Mishler 1In R. Wilson (ed.), Species: New Interdisciplinary Essays, pp.307-315. MIT Press. 1999Getting Rid of Species?byBrent D. MishlerUniversity Herbarium, Jepson Herbarium, and Department of Integrative Biology, University ofCalifornia, Berkeley, CA 94720, U.S.A.Ab stract:This paper explores the implications of generalizing the species problem as a special caseof the taxon problem. Once a decision is made about what taxa in general are to represent, thenspecies in particular are simply the least inclusive taxon of that type. As I favor a Hennigianphylogenetic basis for taxonomy, I explore the application of phylogenetics to species taxa.Furthermore, I advocate a novel extension of the recent calls for rank-free phylogenetictaxonomy to the species level. In brief the argument is: (1) Species must be treated as just onetaxon among many; (2) All taxa should be monophyletic groups; (3) Because of problems withinstability and lack of comparability of ranks in the formal Linnaean system, we need to move toa rank-free formal classification system; (4) In such a system, not all hypothesized monophyleticgroups need be named, but those that are named formally should be given unranked (buthierarchically nested) uninomials; (4) The least inclusive taxon, formally known as "species,"should be treated in the same unranked manner. Finally, I explore the practical implications ofeliminating the rank of species for such areas as ecology, evolution, and conservation, andconclude that these purposes are better served by this move.The debate about species concepts over the last 20 years follows a curious pattern. Ratherthan moving towards some kind of consensus, as one might expect, the trend has been towards an ever-increasing proliferation of concepts. Starting with the widely accepted species concept that had takenover in the 1940's and 50's as a result of the Modern Synthesis, the Biological Species Concept, weheard calls for change from botanists, behaviorists, etc. Despite the babel of new concepts, the BSCcontinues to have fervent advocates (Avise and Ball, 1990; Avise and Wollenberg, 1997), and has itselfspawned several new variants. A recent paper by Mayden (1997) lists no fewer than 22 prevailingconcepts! It appears we can never eliminate any existing concept, only produce new ones.Why is this? The obvious conclusion one might draw, that biologists are contrarians who eachwant to make their own personal mark in a debate and thus coin their own personal concept to defend,is really not the case -- this is no debate about semantics. The conceptual divisions are major, and real.In my opinion the plethora of ways in which different workers want to use the species category reflectsan underlying plethora of valid ways of looking at biological diversity. The way forward is to recognizethis, and face the implications: the basis of this confusion over species concepts is a result of heroic butMishler 2doomed attempts to shoehorn all this variation into an outdated and misguided classification system, theranked Linnaean hierarchy. Most of the confusion can be removed simply by removing the ranks. Theissues that remain can then be dealt with by carefully considering what we want formal classification torepresent as the general reference system, and then carefully specifying criteria for grouping organismsinto these formal classifications.To develop this argument, I will first make the case for generalizing the species problem as aspecial case of the taxon problem. For a consistent general reference classification system, all taxa mustbe of the same type; species should be regarded as simply the least-inclusive taxon in the system. ThenI will review the reasons for why phylogeny provides the best basis for the general purposeclassification; species should thus be considered as just another phylogenetically-based taxon. Next, Iwill address the recent calls for rank-free classification in general, and pursue the central thesis of thispaper: the species rank must disappear along with all the other ranks. Finally, I will explore thepractical implications of eliminating the rank of species for such areas as ecology, evolution, andconservation.Species as just another taxonMany authors have made a firm distinction, in their particular theories of systematics, betweenspecies and higher taxa (e.g., Wiley, 1981; Nelson and Platnick, 1981; Nixon and Wheeler, 1990); seediscussion by De Quieroz, this volume). The idea is that somehow species are units directlyparticipating in the evolutionary process, while higher taxa are at most lineages resulting from pastevolutionary events. However nicely drawn this distinction is in theory, these arguments have resultedmore from wishful thinking than from empirical observations. When anyone has looked closely for anempirical criterion to uniquely and universally distinguish the species rank from all others, the attempt hasfailed.One early suggestion was phenetic: a species is a cluster of organisms in Euclidean spaceseparated from other such clusters by some distinct and comparable gap (e.g., Levin 1979). This hasbeen clearly shown to be mistaken -- phenotypic clusters are actually nested inside each other withcontinuously varying gap sizes. Current entities ranked as species are not comparable either in theamount of phenotypic space they occupy or the size of the "moat" around them, nor can they be madeto be comparable through any massive realignment of current usage. Another suggestion for a unique ranking criterion for species is expressed in the biologicalspecies concept: a species is a reproductive community separated by a major barrier to crossing withother such communities (Mayr, 1982). Like the phenetic gap, this view (nice in theory perhaps) failswhen looking at real organisms. Despite the publication of many conceptual diagrams that depict adistinct break between reticulating and divergent relationships at some level (Nixon and Wheeler, 1990;Roth, 1991; Graybeal, 1995) actual data suggests that in most groups the probability of intercrossabilitydecreases gradually as you compare more and more inclusive groups (Mishler and Donoghue 1982;Maddison 1997). There usually is not a distinct point at which the possibility of reticulation dropsprecipitously to zero.Mishler 3Similar suggestions have been made based on ecological criteria: a species is a group oforganisms occupying some specific and unitary ecological niche (Van Valen, 1976).
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