ? Fo r a n ecol - ogist , th e empirica l observation s consis t o f comparison s acros s variou s assemblage s wit h respec t t o thei r present-da y taxonomi c

A. Tregellasia

- nan a AT H capit a E Q alblgularf s C Y leucop s N G B. Sericornis - c vlrldlo r AT H magnifostri s EQ.S E becail l CY.N G nouhuys l N G loevlgaste r E Q front * S E humill s TA S ker i AT H arfakensl s N G perspeclllatu s N G C. Psophodes - laterali s AT H D. Ptiloris vlctorla e AT H paradlseu s E Q magnlficu s N G

Interceden

s N G albert i C

Yollvaceus EQ.SEnigrogularis SW

leucogaste r ADEL.6 P E . Genera l area-cladogra m NE W GUINE

A (NG)

CAP E YOR K PENINSUL

A (CY)

ATHERTO

N PLATEA

U (ATH)

EASTER

N QUEENSLAN

D (EQ)

SOUTHEASTER

N FORES

T (SE)

FIG

. 6. Phylogenetic hypotheses for four genera ofAustralian birds having endemics in eastern Australiaareas of endemism. A, C, and D from Cracraft (1986);

B

, from Christidis el ai (1988). E, a general area-clado-gram for five areas of endemism inferred from thesephylogenetic hypothesis (see Cracraft, 1986). Abbre-viations for areas of endemism: ATH, Atherton Pla-teau; NG, New Guinea; EQ, Eastern Queensland; CY,Cape York Peninsula; SE, Southeastern Forest; TAS,Tasmania; ADEL, Adelaide; EP, Eyre Peninsula; andSW, Southwestern Forest. See text.

compositio n an d ma y includ e som e knowl - edg e o f th e bioti c interaction s amon g pop - ulation s o f thos e specie s (onl y rarel y i s informatio n abou t th e actua l histor y o f spe - cie s assembl y available). Thes e pattern s ar e the n mos t ofte n explaine d b y model s tha t assum e specie s assemblage s ar e structure d b y colonization . Tha t is , th e observe d tax - onomi c (an d ecological ) compositio n o f th e loca l communit y i s th e resul t o f a series of colonization s o f founder s draw n fro m a poo l o f specie s locate d outsid e th e are a i n ques - tio n (se e reference s cite d earlier) , wit h th e succes s o f th e colonize r bein g largel y deter - mine d b y bioti c interaction s wit h residen t species . A n alternativ e explanatio n fo r th e assem - bl y o f a loca l communit y ca n b e expresse dDownloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

44JOEL CRACRAFT

entirel y withi n a historica l framewor k an

dsees observed taxonomic composition as acontingency of the history of the larger regionwithin which the local community resides.

Thus

, species within an assemblage eitherdifferentiated within the region followingvicariance or they were added to the as-semblage following hybridization of twopreviously separated biotas. In this view,long-distance invaders from an externalspecies-pool play only a marginal role in thespecies composition of local communities.Testing the hypothesis that communitiesare colonization-structured is difficultbecause observations on present commu-nities can at best only elucidate the ecolog-ical mechanisms that might be maintainingongoing community structure (Drake,1990a, b, 1991). The historical alternative,on the other hand, can be evaluated to asignificant degree by the methods outlinedin the preceding sections.

Historical

structure of assemblages at small-scale: an example T o wha t exten t d o assemblage s a t smal

lspatial scales - say, within ecosystems orcommunities - exhibit significant historicalstructure? That is, to what extent can thepresence of those taxa be explained merelyas a consequence of their phylogenetic andbiogeographic histories? A preliminaryanswer to these questions can be sought byway of an example.

Th e exampl e chose n i s th e rainfores t bir

dcommunity of Lamington National Park insouthern Queensland, Australia. The claimcannot be made that birds represent a com-munity in the strict sense, but the notion ofa taxonomic "community" has been usedwidely in ecology. This assemblage is suit-able for present purposes because it consistsof a relatively small number of species, thereare preliminary data on the phylogeneticrelationships of some of the component spe-

cies

, and there exists a basic understandingof the historical biogeography of easternAustralia (Cracraft, 1986, 1991). Still, ourknowledge of relationships is incomplete formany of the species and their close relatives,thus the conclusions have to remain ten-tative.Lamington National Park is situated in

th e Easter n Queenslan d are a o f endemis m(Cracraft, 1991

; Fig. 5). Some species foundin this area have their closest relatives inareas to the north, including the AthertonPlateau, Cape York, and areas to the west(Kimberley Plateau, Arnhem Land), as wellas in New Guinea; others have relatives dis-tributed to the south, in the SoutheasternForest, Tasmania, and the SouthwesternForest areas of endemism (Cracraft, 1986).The Eastern Queensland area of endemismis therefore a "hybrid" region where biotashave intermingled, and it may eventuallybe shown to be a composite of two or moresmaller areas of endemism.

