5 Cracraft, J. (1983) Species concepts andspeciation analysis. Curr. Ornithol. 1, 159–187

6 Ghiselin, M. (1974) A radical solution to thespecies problem. System. Zool. 23, 536–544

7 Hull, D. (1976) Are species really individuals?System. Zool. 25, 174–191

8 Sober, E. (1984) The Nature of Selection:Evolutionary Theory in Philosophical Focus,Massachusetts Institute of Technology Press

9 Mayr, E. (1963) Animal Species and Evolution,Belknap Press

10 Templeton, A.R. (1989) The meaning of speciesand speciation: a genetic perspective. InSpeciation and its consequences (Otte, D. andEndler, J.A. eds), pp. 3–27, Sinauer

11 Bush, G.L. (1994) Sympatric speciation inanimals: new wine in old bottles. Trends Ecol.Evol. 9, 285–288

Published online: 21 August 2002

Species conceptsversus species criteriaResponse from Werner Kunz

Richard Paul’s letter is of great importancefor parasitologists because it highlightsthe species debate. Within a group ofsimilar parasitic individuals, some can bespecific for one host, others for another;some can be pathogenic, others not; somecan be sensitive to a drug and others areresistant. As a result of these data,individual parasites can be classified intogroups. In several cases, host specificity,pathogenicity and drug resistance cannotbe seen immediately with our eyes whenwe observe a parasite in a test tube or underthe microscope; therefore, we prefer to usemorphological or molecular traits as a‘number plate’to recognize which group therespective individual belongs to. However,biodiversity is much more heterogeneousand complex than taxonomists want fortheir aim of classification, particularly inlight of the fact that each individualdiffers from the other by thousands, if nottens of thousands, of different alleles [1].Taxonomists have to find out whichindividuals, having one or a few traits incommon, belong to a common species.

If we are to encourage parasitologistsinto the species debate and the study ofspeciation, it is essential to provide thefoundations. Otherwise, there is a seriousdanger of running into contradictions.Paul’s letter contains some contradictions.If a species is a ‘cross-section in anevolutionary lineage’, it cannot be at thesame time, ‘a chunk of phylogenetic nexus’.

Which time interval is covered by this‘cross-section’? One second, or a thousandyears? Even during a short time interval,mutation and selection can change severaltraits in some individuals. Consequently,after a certain time, several genes andphenotypic characters are not the same asthey were before. So, when does a speciesbegin and when does it end? We cannotdefine a species as a slice of a sausage,while at the same time, we acknowledgeits spatiotemporal dimension.

Furthermore, what does ‘a single line ofdirect ancestry and descent’mean? Thiswould be easy to understand if we look atclonal propagation. But what are separatelineages in highly polymorphic populations?Are they gene lineages or lineages ofspecies? These are not necessarily thesame [2]. Sexuality, in addition, oftenignores separate lineages because in certaingroups of organisms, or under certainconditions, hybridization among differentlineages occurs, resulting in geneticintrogression of parts of the genome intonon-directly related organisms. In severalplants, in some animals, particularly inparasites, and, to a spectacular extent, inprotists, new groups of organisms canarise by hybridization of distantly relatedgroups, replacing the radiation principleof the evolutionary tree by a ‘modularprinciple’, contradicting the branching treeof Darwin and Haeckel [3]. Therefore,within certain limits, the formation ofnetworks obscures proper lineages.

Does Mayr’s biological species concept(population thinking) offer a solution? First,it is most important to discriminate betweenthe properties of a population and those ofits individuals. The properties of apopulation are, for example, populationdensity or balanced polymorphism. Thebearers of properties (such as colours,feathers, sexuality and reproduction) are notpopulations or reproductive communities,but only the individuals of populations.Individuals can mutate, populations cannotand, consequently, almost all the targets ofselection are the traits of individuals, notthose of the population. In other words, apopulation can evolve only in an indirectsense, namely as a consequence of theevolution of the individuals, which belongto that population [4].

Unfortunately, our daily language is notprecise. We say that a certain species has ayellow bill. In fact, it is the individuals, notthe species, which have a yellow bill. Undernormal conditions, our abbreviated manner

of speaking is harmless because we are ableto state it more precisely, if necessary.However, such manner of speaking canbecome dangerous, when we constructtheories or definitions. With this intention,it is a logical mistake to attribute theproperties of the members of a class tothe class [4].

The species is not a material object. It isthe result of a conceptual operation thatcombines individuals into a set in amathematical sense. Of course, cladisticlineages and reproductive communitiesare real, but they are completely differentfrom taxa. In some examples, a taxon canbe understood as the transfer of suchnatural things into an artificial taxonomicsystem [5]. All real objects that will beused (and misunderstood) as species areonly useful as an indicator for subsequenttaxonomic classification. The reproductivecommunity, for example, sometimes canbe used to group individuals into a speciestaxon. However, in the majority oforganismic biodiversity, particularly inplants, but also in several animal groups,a reproductive community either does notexist, or it represents a group of organismsthat is not suited by any means to be usedas a species taxon. Reproductive barriersare often not the cause of divergentevolution, nor are they essential for it tooccur [6]. Also, sexual interaction is not adefinition for a species because it does nottell us what a species is.

Werner Kunz

Section of Genetic Parasitology, Heinrich Heine University, Institute forGenetics, Universitätsstrasse 1, D-40225 Düsseldorf, Germany.e-mail: kunz@uni-duesseldorf.de

References

1 Powell, J.R. (1997) Progress and Prospects inEvolutionary Biology – The Drosophila Model.Oxford University Press

2 Nichols, R. (2001) Gene trees and species trees arenot the same. Trends Ecol. Evol. 16, 358–364

3 Doolittle, W.F. (1999) Phylogenetic classificationand the universal tree. Science 284, 2124–2129

4 Mahner, M. and Bunge, M. (1997) Foundations ofBiophilosophy. Springer-Verlag

5 Mahner, M. (1998) Why evolution still exists if species do not evolve. Theory Biosci. 117,173–199

6 Mishler, B.D. and Donoghue, M.J. (1994) Speciesconcepts: a case for pluralism. In: ConceptualIssues in Evolutionary Biology. (Sober, E. ed.),pp. 217–232, Massachusetts Institute ofTechnology Press

Published online: 21 August 2002

TRENDS in Parasitology Vol.18 No.10 October 2002

http://parasites.trends.com 1471-4922/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S1471-4922(02)02320-6

440 News&Comment