What is a fish species?

Reviews in Fish Biology and Fisheries 9: 281–297, 1999. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.

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What is a fish species? George F. Turner School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, UK (E-mail: [email protected]) Accepted 25 November 1998

Contents Abstract Introduction Why do we need a species definition? Species concepts Typological species concept Darwinian species concept Biological species concept Recognition species concept Phylogenetic species concept Other species concepts Back to the Darwinian concept Molecular measures of species distinctness The nature of fish species Biological species Self-fertilizing species Hybridization and introgression Unisexual species Allopatric populations and the problem of incipient species Sympatric morphs and species Parallel speciation Sibling species Mate recognition, sexual selection and species The biological and Darwinian species are testable hypotheses Concluding remarks Acknowledgements References

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Abstract The formal processes of alpha-taxonomy ensure that species have unique names and can be identified. No similar process is mandatory for infraspecific variation, so the species is a uniquely important practical term. At present, there is little agreement of the definition of a species. In the last 30 years, numerous concepts have been proposed. The nature of fish species is reviewed. Clonal inheritance of nuclear genes occurs in several lineages. Hybridization is frequent, often leading to introgression, which may lead to extinction of species. Species may have hybrid origins. There is good evidence for parallel speciation in similar habitats. There are clearly exceptions to the cladistic assumption of dichotomous branching during speciation. Sibling species may exist with no discernible niche differentiation. Basic assumptions are violated for the recognition, phylogenetic, ecological and some formulations of the evolutionary species concepts. The most satisfactory definitions are two of the earliest proposed in the light of evolutionary theory. The Darwinian view is that species are recognizable entities which are not qualitatively distinct from varieties. A restatement of this concept in genetic terms provides a means of dealing with all forms of species known in present-day fishes. This modified Darwinian concept is operated through the application of fuzzy logic

282 rather than rigid definition. This involves a search for discontinuities between species, rather than an a priori definition of how boundaries are to be determined. A subset of Darwinian species are Mayrian or ‘biological species’, which are characterized by their demonstrable reproductive isolation from other species. The status of a population as a Mayrian species is a testable hypothesis. Molecular techniques allow this hypothesis to be tested more easily than previously, at least when dealing with sympatric populations. Key words: fish, gene flow, hybridization, molecular phylogenetics, speciation, species concepts

Introduction Most researchers working with fishes do not concern themselves over the nature of fish species, but many taxonomists and evolutionary biologists vigorously disagree with each other about their definitions. The purpose of this review is to outline why it is important to know what a species is, to review the various kinds of population units in which fish actually exist, to summarize some of the contending species concepts, and to suggest a way in which a flexible hierarchical approach to definitions may be helpful. The ‘species’ is both a scientific/philosophical category (and it is fiercely debated whether it is a real entity or an artificial term of convenience) and a practical working unit used by taxonomists. These are not the same thing. Many systematic biologists have a strong commitment to a ‘biological’ species concept based on reproductive isolation, but in practice, generally make an educated guess at the specific status of the taxa they are working on, defining them on the basis of morphological traits which are often not central to their species concept. This review aims to address both uses.

Why do we need a species definition? Everything has to be named, so we all know what we are talking about. If you carry out a study of the population dynamics of species A, and later find out that you had lumped together two distinct species with different growth and mortality rates, size at maturity, breeding seasonality and habitat use, you would have been wasting your time. Virtually all other disciplines in biology depend on alpha-taxonomy – the naming of species and the development of identification keys and other diagnostic tools. Infraspecific diversity is also important: within species, individuals often differ in behaviour, morphology and life histories (behavioural examples: Magurran, 1993). Molecular ecologists are becoming increasingly concerned that diversity at levels below

the species is in danger of being reduced through anthropogenic environmental change (Ferguson, 1989; Ryman, 1991; Carvalho and Hauser, 1994, 1995). Despite the obvious significance of intraspecific diversity, there are reasons why the species is of particular importance. 1. Factors affecting some members of a species may affect conspecific individuals more directly and more fundamentally than non-conspecifics, for example, absence of conspecifics of the other sex may severely limit the reproductive potential of an individual. 2. The species is a political concept. Policy-makers and conservationists recognize and give more value to the species than to lower taxonomic categories. 3. Description of a species has to follow certain rules, which should guarantee that the species can be unambiguously diagnosed and that all synonyms are identified. Other interested parties can then identify it from a published key, a list of diagnostic features, an illustration or by examination of type material in a museum collection. While the formal process of alpha-taxonomy ensures that diversity at the species level is well documented, no such process is mandatory for intraspecific diversity. Local variants, races or subspecies need not be described or named and frequently no material is deposited in museum collections. Indeed, some taxonomists are campaigning to have the subspecies abolished as a zoological category (Mallet, 1995). Although workers in other disciplines commonly appreciate the importance of alpha-taxonomy, it is often seen as a moribund science. This is essentially a biased view held by scientists working in the northern temperate regions, where most taxonomic problems are solved. Most tropical freshwater fish species are either undescribed or so poorly known that they cannot be reliably identified in the field. Tropical freshwater fishes comprise the vast majority of endangered fish species. While conservation of plants

283 and invertebrates is mainly a question of preserving habitats without necessarily knowing much about the species, the same is not true for fish which are hunted directly for food. Alpha-taxonomy is also unfashionable because it uses intellectually undemanding, cheap, old-fashioned techniques and does not require big grants or generate high-impact papers.