Th e avifaun a o f Lamingto n Par k consist

sof approximately 45 species that are hereclassified as being a part of the "rainforestbird community" (Appendix 1), althoughsome species certainly range into less moisthabitats. For the purposes of this example,however, it is not critical that habitat spec-ificities might be discriminated somewhatbroadly, nor is it consequential if some spe-cies that are sometimes found in rainforestare omitted.

O f thes e 4 5 species , abou t 1 7 (38% ) ar

epostulated to have their phylogenetic andbiogeographic relationships to taxa in morenorthern areas of endemism, whereas a nearequal number (15 species, 33%) have affin-ities to more southern areas. Nine species(20%) are broadly distributed but restrictedto eastern Australia, and therefore haveuncertain relationships, and only four spe-cies (slightly less than 10%) are classified asbeing widespread. Of the 45 species, five areconsidered to be endemic to the region, withthree of these being northern and two south-ern in their relationships.

Significantly

, non e o f th e specie s appear

sto be an outlier, or show evidence of beingthe result of long-distance dispersal fromsome distant source area. The large majorityhave species-level close relatives either inareas to the north or to the south, and thosearea-relationships are not unexpected givenwhat we can infer from phylogenetic anal-yses of species groups that have taxa in thisassemblage (Cracraft, 1986). For example,phylogenetic hypotheses for four cladesDownloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

EVOLUTION OF BIOTAS45

havin g representative s i n thi s communit

yare shown in Figure 6. The Eastern Queens-land species have their closest relative eitherto the north in the Atherton Plateau area ofendemism (in

Tregellasia, Ptiloris) or to thesouth in the Southeastern Forest area (in

Sericornis). Some of the species in EasternQueensland are also distributed in theSoutheastern Forest. A general hypothesisthat seems to summarize the area-relation-ships is shown in Figure 6E. It should benoted that even this hypothesis may be toosimple, as a number of groups (includingsome not shown in Fig. 6) have species thatare distributed in other areas as well. Thus,some clades like Psophodes (Fig. 6C) haverepresentatives in southern areas, whereasothers have representatives in northern areassuch as Arnhem Land and the KimberleyPlateau (Cracraft, 1986).

Th e importan t conclusio n suggeste d b

ythese data is that the species in the Lam-ington rainforest community have been his-torically tied to one another for quite some

time

. At the spatiotemporal scale of thisanalysis, we can see no evidence for assem-bly of the community by long-distance dis-persal from an outside species pool.Repeated cycles of biotic dispersion, vicar-iance, and differentiation are necessary andsufficient to explain the assembly of thisensemble of species.

CONCLUSION

S I f ther e ar e an y ne w insight s int o bioti

cevolution contained in this paper they are,first, that we can expect biotic dispersionitself to leave a significant historical imprinton the evolution of species assemblages.Coupled with subsequent vicariance, thesetwo processes will result in biotas that arehighly structured historically. Second, evena biotic assemblage of small spatiotemporalscale is composed of species whose presencereflects the spatial history of the area ofendemism within which the assemblage isembedded. Thus, the taxonomic composi-tion of small-scale species assemblages arephylogenetically nonrandom. In the case ofthe Australian rainforest community dis-cussed above, vicariance and biotic disper-sion are necessary, and apparently suffi-cient, to explain the ensemble of species inthat community. If ecological models of

assembl y ar e operable , the y appea r t o b

econfined to much smaller spatiotemporalscales and perhaps are applicable to explain-ing relative abundances and other aspectsof community structure that have influenceon extinction processes.

