geographic distributions of homosporous ferns: does dispersal obscure evidence of vicariance?

Geographic distributions of homosporous ferns:does dispersal obscure evidence of vicariance?P. G. Wolf1, H. Schneider2 and T. A. Ranker3* 1Department of Biology, Utah State

University, Logan, UT 84322 USA, E-mail: [email protected]; 2Department of Botany,

The Field Museum, 1400 S. Lake Shore Drive, Chicago, Illinois 60605±2496, USA, E-mail:

[email protected]; 3University of Colorado Museum and Department of EPO

Biology, Campus Box 350, Boulder, CO 80309, USA, E-mail: [email protected]

Abstract

1 The central problem in biogeography is that interactions between different processesresult in the formation of historical patterns, such that it is dif®cult to discriminate therelative roles of vicariance and dispersal. Ferns are distributed by small wind-dispersedpropagules that are produced in very large numbers and capable of dispersing thousandsof kilometers. Thus, most taxon distributions in ferns are assumed to be a function ofdispersal rather than vicariance. Here, we review some case examples that provide goodevidence for vicariance and dispersal in ferns. We then ask whether dispersal is soextensive in ferns that vicariance is no longer detectable in most cases. Although wethink that too few studies have been carried out to make generalizations at this stage, weoutline the criteria for an effective research programme that can address this issue.Phylogenetic and distributional data are needed, not only because they are lacking in anevolutionarily important group of organisms, but also because data from ferns and othercryptogams are likely to be crucial in making broad biogeographic statements.

Keywords

Long-distance dispersal, reproductive biology, phylogeny, spore dispersal, pterido-phytes, biogeography, vicariance.

INTRODUCTION

Taxon distributions are produced by several biological andgeological processes. In relatively simple cases, it may bepossible to infer a single process, such as dispersal to anoceanic island or disruption of a former range by vicariance.However, in most cases it is dif®cult to assess the role ofvicariance because patterns have been formed by theinteraction of dispersal, extinction, and vicariance. Herewe explore the origins of geographical ranges of homospor-ous fern taxa. Distributions of ferns on islands suggest thatferns can indeed disperse thousands of kilometers (Tryon,1970). Thus, it is often assumed that taxon ranges in fernsare a function of dispersal rather than vicariance (Tryon,1985). However, recent dispersal (which itself cannot bedoubted) need not completely obscure ancient vicariancepatterns. Combining information from phylogeny, distri-

bution, and the fossil record, we ask whether it is possible todiscern vicariance patterns in the face of long-distancedispersal in ferns. We then examine distributions of fernspecies and higher taxa, in the light of phylogeneticinformation, using several case examples to test vicariance.We ®nish with a prospectus for future work in this area.

There are about 11,000 species of homosporous, leptosp-orangiate ferns (Polypodiidae), in about 30 families (Kramer,1993). Sexual reproduction in these plants differs fromheterosporous seed plants, which have diploid propagules.Homosporous ferns produce a single type of haploid sporethat is dispersed by wind and capable of traveling longdistances. The spores germinate to produce the haploidgametophytes which develop the reproductive structures.Motile sperm, that require an external source of water,fertilize an egg cell, after which the more familiar, and larger,diploid sporophyte grows directly out of the gametophyte. Inthe majority of leptosporangiate ferns, the sporophyte is long-lived (several years) in comparison with the more ephemeralgametophytes. Despite the requirement for seasonal sourcesof water, homosporous ferns have exploited a wide range ofhabitats, tropical and temperate, desert to rainforest. Many

*Correspondence: University of Colorado Museum and Department of EPO

Biology, Campus Box 350, Boulder, CO 80309, USA. E-mail: tom.ranker@

Colorado.edu

Journal of Biogeography, 28, 263±270

Ó 2001 Blackwell Science Ltd

fern taxa are old; representatives of c. 7±12 extant familiescan be found in deposits from Permian to Jurassic (Collinson,1996). Fossil evidence from several taxa such as Matoniaceae(Tidwell & Ash, 1994) shows evidence of former globaldistributions prior to breakup of the continents.