Species concepts Biologists have always recognized that species exist, but have seldom agreed about how they can be defined. Many species concepts have been proposed. Typological species concept Initially, species were considered to be manifestations of ‘universals’ (Mayden and Wood, 1995). The idea of universals emanates from Platonic philosophy, where a universal is supposed to have an ideal nature and form which exist independently from their material manifestations in any one object (Russell, 1945). Each individual represents an imperfect realization of some of the elements of the perfect universal nature. This view, which predates the development of evolutionary theory, is really only compatible with the creationist idea that species are unchanging entities. However, it is worth emphasizing that the normal working practice of most taxonomists is to define species in terms of morphology, based around a type specimen. For the most part, this is the only practical thing to do. There is no inconsistency between this practice and the fact that few taxonomists would accept the philosophical implications of the typological concept. I will not consider this concept further. Darwinian species concept Darwin (1859) maintained that species were simply well-marked varieties and that there was no discontinuity between individual variation and variation at the level of species or higher taxa. This implies that the species is fundamentally an artificial category. If we consider a species at the moment of splitting into two, it is never possible to say when one species has become two, although it may be possible to say so retrospectively (Dennett, 1995). Darwin was content to leave diagnosis of specific status to whoever was a competent authority in the particular group concerned. In practice, Darwin’s concept of a species was the

same as the typological concept, but the underlying philosophy is different. Biological species concept The dominant species concept of the last 50 years has been based around the idea that a species is a population (or group of populations) within which there is interbreeding (or there would be interbreeding if they were not geographically separated), but which does not interbreed with other populations. The importance of interbreeding in the definition of species was stressed by Dobzhansky (1937), although the idea was much older (see Mallet, 1995). It was crystallized, named and popularized by Ernst Mayr (Mayr, 1942, 1963) as the rather decisively titled biological species concept (BSC), renamed the isolation concept by its enemies, principally H.E.H. Paterson and his followers (see below). Although slightly changing over the years, the fundamental emphasis of the BSC has always been on reproductive isolation (caused by ‘isolating mechanisms’) as the essential distinction of a species. The main criticisms levelled at the BSC are: (1) it does not apply to asexual organisms; (2) there is a practical difficulty in determining if fully allopatric forms are species or not; (3) a species can be defined only in relation to other species; (4) the term ‘isolating mechanisms’ implies that these are selected for in order to cause reproductive isolation; and (5) it has difficulties in dealing with clearly defined forms that undergo introgression. The first two arguments were clearly recognized and dealt with by Mayr (e.g. 1942) – the first by simply excluding asexual taxa and the second by the use of the phrase ‘potentially interbreeding’ in his definition (1942) and later by stating that the concept is best applied to sympatric taxa (1963). The third and fourth criticisms derive mainly from Paterson and his followers (see below). Both appear rather more semantic or philosophical than scientific criticisms, and both can be removed without changing the meaning of the concept, for example simply by substituting a word without the functional connotation of ‘mechanisms’, such as ‘traits’ (Turner, 1994) or ‘barriers’ (Avise, 1994). However, Paterson’s criticisms were worth making, as it is clear that Mayr, Dobzhansky and many others sometimes seemed to view isolating mechanisms as existing in order to protect co-adapted gene complexes from the detrimental effects of gene flow (Coyne, 1994), although in his later writings Mayr was sceptical of the idea of

284 reinforcement. The problem of introgression is empirical rather than semantic, and therefore more significant. If there can be long-term stable coexistence of two ecologically distinctive and morphologically recognizable taxa despite a non-zero level of gene flow, then the basic assumption of the BSC is violated. Species need not be ‘protected’ by reproductive isolation, but may be maintained by selection. What, then, is the difference between species and morphs? Despite these criticisms, the BSC still has many supporters and remains the most widespread view of the nature of animal species (King, 1993; Coyne, 1994). Later formulations of the BSC incorporated aspects of the ecological species concept (Mayr, 1982), a move which has been heavily criticized (King, 1993) – rightly, in my view. All subsequent references to the BSC are to the original version. Recognition species concept Paterson (1980, 1993), like Mayr, is a firm believer in that virtually all speciation is allopatric. He realized that if ‘ isolating mechanisms’ evolve so as to establish reproductive isolation (as Mayr and Dobzhansky sometimes stated), then they must evolve at least partially in sympatry. If they evolve in allopatry to prevent hybridization, evolution is predicting the future. This is clearly nonsensical, except to those