Curren

t theor y abou t specie s assembl

ywithin community ecology seems to havebeen developed with island biotas in mind,starting with island biogeographic theory(MacArthur and Wilson, 1963, 1967) andcontinuing to the present

{e.g., see earliercitations). Much less attention has been paidto the evolution of continental biotas. Thisemphasis on island biotas may have biasedcurrent theory in favor of long-distance dis-persal as the most important process incommunity assembly. The results of thispaper, in contrast, imply that communitieswithin continental biotas may be taxonom-ically structured by very different processes.In fact, this conclusion may actually be moregeneral, for as island biotas are studied inmore detail, their species composition

i

s alsoseen to have been strongly influenced byprocesses other than long-distance dispersal(Schuh and Stonedahl, 1986; Cracraft,

1988*

; Andersen, 1991; Muona, 1991, andreferences therein). Thus, the application ofsystematics to the problem of speciesassembly, across a breadth of spatiotem-poral scales, is indicating that the world'sbiotic assemblages are far more historicallystructured than is sometimes currentlythought.

ACKNOWLEDGMENT

S I a m gratefu l t o Drs . Georg e Barrow - clough , Jame s Carpenter , Harr y Greene , an d Norma n Platnic k fo r givin g m e th e benefi t o f thei r wisdo m an d improvin g th e manu - scrip t significantly . I als o than k Dr . M .

Patrici

a Mors e an d th e America n Societ y o f

Zoologist

s fo r invitin g m e t o participat e i n th e SAAWO K biodiversit y symposium . Thi s pape r ha s bee n gestatin g fo r severa l year s an d researc h suppor t contributin g t o i t ha s bee n provide d b y grant s fro m th e

Nationa

l Scienc e Foundatio n (BSR -

8805957

, BSR-9007652 , DEB-9119539) .Downloaded from https://academic.oup.com/icb/article/34/1/33/111725 by guest on 25 August 2023

46JOEL CRACRAFT

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APPENDI

X 1 Species in rainforest bird community ofLamington National Park, Queensland, Austra-lia.'"

Pigeon

s (Columbidae) : Ptilonopus superbus (W), P. regina (N), P. magnificus (N), Columba leucomela (EA)

, Lopholaimus antarcticus (EA), Macropygiaamboiensis (EA), Chalcophaps indica (N), Leuco-sarcia melanoleuca (S?); parrots (Psittacidae):Calyptorhynchus funereus (S), Cacatua

galerita (W),

Alisterus

scapularis (EA), Platycercus elegans (S);cuckoos (Cuculidae): Cuculus pyrrhophanus (N), Chrysococcyx lucidus (S); pittas (Pittidae): Pitta ver- sicolor (N); lyrebirds (Menuridae): Menura alberti (E,S)

, Atrichornis rufescens (E,S); trillers (Campe-phagidae): Lalage leucomela (N); flycatchers("Muscicapidae"):

Petroica rosea (S), Eopsaltria australis (S), Tregellasia capito (N), Falcunculusfrontatus (W) , Pachycephala olivacea (S), P. pectoral- is (S) , Monarcha melanopsis (EA) , M. trivirgatus (N?) , M. leucotis (N), Rhipidura rufifrons (N), R.fu/iginosa (W); logrunners, whipbirds (Orthonychi- dae) : Orthonyx temminckii (N), Psophodes oliva-

ceus (S); scrubwrens, thornbills (Acanthizidae): Seri-cornis magnirostris (N), S. frontalis (S), Acanthizapusilla (S); treecreepers (Climacteridae): Climacte-rus

leucophaea (S); honeyeaters (Meliphagidae):Manohna melanophrys (S?), Meliphaga lewinii (EA) , Acanthorhynchus tenuirostris (EA), Myzomela sanguinolenta (EA); pardalotes (Dicaeidae): Pardalo- tus punctatus (N?) ; grass-finche s (Ploceidae) :Emblema temporalis (N?); bowerbirds (Ptilonorhyn-chidae): Ptilonorhynchus violaceus (EA), Sericulus chrysocephalus (E,N), Ailuroedus crassirostris (E,N);riflebirds (Paradisaeidae): Ptiloris paradiseus (E,N). " Base d o n unpublishe d specie s list s fo r Lamingto

nNational Park (Green Mountains Natural HistoryAssociation) and personal observations. This listexcludes raptors, migrants, and introduced species.

b Abbreviations : E , endemi c t o th e Easter n Queens

-land area of endemism; N, the Eastern Queenslandspecies or subspecies is hypothesized to show vicariantrelationships to taxa in northern areas of endemism;S, the Eastern Queensland species or subspecies ishypothesized to show vicariant relationships to taxain southern areas of endemism; EA, the species

i

s foundin eastern Australia from the Cape York and/or Ath-erton areas of endemism south to the SoutheasternForest and/or Tasmania areas of endemism, and rela-tionships are considered uncertain; W, widespreadacross much of Australia, and relationships are con-sidered uncertain.Downloaded from https://academ

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