Attempts to describe and explain the global distribution offern taxa are not new (Lyell, 1870; Winkler, 1938). Christ(1910) was one of the ®rst to discuss the distribution of ferngenera, but his inferences were constrained both by a lack ofdata on a global scale and poor knowledge of fern phylogeny(Smith, 1972). Although the situation is improving, bothproblems still exist today because accurate distributionaldata for species are often collected only at local or regionallevels, whereas many fern taxa, even species (such asPteridium aquilinum and Pityrogramma calomelanos), arefound on several continents. Also, although our knowledgeof fern phylogeny is improving rapidly (Hasebe et al., 1995;Pryer et al., 1995), genus-and species-level studies are few.Early studies of fern distribution focused on ecologicalexplanations for species presence, especially rainfall (e.g.Christ, 1910). Later studies on the subject began to focus ondispersal in ferns. Many fern species have large global-scaledistributions with wide disjunctions. Several authors haveattributed these disjunctions to long-distance dispersal(Tryon, 1970; Smith, 1972; Tryon, 1985). However, thismay be a result of emphases on the distribution of ferns onoceanic islands, especially in comparison to angiosperms.Nevertheless, Smith (1972) also examined continental dis-tributions and inferred a stronger effect of long-distancedispersal in ferns relative to angiosperms (Wagner, 1972).Starting in the 1980s, pteridologists followed the generalshift by evolutionary biologists from an emphasis ondispersal to a consideration also of vicariance to explaindisjunct distributions. This is exempli®ed by a series ofpapers published in the Journal of Biogeography in 1993(Barrington & Kato, 1993; Kato, 1993; Smith, 1993). Forexample, Kato (1993) argued that both dispersal andvicariance were required to explain the distribution ofDrynarioideae and Pyrrosia Mirb., whereas the distributionof the Old World genus Taenitis Willd. ex Spreng. could beexplained primarily by dispersal. Kato (1993) also erected aseries of alternative hypotheses for which phylogenetic datawere needed to distinguish competing hypotheses of vicari-ance vs. dispersal in other taxa. We shall return to some ofKato's examples to examine progress in this ®eld of research.

The most recent consideration of factors responsible forfern distributions occurred at the 1999 International Botan-ical Congress. In a symposium organized by R. C. Moranand B. éllgaard, the general theme appeared to havereverted to an emphasis on dispersal (Dassler & Farrar,1999; Gradstein & van Zanten, 1999), even for explainingthe occurrence of species and species pairs with an Africa/South America disjunction (Moran & Smith, 1999).Regardless of the pervasive philosophical in¯uence, we thinkthat attempts to explain the origin of fern distributions areuseful, both for understanding the biology of the organisms,but also as a contribution to the broader ®eld of biogeog-raphy. It may not be possible to make useful generalizations

about such a large group of plants. However, a case-by-caseanalysis might provide the most practical insight into fernbiogeography.

To understand the origins of taxon distributions, themechanisms for changes in ranges must be examined. We®rst deal with range expansion via dispersal. Successfulrange expansion requires success at a series of stages:production of large numbers of propagules, actual transportover long distances, viability of propagules after dispersal,landing in suitable sites, establishment of a reproductiveindividual, and establishment of a stable population. Anyone of these stages can restrict range expansion. Homos-porous ferns are dispersed primarily by haploid spores thatare released from the leaves of the sporophyte. Fern sporesare small (30±100 lm) (Tryon & Lugardon, 1990) andproduced in large numbers. Conant (1976) estimated that asingle individual of Cyathea arborea (L.) Sm. can release3 ´ 1011 spores in a season. Peck et al. (1990) surveyed 14species from Iowa, USA, and found that spore productionper plant ranged from 5.4 ´ 104 to 3.3 ´ 108, with mean of6.0 ´ 107 (SE 2.4 ´ 107). Furthermore, most ferns are longlived perennials and produce multiple fertile leaves per year.However, most propagules (spores and seeds) land close tothe parent plant (leptokurtic dispersal; Conant, 19762 ; Pecket al., 1990) so that the actual proportion of propagulesdispersed outside the immediate neighbourhood of theparent will be low. Mechanisms for spore release varyamong fern groups, but in terms of species numbers, mostare leptosporangiate ferns in which a drying annuluscatapults the mature spores from the sporangium. Sporerelease usually requires dry conditions but the spores mustthen catch passing air¯ows to be transported away from theparent (Cionco, 1965). This is probably easier for tallerferns growing in more open habitats or epiphytic speciesthan it is for low-growing, terrestrial species under a forestcanopy (Raynor et al., 1976). Extreme long-distance dis-persal, for example over 1000 km, requires access for thespores to higher atmospheric winds (Puentha, 1991).Indeed, fern spores have been detected at high altitudes,including the jet stream (Erdtman, 1937; Aylor & Ferran-dino, 1985). But how viable are fern spores after such longperiods of exposure to high levels of UV and other extremeconditions? This question was addressed recently by Grad-stein & van Zanten (1999) who attached spores of severalfern species to the wings of passenger aircraft travellingabove 10,000 m. Survival of some widespread and alsoalpine species was detected and after transatlantic exposurethe spores were able to germinate and develop apparentlynormal gametophytes. However, most species appeared tobe very sensitive to high UV-light levels in the jet stream, andthe authors suggest that lower level transport would be moresuccessful for most species. Most fern species have non-greenspores with viabilities of several years. This is particularlyuseful for long-distance dispersal because it means thatthese species are not constrained to rapid acquisition of asuitable site. However, some ferns, for example the Gram-mitidaceae, have green spores with short-term viability,often only a few days (Lloyd & Klekowski, 1970).