285 Some formulations of the phylogenetic concept do not involve the monophyly criterion, but then the concept loses its distinctive nature, so I will not consider these further. Other species concepts The evolutionary species concept was proposed by Simpson (1961), largely to enable paleontologists to designate specific status on the basis of anagenetic change (change within lineages, without splitting) in fossils. Wiley’s (1978) re-definition is pretty inclusive: “a single lineage of ancestor – descendent populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and fate”. Simpson’s and Wiley’s usages are completely different, although based on similar definitions. Morphologically distinct forms occurring at different times within the history of a single lineage are species according to Simpson, but not Wiley. Wiley recognized morphologically similar, but reproductively isolated forms as species; Simpson did not. In the following discussion, I use Wiley’s formulation. Mayden and Wood (1995) put forward a vigorous argument in its favour, largely through a critique of other concepts. Van Valen’s (1976) ecological species was defined on the basis of its occupation of a distinct ecological niche. Templeton (1989) defined a cohesion species as “the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability”. This is able to incorporate asexual species, and is essentially a hybrid between the recognition and ecological concepts intended to make the former more inclusive. However, it does not seem to have gained widespread use and it seems extremely vague. Back to the Darwinian concept Mallet (1995) defined a species as a recognizable ‘morphological and genotypic cluster’, meaning that a species can be recognized by discontinuities in frequency distributions of such traits. He suggested that this corresponds to taxonomic practice of most workers, irrespective of their theoretical concepts, and explicitly acknowledged that this is really a return to Darwin’s concept.

Molecular measures of specific distinctness Recent advances in molecular techniques are having a profound influence on phylogenetics and evolutionary biology. Detailed discussion of molecular techniques and their applications can be found elsewhere (Avise, 1994; Hillis et al., 1996), but the principal division in molecular markers is between mitochondrial DNA (mtDNA), which is usually maternally inherited and does not recombine, and nuclear genes together with the proteins they code for, which are transmitted via sexual reproduction in sexual species. Although changes in nuclear or mtDNA structure can affect the organism in ways which may be amenable to selection, most of the uses of molecular markers rely on the assumption that the variation chosen for analysis is selectively neutral. This assumption appears to be valid in most cases, although there are some spectacular examples to the contrary, at least in the case of protein polymorphisms (Mitton, 1997). Critically, molecular techniques provide a huge number of independent characters for cladistic analysis. Using protein (allozyme) electrophoresis, Avise and Smith (1977) found that, among North American sunfish (Lepomis, Centrarchidae), the mean genetic identity (Nei, 1972) of species was 0.54, of subspecies 0.84 and of geographically isolated populations 0.98. Genetic identities of congeneric species were very similar for the slowly speciating sunfish and the more rapidly speciating North American minnows (Notropis, Cyprinidae) (Avise and Ayala, 1976). Reviewing a large body of allozyme studies, Thorpe (1983) concluded that 97% of congeneric species had identities of less than 0.85, while 98% of conspecific populations have identities of over 0.85. This was held to suggest that measures of genetic identity can usually indicate if two populations are conspecific, even if they are completely allopatric (Thorpe, 1983; Richardson et al., 1986). However, four clearly differentiated congeneric species of the cichlid genus Pseudotropheus from Lake Malawi showed genetic identities averaging 0.95, while intergeneric comparisons gave identities averaging 0.93 (data reanalysed for changed generic placement of species, from Kornfield, 1978). Verheyen and Van Rompaey (1986) found a mean identity of 0.89 in comparisons between a Lake Malawi cichlid and seven species from Lake Victoria. Taken together, the entire Malawian and Victorian radiations, about 1500 species, are about as genetically

286 diversified as a pair of subspecies of North American fishes, while biological species within lakes are as similar as pairs of morphologically undifferentiated geographic populations of other fish. Mitochondrial DNA studies have given a similar picture (Meyer et al., 1990; Moran and Kornfield, 1993). Furthermore, founder effects may lead to rapid changes in genetic similarity, without concomitant changes in morphology or other traits (Carvalho et al., 1996). Clearly, overall genetic distinctness is not a good guide to whether populations are species or not, unless measures of distance are themselves to be used as the principal criterion in the definition of species. I would consider this unwise, because genetic distance does not represent anything very important biologically, and in any case different molecular markers or different loci give different and often inconsistent distance measures. A much better use of molecular techniques is to test the hypothesis that two populations are interbreeding. If a large sample of two sympatric morphotypes indicates that they share no alleles at a nuclear DNA locus, they are almost certainly reproductively isolated. The only exception would be in the highly improbable event that the locus examined was itself responsible for the morphological variation observed, or was very tightly linked to it. This can be easily overcome by examining several unlinked loci. Even if there are no fixed differences, statistically significant differences in allele frequencies among sympatric taxa can demonstrate at least partial assortative mating, provided that certain assumptions are met. Even where there is no obvious morphological trait to categorize population subdivisions, molecular techniques can still demonstrate non-assortative mating within a population. With nuclear genes, the frequencies of homozygotes and heterozygotes should lie at the Hardy-Weinberg equilibrium, so a deficiency of heterozygotes indicates either spatial structuring and local inbreeding or assortative mating within a geographically mixed population (Avise, 1994). In both these cases, it is assumed that there is no selection on the markers or linked genes, and this is especially likely to be true if multiple unlinked loci give the same pattern. Molecular techniques also provide a powerful tool for the determination of paternity in laboratory studies of assortative mating (Knight et al., 1998). This kind of molecular analysis provides quick answers in comparison with behavioural or ecological studies, which would require extensive field work over many years. Also, the molecular approach represents

a sample of gene flow taken over a long time period rather than just the season when the observations were collected.