Ó Blackwell Science Ltd 2001, Journal of Biogeography, 28, 263±270

264 P. G. Wolf, H. Schneider and T. A. Ranker

The next critical stage in long-distance dispersal is for thespore to land in a suitable environment for germination. Thisis obviously another random process and most spores thatdisperse probably fail to reach a suitable place. Cousenset al. (1988) discussed these safe sites and noted that speciesvary considerably in their requirements for germination andgametophyte survival. Dyer & Lindsay (1992) examined therole of spore banks in the soil. Their ®ndings suggest amechanism whereby long-term survival in the soil can bufferagainst temporal variation for suitable germination condi-tions.

Once a spore has germinated and a mature gametophytehas developed, sperm cells must meet egg cells for theprocess to continue, at least in sexual species (probably mostfern species). Baker (1955) and Carlquist (1966) discussedthe problem of mating after long-distance dispersal for seedplants. However, the situation is somewhat more problem-atic for homosporous ferns that are dispersed by haploidpropagules. A single isolated spore gives rise to a gameto-phyte that can, in many species, produce both sperm cellsand egg cells. Because they are genetically identical (i.e.sperm and egg cell nuclei result from mitosis in gametophyticcells) a sporophyte resulting from intragametophytic sel®ngwill be homozygous at all loci (Klekowski, 1969). This is farmore extreme than sel®ng of a diploid sporophyte of a seedplant. Thus, only habitually sel®ng species (perhaps colon-izers), that have lost their genetic load, will be capable ofsuccessful intragametophytic sel®ng. Species that normallyoutcross are unlikely to survive the expression of manyrecessive deleterious alleles, so that successful colonizationmay require the involvement of more than one gametophyte(Crist & Farrar, 1983). Mating can be further constrained inferns by the release of the pheromone antheridiogen thataffects sexual expression (Schneller et al., 1990). Thisfurther reduces the overall probability of successful long-distance dispersal. The ®nal hurdle requires establishment ofa new population. Thus, environmental conditions must besuitable not only for gametophytes and mating, but also forsporophytes to grow and produce spores. Our take-homemessage is that although most sexual ferns produce abun-dant spores capable of travelling long distances, the actualchances of establishing new populations are low. Overevolutionary time, ferns on the whole may indeed be morecapable of dispersal than seed plants. However, like seedplants, fern species vary in their ecological and reproductivecharacteristics so that the ability to succeed at all stages ofdispersal may be limited to a relatively small proportion ofspecies.

What evidence exists for successful long-distance disper-sal? The information takes several forms: direct observationof successful dispersal, evidence from distributions onislands, evidence from distributions on continents andphylogenetic evidence combined with distributional data.These types of data deal with different aspects of the processand we will discuss each in turn.

Numerous reports exist that describe the occasionaldiscovery of ferns outside their normal range (Smith, 1972,1993). Sometimes these result in eventual range expansion.

For example, Thelypteris opulenta (Kaulf.) Fosberg hasbecome naturalized in the New World this century (Smith,1971). Often, however, despite long-distance dispersal, theimmigrant species fails the ®nal stage of establishment. Thishas been documented for Dicranopteris ¯exuosa (Schrad.)Underw. in Florida (Novak & Cousens, 1985), whicharrived on the coast of Florida, probably following dispersalfrom the Greater Antilles by hurricane storms. Isolatedpopulations became sexual, producing male, female andbisexual gametophytes. Young sporophytes were also repor-ted from the ®eld (Novak & Cousens, 1985) but thepopulation failed to establish, probably because of killingfrosts. This highlights the importance of all conditions beingnecessary for successful establishment following long-distance dispersal.

Several authors have discussed the high proportion ofpteridophytes on oceanic islands (Tryon, 1970; Smith, 1972;Wagner, 1972). Smith (1972) compiled data from severalislands and showed that ferns have much lower levels ofendemism than angiosperms at both the genus and specieslevels. For example, on Hawaii, about 16% of angiospermgenera are endemic vs. 6.7% of fern genera. The interpret-ation for this is two-fold: ®rst, ferns are more capable oflong-distance dispersal and establishment, second, continuedgene ¯ow from mainland sources may slow speciation ratesfor ferns, thereby constraining the evolution of endemic taxa(Ranker et al., 1994).

Smith (1972) produced one of the more recent compila-tions for continental distributions at the generic and familiallevels. He found that at the generic levels ferns were morewidely distributed than angiosperms. For example, 3% ofangiosperm genera vs. about 20% of fern genera havedistributions that are cosmopolitan, subcosmopolitan, orpantropical. Concomitantly, Smith (1972) estimated that80% of angiosperm genera are endemic, vs. 34% for ferns.Although there are numerous ways to de®ne endemism, thevalues are still useful for comparative purposes. The patternsat the generic level generally extrapolated to families also.Smith (1972) proposed several explanations for the patternsof genera and families. One hypothesis is essentially avicariance argument: fern taxa are older, some of thempredating the major continental drift events. This is certainlythe case for the older (`basal') fern families such asDicksoniaceeae, Gleicheniaceae, Hymenophyllaceae,Osmundaceae and Schizaeaceae, that have fossils dating atleast to the Triassic but several modern fern families mayre¯ect Cretaceous and Quaternary changes of the position ofthe continents such as the breakup of the Gondwanacontinent. However, most `modern' fern families may havearisen at the same time as many angiosperm families. Smith(1972) also argued that ferns may evolve slower thanangiosperms; speciation rates may be different as a functionof differences in reproductive biology and ecology. The thirdexplanation is that ferns disperse more readily than angio-sperms. This would apply especially to the `modern' groupswhere ancient vicariance is dif®cult to justify. Kramer (1993)used an updated data set, ®nding similar patterns to Smith(1972), and provided an additional explanation: that fern


Dispersal and vicariance in ferns 265

genera and families are not equivalent to angiosperm taxa ofequal rank. However, the difference between this and anevolutionary rate argument appears to be little more thansemantics.

Clearly, if we are to distinguish between dispersal andvicariance to explain distributions then we need phyloge-netic information. To this end, Ranker et al. (19993 ) presen-ted a molecular phylogenetic analysis of two closely relatedmodern fern families Polypodiaceae and Grammitidaceae.From a consensus cladogram, they produced an areacladogram from which they inferred historical biogeo-graphical processes. Their results (Fig. 1) suggested that amonophyletic Grammitidaceae evolved from within a para-phyletic Polypodiaceae and that the entire group probablyhad a palaeotropical origin. Based on the geographicaldistributions of constituent clades, Ranker et al. (1999)inferred that Grammitidaceae are sister to neotropicalPolypodiaceae and that there was a single ancient migration

to Hawaii and a second more recent migration. Further-more, a more recent migration occurred back to thepalaeotropics. Thus, phylogenetic data can be used to inferancient dispersal events. The deeper split of the Polypodi-aceae into palaeotropical and neotropical clades may beexplained as an older dispersal event from the palaeotropicsto the neotropics, but an alternative explanation of contin-ental drift disrupting the distribution area can not bedismissed.