The nature of fish species Most animal biologists work on mammals, birds and insects. They have tended to favour rigid definitions such as the biological and recognition concepts. Botanists, observing extensive hybridization and with clearly demonstrable cases of sympatric speciation, are less comfortable with the BSC. How do the various concepts fare when applied to fish? Biological species Many species of fish look different, are genetically different and have different behaviour and ecology from other species existing in the same place, and it is a reasonable conclusion that each species is a reproductively isolated biological species. If hybrids are produced naturally, they are often infertile or fail to produce viable or fertile offspring. In other words, there is no introgression and populations are good biological species. Although most biologists have used the BSC when they think about species, reproductive isolation was rarely tested directly, but generally inferred from morphological and ecological differences between forms. Molecular techniques can now provide a way to directly test for reproductive isolation (Avise, 1994), leading to an upsurge in the number of studies actually demonstrating that populations represent biological species. The BSC has thus become a testable hypothesis, rather than a theoretical concept. However, it is clear that not all fish populations behave as biological species. Self-fertilizing species The mangrove killifish, Rivulus marmoratus (Aplocheilidae), has gonads containing male and female tissues (Harrington, 1961; Kallman and Harrington, 1964). Although males are known from natural populations (Davis et al., 1990) and females from laboratory stocks (Cole and Noakes, 1997), most natural populations appear to consist entirely of self-fertilizing hermaphrodites (Turner et al., 1992). In such cases, because there is no interbreeding and each individual is reproductively isolated, such forms

287 cannot be classified by biological or recognition concepts. Hybridization and introgression Many organisms hybridize in nature, fishes probably more than most other animals. Hybrids have generally been treated as an anomaly, like albinos or melanistic varieties. Early work tended to emphasize the high frequency of sterility or inviability of hybrids (Hubbs, 1955). Although this is indisputably true in animals, including fish, some hybrids are able to interbreed successfully with one or both parental forms, leading to introgression. By contrast, the same process within species is termed ‘gene flow’ (Mayr, 1963), although it is not clear how we are supposed to distinguish between these two phenomena (Verspoor and Hammar, 1991). Evidence for hybridization and introgression in fish is increasing rapidly, much of the information coming from molecular studies. Verspoor and Hammar (1991) list 24 cases of natural hybridization not known prior to molecular analysis. In 19 cases, there is evidence of introgression. Smith (1993) lists 49 cases of introgression, although some of these are based on inferences from morphology alone. Molecular identification of hybrids is much more convincing, as hybrids are not necessarily intermediate forms and may exhibit novel morphologies outside the range of the parental species (Crapon de Caprona and Fritzsch, 1984; McElroy and Kornfield, 1993). The vast majority of all known cases of hybridization involve North American or European freshwater fishes, almost certainly because there is more research effort in these geographic regions, and because freshwater fishes are easy to study. The true number of introgressing species is likely to be much larger. Where two populations are largely allopatric, but hybridize along a narrow area, a hybrid zone or contact zone is formed. These are generally assumed to have been caused by secondary contact between populations or species which previously diverged allopatrically. While this is often likely to be the explanation, it is not clear how it could be distinguished from the alternative possibility that during speciation, the situation was just as it is now and the populations could have diverged while in contact, through a process of parapatric speciation (Endler, 1977). A contact zone may be massive, as between the subspecies of the largemouth bass (Micropterus salmoides, Centrarchidae) in the USA (Philipp et al.,