Some taxon distributions may imply ancient vicariance,others are most easily interpreted as dispersal. More often,however, it is not possible to chose between alternativehistories without some form of biogeographical analysis.This is not the place for a rigorous review of methods as thesubject is covered by several books (Nelson & Platnick,1981; Humphries & Parenti, 1999) and is the central focusof at least one journal. However, a brief (and probablybiased) summary may be necessary before we can addressthe question we posed at the start. Morrone & Carpenter(1994) stated that a general goal in analytical biogeographyis to use taxon cladograms to infer biogeographic patterns.This involves a stepwise procedure of converting taxoncladograms to taxon-area cladograms, which are then usedto generate resolved area cladograms (RACs) from whichgeneral area cladograms can be inferred if multiple inde-pendent taxon groups are examined. If each taxon isendemic to a unique area and every area has only onetaxon, then the entire procedure is (relatively) trivial.Complications for generating RACs arise when an area isnot represented by any taxon (in a speci®ed group) orcontains multiple taxa (redundant distribution). In thesecases a series of assumptions come into play (Nelson &Platnick, 1981) that incorporate mechanisms other thanvicariance. Several alternative procedures can be used toinfer the general area cladograms (e.g. Page, 1988; Bremer,1995). If the data are straightforward, as described above,then component analysis (Page 1988) and various parsimonyapproaches give the same result. However, the differentapproaches have been shown to give different resultsdepending on the implementation of assumptions (Morrone& Carpenter, 1994). A more troubling philosophical issue isthe general application of cladistic notation to the descrip-tion of `Earth History' phenomena (Sober, 1988; Hovenk-amp, 1997; Ronquist, 1997). Hovenkamp (1997) arguedconvincingly that area cladograms are not appropriate fordepicting geological events, and that vicariance events ratherthan areas should be examined. This approach couldprobably be used both to search for biogeographic patternsin taxon cladograms, as well as test to see if groups ofunrelated taxa conform to vicariance or dispersal patterns.Several studies have used data from multiple groups toexamine this issue from both sides. For example, Enghoff(1995) incorporated presumed geographical patterns (landconnection changes over time) to infer general holarcticbiogeography based on published phylogenetic data from 73animal taxa. Although distinct patterns can be observed insuch analyses, an emerging problem is that taxon groupswith the same area cladograms may still have had very

Figure 1 Phylogenetic hypothesis derived from a consensus clado-gram of 248 equally parsimonious trees based on sequence variationof a 1350 base-pair segment of the rbcL gene of 117 species.Outgroups were Davallia (Davalliaceae) and Oleandra (Oleandra-ceae). Abbreviations: Polypod � Polypodiaceae; Grammi-tid � Grammitidaceae; NT � neotropics; PT � palaeotropics;HI � Hawaii; N. Temp. � north temperate. Major inferred disper-sal events are indicated on cladogram. (Modi®ed from Ranker et al.,1999.)



different biogeographic histories (Enghoff, 1995; Xianget al., 1998). Ronquist (1997) recognized this problem anddeveloped a method, dispersal-vicariance analysis, thatalllows for multiple reticulate relationships among areas. Arelated issue is that dispersal is not the only mechanism thatcan obscure vicariance: lineage sorting and certain patternsof extinction can have the same effect. Furthermore,inferring taxon cladograms (from which all other inferencesare derived) is itself a shaky business at best. Thus, manyindependent studies may be needed before reliable patternsand generalizations emerge.

We now return to our original question of whetherrampant long-distance dispersal has obscured vicariancepatterns for fern taxa. We will use vicariance as a nullhypothesis (Ronquist, 1997), so that other non-dispersalevents will be included under the umbrella of `vicariance-obscuring' mechanisms. We hope it may be possible toidentify speci®c ecological and biological attributes thatmake a fern group more likely to have retained evidence forvicariance. If we can achieve this then we could moreeffectively use data from ferns to examine general biogeo-graphic principles. Thus, here we examine available evidencefor vicariance in fern taxa (in the form of case examples),then ®nish with recommendations for future research on fernbiogeography.