1983), or narrow, with little evidence for introgression outside the immediate vicinity of the zone, as occurs in the northern part of the zone of contact between Notropis cornutus and N. chrysocephalus (Cyprinidae) in the midwestern USA (Dowling and Hoeh, 1991). The Atlantic salmon (Salmo salar, Salmonidae) and the brown trout (Salmo trutta, Salmonidae), surely two of the best-studied fish species, were not known to hybridize until comparatively recently (Payne et al., 1972). In one river in Sweden, 23% of the Salmo population were found to be hybrids (Jansson et al., 1991). This is not a contact zone, and there is now considerable evidence that introgression is occurring sporadically in a number of locations throughout the widely overlapping and extensive ranges of these species (Verspoor and Hammar, 1991). Brook trout (Salvelinus fontinalis, Salmonidae) and Arctic charr (Salvelinus alpinus, Salmonidae) are widespread species with considerable overlap in their ranges. They often hybridize, but are usually readily distinguished. In Lake Alain in Quebec, Bernatchez et al. (1995) found a population in which all individuals look like brook trout and have brook trout allozymes, but every one of the 48 individuals examined had Arctic charr mtDNA. There are no charr in the lake or its catchment area, suggesting that some time in the past, there was hybridization and introgression between charr and trout. The nuclear genes of the trout have swamped the charr genes (presumably because most of the resident population were initially brook trout), but the mtDNA is all charr, perhaps from a single distant female ancestor. Using similar techniques, Dowling and De Marais (1993) found that a number of distinctive species and subspecies of the genus Gila (Cyprinidae) showed evidence of past introgression between extant species, and suggested that these species may have arisen through hybridization. Similarly, Giuffra et al. (1996) proposed a hybrid origin for Salmo carpio (Salmonidae), a species of trout endemic to Lake Garda in Italy. These studies illustrate the potentially long-term effects of introgressive hybridization. So, hybridization is not uncommon in fishes and may lead to introgression of varying degrees. It is unclear how much reproductive isolation is needed before forms should be regarded as good biological species. Where introgression is infrequent, it is often assumed to be of little importance, and under the biological concept, the two populations are treated as species. Where introgression is on a large scale, it has been assumed that gene flow will eventually lead to a

288 loss of distinctiveness of populations, which are thus considered to be geographical variants or subspecies. This view rests on the assumption that gene flow will generally be a more powerful force than selection. This is an assumption, not a demonstrated fact. According to some followers of the recognition concept, any pair of populations that hybridize at all in nature are conspecific, even if hybrids have zero fitness (Spencer et al., 1987). Fish biologists should consider the great practical advantages of adopting this practice, as it would eliminate difficulties in distinguishing Atlantic salmon and brown trout parr – they would be conspecific, as would most European cyprinids, such as roach, rudd, bream, carp, dace, bleak, chub! I am, of course, only joking (one of the referees of this paper seemed unsure of this). This move also makes the identification of species entirely dependent on field behavioural observations to determine specific status. Molecular evidence would be irrelevant, because sympatric populations fully isolated by postzygotic means would be considered conspecific. Laboratory tests of sterility would likewise be uninformative. It seems unlikely that many fish biologists would want to adopt such a radical proposal. Hybridization provides a problem for the phylogenetic concept, and indeed the whole process of phylogenetic reconstruction (Smith, 1993), in that introgression may lead to previously autapomorphic traits being transferred between species. The exchange of the entire mtDNA genome between trout and charr is an extreme example. If hybridization leads to speciation, this invalidates the monophyly criterion of cladistics and the phylogenetic concept. A phylogenetic species would have to include the entire group of populations that have undergone introgression in the past, and also any non-introgressing populations that are conspecific with any hybridizing ones. Sympatric, reproductively isolated populations could be held to be conspecific, which seems unsatisfactory. Such reticulate evolution would not be accommodated within the evolutionary concept’s requirement for a species to be a single lineage. Mallet’s reworking of the Darwinian species concept holds that species can be defined as genotypic clusters, statistically distinguishable groups of individuals with similar genotypes separated from other groups by a gene-space containing few individuals. This view could accommodate introgressive hybridization without too much trouble, provided that the two populations remain in spatial contact. Distinct species could be recognized from a frequency distribu-

tion of genotypes, although it may not be possible to assign every individual to one species or another. An allopatric sexual taxon of hybrid origin might also be recognized as a species, at least after enough time had elapsed to produce genetic divergence by drift, after the loss of gene flow to the parental populations. In conclusion, the BSC is vague about how to treat introgressive hybridization, and phylogenetic and evolutionary concepts cope poorly. Ecological or cohesion concepts can accommodate hybrid species, and the Darwinian concept provides a blurred guide to their identification from genetic traits. Unisexual species Unisexual fishes exist in all-female populations. Although these females copulate with males, the male genes do not contribute to the gene pool of the population in the long term. They are essentially asexual parasites of coexisting sexual species. In the desert streams of Sonora, Mexico, Poeciliopsis monacha (Poeciliidae) predominates in pools in springs and temporary waters, while P. lucida generally inhabits permanent streams (Vrijenhoek, 1989). Both species are diploid, and reproduce by internal fertilization. Where they overlap, they hybridize. Some hybrids are diploid (P. monacha-lucida), others triploid (P. monacha-2 lucida or P. 2 monacha-lucida). Triploid forms copulate with males of the most similar parental form, for example P. 2 monacha-lucida with male P. monacha. Sperm is essential to trigger development of the eggs, but the offspring are genetically identical to their mother. The diploid is a somatic hybrid, but produces haploid ova containing only P. monacha genes, expelling the P. lucida genome before recombination. The ova are fertilized by sperm from male P. lucida, producing more diploid hybrids, a ‘hemiclonal’ mode of reproduction. Each of these hybrid forms exists as several clones, two or more often occurring sympatrically, which have arisen from a number of independent hybridization events. Similar female-only forms, probably arising from hybridization, also occur in mollies (Poecilia formosa, Poeciliidae), wild goldfish (Carassius auratus gibelio, Cyprinidae), bleak (Rutilus alburnoides, Cyprinidae), American dace (Phoxinus eos × P. neogaeus, Cyprinidae), loaches (Cobitis spp., Cobitidae) and silversides (Menidia clarkhubbsi, Atherinidae) (Kallman, 1962; Echelle and Mosier, 1981; Goddard and Dawley, 1990; Purdom, 1993).