The best place to start is with the clear cases of vicariance.One of the best-documented examples involves species thatare found only as gametophyte populations in temperateregions with the closest sporophytic populations in tropicalareas (Farrar, 1967). Several taxa, notably Vittaria, occur asindependent populations, restricted to certain habitats (usu-ally in, or close to, caves). The simplest explanation is thatsuch habitats have acted as refugia during cooling periods inthe northern hemisphere (Farrar, 1967). Spores or gameto-phytes may have survived in the refugia while populationsthat retain sexual reproduction and sporophytes have beenforced south, with the intervening temperate areas acting asbarriers. The fact that so many examples have beendocumented, combined with current molecular and anatom-ical tools for identifying gametophytes (Farrar, 1978), makethese independent gametophyte populations very likelycandidates for vicariance. Although dispersal of gameto-phytes within the Appalachian region has been implicated(Farrar, 1990), non-vicariance arguments to explain thecurrent disjunction from tropical areas seem unlikely.

Vicariance is a more likely explanation for currentdisjunct distributions for taxa where former ranges wereextensive. Such evidence can come from two sources: currentwide distribution of an entire group of taxa, or fossilevidence; both situations are well represented in ferns. Oneof the most widely distributed plants is the bracken fern,Pteridium aquilinum (L.) Kuhn (Page, 1976). Whetherbracken is a single species (Tryon, 1941) or several (Mickel& Beitel, 1988) does not alter the global distribution of theentire group. With only a few exceptions, most Pteridiumtaxa (whether they be species or subspeci®c taxa) arerestricted to distinct regions and most regions of theworld are represented. A phylogenetic analysis of Pteridium

(Tan & Thomson 1990) yielded distinct geographical clades.The lowest branches separated a northern hemisphere clade,corresponding to a Laurasian distribution, from the Gondw-ana groups. Some of the tips of the overall tree were notwell-resolved by the restriction site data used, but ongoingstudies by Speer (2000), using rapidly evolving chloroplastgenes should help to resolve further the biogeographicpatterns of Pteridium.

Other good candidates for vicariance are the onocleoidferns. Kato (1993) discussed possible biogeographic scenar-ios but realized that a cladistic analysis was required todistinguish between alternatives. Recently, Gastony &Ungerer (1997) used rbcL nucleotide sequences to estimaterelationships. Their results support a vicariance scenario forthe disjunct distribution of Onoclea sensibilis L. in easternAsia and eastern North America. This is consistent with afossil record dating to the Palaeocene for Europe (Boureau,1970) and western North America (Rothwell & Stockey,1991). Another fern taxon with a circumboreal distributionis the Adiantum pedatum L. complex. Systematic studies ofthe group (Paris & Windham, 1988) support a vicariancehistory to explain the current disjunctions.

Biogeographic studies of ferns have also been conductedon tropical groups and here we will illustrate with twoexamples. The genus Pyrrosia Mirb. is distributed through-out south-east Asia and biogeographic analysis supports ahistory that involves vicariance, overlaid with extensivedispersal (Hovenkamp, 1986). Several distinct vicarianceevents are identi®ed and these generally conform to resultsfrom other groups of organisms (Balgooy et al., 1996;Hovenkamp, 1997). For example, concordance amongseveral Pyrrosia clades supports a basal split betweencontinental Asia and Malaysia, followed by a split betweeneast and west Malaysia and ®nally a split between cladesfrom the Philippines and New Guinea. Ongoing work bySchneider (in press), on the fern family Pteridaceae (Cheil-anthoideae and Taenitidoideae) supports many of thepatterns inferred from studies of angiosperms (e.g. vanSteenis, 1979; Balgooy et al., 1996; Ridder-Newman, 1996;van Welzen, 19974 ) and ferns such as Pyrrosia (Hovenkamp,1997). The Malesian distribution pattern of some genera,such as the mountain fern Coniogramme FeÂe, re¯ects mainlydispersal events, while others show vicariance patterns. Thedistribution of the genus Aspleniopsis Mett. ex Kuhn( � Austrogramme E. Fourn. subgen. Aspleniopsis (Mett.ex Kuhn) Hennipman) is explained as two vicariance events.The genus, consisting of three species, occurs from NewCaledonia through the New Hebrides and New Guinea tothe Moluccas (Ambon, Seram). One species (A. boerlageana(Alderw.) Pic. Serm.) is found only in Ambon and Seramandand it is a sister species of A. asplenioides (Holttum) Pic.Serm., which is endemic in New Guinea. The third speciesA. decipiens (Mett.) Kuhn occurs in the New Hebrides andNew Caledonia. As Kato (1993) suggested, the distributionof the genus Taenitis Willd. ex Spreng. re¯ects the migratingfrom East to West Malesia. However, it is also possible todetect several vicariance events at the species level, whichmay re¯ect the separation of forest refugees during the last