289 How do taxonomists deal with these female-only forms of hybrid origin? Sometimes they have been described as species, such as Poecilia formosa or Menidia clarkhubbsi. Asexual goldfish have been given subspecific status (Carassius auratus gibelio and C. a. langsdorfi)- an unusual use of the term, as they are not geographically separated from the sexual subspecies. Poeciliopsis and Phoxinus forms have been given hyphenated epithets derived from the specific names of the two parental species. None of these forms are generally designated by the standard hybrid notation of a cross, ‘×’, between the specific names of the parental species, indicating a recognition that these are not simply hybrids of a transient nature, but discrete populations, breeding true and largely isolated from the parental species by postzygotic means. Are they species? The biological species concept cannot accommodate them because they do not interbreed. The recognition concept would consider all the asexual lineages and both parental species as a single species. This could become difficult if one parental species interbreeds with two others, but the other two will not interbreed with each other. A phylogenetic concept might class them as species on account of their apomorphic reproductive strategies, but they are not generally monophyletic. Inclusion in either parental species would also undermine monophyly. If the two parental species were sister taxa (each other’s closest relatives), then both parental species and all hybrids could be combined in a single species. However, there is no a priori reason why hybridizing forms should always be sister taxa – for example, Poeciliopsis monacha will hybridize with both P. lucida and P. occidentalis (Vrijenhoek, 1994). It is not clear how the evolutionary species concept would treat them, as each clone is certainly an ancestor-descendant lineage, but do they have their own evolutionary tendencies and fate? They cannot exist without the appropriate parental species to mate with, so they cannot have their own evolutionary fate. A Darwinian species concept could accommodate these forms as species without too much difficulty. They can be clearly characterized by their reproductive behaviour and genotypes taken in combination. They do not intergrade with either of the parental species.

Allopatric populations and the problem of incipient species Most species exist as a number of geographically separate populations. Except when there are major alterations in the course of rivers or streams, many populations of freshwater fishes are completely isolated from one another. Many of these populations are morphologically and ecologically differentiated. How do we know which to class as species? Measures of genetic distance can provide no decisive answer, at least if we use any of the currently popular species concepts. Field studies of reproductive isolation cannot be used either. If laboratory studies showed hybrid or backcross sterility, or absolutely inviolable pre-mating barriers, we would have proven specific status in terms of the BSC. However, the reverse is not true: successful hybridization in the laboratory does not prove that populations would hybridize in the field, because reproductively isolated sympatric species may freely hybridize in the lab (Crapon de Caprona and Fritzsch, 1984). This practical difficulty with the diagnosis of allopatric species is a major problem for the biological species concept (Cracraft, 1989; Mayden and Wood, 1995). Advocates of the recognition concept claim that it is non-relational, so there should be no need to test for reproductive isolation between species. It should be sufficient to study the specific mate-recognition systems of allopatric populations and decide whether they would recognize each other as conspecific or not. That no such studies have apparently been performed on fishes may be testimony to the impracticality of such an approach. It is not easy to see how an experimenter could determine which are the key features of the mate-recognition system. If the SMRS is so important for mating success, it should be under strong stabilizing selection (Ribbink, 1986), so it would be unlikely that there would be heritable variation remaining in a natural population, making it impossible to observe individuals rejecting some conspecifics because they lack essential features of the SMRS. In some circumstances, particularly where recognition is via acoustic cues, aspects of the SMRS could be experimentally altered. However, Paterson (1993) explicitly rejects laboratory studies of any kind, and maintains that the SMRS only functions properly in the natural habitat. Thus, the recognition concept is even worse than the BSC in the diagnosis of allopatric species.

290 While fixed allelic differences can indisputably be used to aid the diagnosis of sympatric species, some authors have argued for the assignation of specific status to allopatric populations on the same basis, even if they are morphologically indistinguishable ‘cryptic species’ (Musyl and Keenan, 1996). This is an implicit and extreme use of a phylogenetic species concept, where even a single apomorphic protein allele is taken as sufficient to warrant formal species description. Because these allelic differences will normally have no influence on reproductive isolation following secondary contact, such species can be expected to disappear through introgression. This is not necessarily a criticism, but it is inconsistent with the phylogenetic species concept itself, in that introgression leads to violation of the monophyly criterion. Seehausen and co-workers have found two sympatric species of rocky shore cichlids in Lake Victoria (Pundamilia nyererei and P. pundamilia) which are usually clearly distinguishable in the field on the basis of male courtship dress, and small differences in morphometrics (Seehausen et al., 1997). They court assortatively in dichotomous-choice laboratory experiments (Seehausen and van Alphen, 1998). However, in a few geographically isolated areas of low water clarity, a range of intermediate forms is found (Seehausen et al., 1997), and in laboratory experiments under orange lighting, females respond equally to males of both species (Seehausen and van Alphen, 1998). Had these fishes not been investigated in areas of low water clarity, or had they never colonized these habitats, there would be no question that they would be regarded as biological species. Not only are these taxa formally described as species (Seehausen et al., 1998), but they are not even considered to be particularly closely related within the genus (Seehausen, 1996), which contains 5 described and at least 14 undescribed species (Seehausen et al., 1998). They are probably not sister taxa. This would seem like a clear demonstration of the temporary nature of many species. Mallet (1995) says that if we accept that that species can be lost through extinction, we should also accept that they may disappear through hybridization. Thus, the Darwinian species concept has no difficulty with accommodating cases like Pundamilia spp. Interestingly, if we accept that valid species can later disappear through introgression, we need not be so frustrated by the practical difficulty in assigning biological species status to allopatric populations, when such assignations may be falsified by introgression following range changes. Such an outcome

should be regarded as an inevitable consequence of the arbitrary and temporary nature of the species. During the origin of species there is a phase when the two varieties can be said to be ‘incipient’ species, which may or may not eventually separate into species (Darwin, 1859), an uncomfortable thought for those who consider species as objective realities. Mayr seemed to resolve this problem by proposing that species are generated more or less instantaneously because they are founded by a few individuals carrying a non-random sample of genes from the ancestral population. New species would be invariably formed by such founder effects in small isolated areas at the periphery of the main range of the ancestral species. Many workers still support these ideas, but others hold that empirical studies of biogeography do not support the peripheral isolate model (Lynch, 1989), and population genetic models suggest that speciation by founder effects and subsequent loss of alleles through drift is an improbable process and unlikely to account for the majority of speciation events (Barton, 1996). Biological and recognition concepts face considerable practical problems with the diagnosis of allopatric species, while the phylogenetic concept may produce self-incompatible results. The Darwinian concept has the advantage of regarding the designation of allopatric species as the arbitrary process that it is, but only at the cost of accepting that species may disappear through introgression. Sympatric morphs and species Fish, like many other animals, often mate in particular locations. Temporal and spatial separation of breeding may be sufficient to create complete reproductive isolation, even among populations that mix together for much of their lives (Ferguson and Taggart, 1991; Svedang, 1992; Taylor and Bentzen, 1993; Carvalho and Hauser, 1994). Such populations are variously, and somewhat inconsistently, regarded as species, subspecies, varieties or ‘stocks’. If such populations are sympatric and completely reproductively isolated by prezygotic means, then they must be designated as species under the biological concept. If they are monophyletic and have fixed autapomorphic traits, they are also phylogenetic species. However, reproductive isolation may be lost through environmental changes, if other barriers to gene flow are absent or small. Elimination of the breeding habitat of one species, climatic changes, or

291 rarity of conspecific mating partners may lead to introgression. Generally, a perception of possible impermanence seems to be the reason why these forms are rarely recognized as species, emphasizing that all of these species concepts rely on some kind of prediction of the future, what Ohara (1993) calls ‘prospective narration’. This notion is made explicit by the definiton of the evolutionary species concept in terms of ‘separate tendencies and fate’. In all cases, it illustrates the somewhat arbitrary nature of species definitions. Many coexisting salmonid populations are reproductively isolated, but show fairly low genetic diversification and can produce fertile hybrids. Behnke (1972) rejects the granting of specific status to these populations on the grounds that it would result in taxonomic chaos. This rejection implies a novel conception of the species – perhaps a ‘convenience concept’. Often these ‘sympatric morphs’ show substantial morphological and ecological diversification. In Lough Melvin in Ireland, there are three reproductively isolated forms of brown trout. The ferox is a large piscivore, the sonaghen a midwater plankton feeder, and the gillaroo a benthic feeder. They differ in morphology of teeth and gill rakers, colour, size at maturity, breeding habitat preference, allozymes and mtDNA (Taggart and Feguson, 1991). The ferox appears to be one of several relicts of an early postglacial invasion of northern Europe, but the sonaghen and gillaroo are part of a much later wave of colonization and have probably diverged within the lake. Taggart and Ferguson maintain that such distinct forms deserve formal taxonomic recognition, because application of the BSC leads to the designation of each sympatric, reproductively isolated population as a species. Workers studying the much more diverse cichlid fishes of the African Great Lakes have been more willing to grant specific status to sympatric forms. Initially, species were described on the basis of morphological features, mainly traits associated with feeding specializations (Eccles and Trewavas, 1989). Subsequently, it has been established that there is frequently full reproductive isolation between sympatric forms differing mainly in male courtship dress, and such forms are typically regarded as species (McKaye et al., 1982, 1984; Ribbink et al., 1983; van Oppen et al., 1998). Researchers working on African cichlids have been ready to grant species status to reproductively isolated sympatric forms, while students of north temperate

fishes have not, at least until recently. One reason is probably that the African cichlids have radiated into an immense number of species, many of which