glacial period. Vicariance patterns are often obscured bydispersal but hybrid zones may indicate overlapping speciesareas. The genus Taenitis may be a good example to studythe dynamic interaction between dispersal and vicarianceevents, which may re¯ect geological events such as climatechanges and continental drift.

Several other good examples of vicariance in ferns exist,but too few to examine the issue more analytically at thisstage. Much of the problem stems from a shortage ofphylogenetic studies at the genus level and below in ferns.Although individual studies are very useful and revealmuch about the biogeography of the group in question, weneed to know at the larger scale whether ferns aredistributed according to pre-de®ned vicariance patternsmore than one would expect by chance alone. Fernbiologists studying vicariance should follow establishedprocedures (e.g. Enghoff, 1995). Regions and subregionsshould be clearly de®ned, as should vicariance events.Appropriate taxa should be chosen for investigatingspeci®c events and good distributional data should beavailable. At least three samples should be used from eachsubregion so that polymorphisms can be interpreted. Mostimportantly, studies should use phylogenetic analyses. Thisdoes not necessarily mean molecular data, but publisheddata should be freely available for further analyses, if onlyto ensure valid comparisons among taxa. Eventually, wewould like to test the null hypothesis that ferns aredistributed by vicariance patterns. Thus we should exam-ine the distribution of cladograms across taxa (andstudies) to see if they ®t vicariance patterns more than isexpected by chance. We hope that data will accumulate sothat we can test if retention of vicariance patterns isdistributed only among certain taxa with speci®c repro-ductive or ecological attributes.

The above prospectus is not a quick solution to theproblem. Recommended procedures will certainly change asanalytical aspects of biogeography evolve further. Some-times, research in a given area appears to lag behind in thecryptogams relative to other groups, such as angiosperms.Whether or not this is entirely true, we can use the problemto our advantage by pre-planning and directing researchpatterns to our bene®t. Ferns and other cryptogams havemuch to offer and it is important that data on theirbiogeography is incorporated into general statements aboutorganisms and earth history.

ACKNOWLEDGMENTS

P.G. Wolf thanks the Laboratoire de Biologie desPopulations d'altitude at UniversiteÂ Joseph Fourier(France), for hosting his sabbatical during which thispaper was written. H. Schneider's research on MalesianPteridaceae was supported by the European Commission(Grant Number ERBFMBICT960900) and the work com-pleted while at the Rijksherbarium, Leiden (The Nether-lands). Thanks to David Hibbett for a constructive reviewof the manuscript.

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BIOSKETCHES

Paul Wolf is interested in plant evolutionary biologyfrom the population scale to `deep' phylogeny ofvascular plants. His most recent focus has been onmolecular systematics of ferns.

Harald Schneider studied taxonomic and phylogeneticaspects of pteridoid ferns in Malesia as a postdoctoralresearch associate at the Rijksherbarium, Leiden (Neth-erlands). He received his PhD from the University ofZurich (Switzerland) and is currently a postdoctoralresearch associate at the Field Museum (Chicago, USA)and the University of California at Berkeley (USA). Hisresearch interests include evolutionary aspects of plantbiogeography in particular of Southeast Asian ferns.

Tom Ranker has been conducting evolutionary studies ofHawaiian ferns since 1988. His research interests includepopulation genetics, biology and evolution of islandferns, biogeography, and phylogenetic systematics.



geographic distributions of homosporous ferns: does dispersal obscure evidence of vicariance?

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