292 1996; Thompson et al., 1997). Because similar selection pressures may occur in different lakes, the outcome tends to be the same. Remarkably, among Canadian sticklebacks, the independently evolved limnetics from several different lakes will readily mate with one another. Likewise, benthics from different lakes also interbreed freely, indicating that the same reproductive isolating barriers have arisen independently in different lakes (Schluter, 1996). Thus, the several independently derived limnetic stickleback populations are conspecific in terms of biological, recognition, cohesion, and ecological concepts, while the benthic forms would all belong to a second species. Such limnetic or benthic species would be polyphyletic, disqualifying it from being regarded as a phylogenetic species. However, each lake’s limnetic or benthic population is monophyletic, and although all share the same suite of morphological apomorphies, they are likely to have different molecular ones, so each is logically a distinct phylogenetic species. However, all benthic and limnetic clades have the same population of generalist riverine sticklebacks as their sister group, which would be rendered paraphyletic by the exclusion of the lacustrine forms. Once more, the phylogenetic concept is logically inconsistent. Classification of such multiple, independently evolved populations as one or more species would be arbitrary under the Darwinian concept, and unclear for the evolutionary concept. Sibling species The ecological species concept, Simpson’s (1961) formulation of the evolutionary species concept, and Mayr’s later (1982) versions of the biological species concept all maintain that species must occupy different niches. This implies the truth of the competitive exclusion principle, which is not universally accepted among ecologists. If reproductive isolation were to take place before niche divergence, it would be possible to have two or more reproductively isolated forms occurring sympatrically but occupying the same niche. On the criterion of reproductive isolation, many such sibling species have been identified and described in many taxa, including fish (Avise, 1994, pp. 274– 278). This would be appropriate under biological, recognition, Darwinian and phylogenetic concepts. It is certainly possible for two phenotypes within a population to occupy distinct niches (as males and females or adults and immatures often do), and no

acceptable level for niche similarity has been, or perhaps could be, proposed. Wiley (1978) has pointed out that according to the ecological species concept, if one population competitively excludes another, they must occupy the same niche. This implies that ecologically equivalent species must be conspecific even if reproductively isolated or indeed quite distantly related. Obviously, this is not what proponents of the ecological concept had in mind, but it is a logical consequence of the definition, and indicates that in practice, an ecological concept operates only with some kind of vague biological or phylogenetic concept lurking in the background. Mate recognition, sexual selection and species According to the recognition concept, a species can be distinguished by its possession of a specific materecognition system (SMRS), which is envisaged as an invariant suite of traits that ensure efficient finding and recognition of mates (Paterson, 1993; Ribbink, 1994; Mayden and Wood, 1995). Behavioural ecologists have found that animals rarely respond to signals from conspecifics in simple, predictable stimulus-response chains. Females are selected to discriminate among males (Turner, 1993), so some males may make all the right species-specific signals and be rejected by every female. Females of some African cichlid fishes prefer to mate with males having several bright spots on the anal fin, a trait that varies genetically, with age and as a result of damage to the fins (Hert, 1989). Sneaker males, which occur in many fish species (Magurran, 1993), need not be recognized by females at all. Males are often very indiscriminate, particularly if they provide no parental care (Turner, 1993). The very existence of parthenogenetic poeciliids depends entirely on the lack of discrimination of males of another species. It is often subordinate males which, lacking alternative means of securing mates, fertilize the hybrid females (Vrijenhoek, 1989). Females of some swordless species of Xiphophorus (Poeciliidae) prefer males that have had swords added to their tails (Basolo, 1990) and indeed may actually prefer heterospecific to conspecific males (Ryan and Wagner, 1987). All of these scenarios are adequately accommodated by the theory of sexual selection, initially proposed by Darwin (1859), which postulates that patterns of mate choice arise from selection pressures favouring future success of an individual’s offspring in the particular social environment it inhabits. In

293 common with Paterson’s recognition concept, sexual selection theory implies that mate choice does not evolve to protect the boundaries of a species’ gene pool as is often assumed (Mayr, 1963; Dover, 1995). However, Paterson (1993) is oddly antagonistic to the whole idea of sexual selection and mate choice. It would seem that sexual selection, like selection pressures imposed by the physical environment, could have the effect of altering the SMRS and so leading to speciation (Ryan, 1990; Ryan and Rand, 1993, Turner, 1994; Turner and Burrows, 1995).

The biological and Darwinian species are testable hypotheses Mayr has always stressed that an important point of the biological species was that it was an objective reality. Yet, in its present use, it is subjective in its application to introgressing and allopatric forms. Proponents of the BSC have always recognized that the biological species concept is inapplicable to asexual forms. Why stop there? Why not retain the objectivity of the BSC by simply restricting its application to sexual populations that are demonstrated to be reproductively isolated from all other populations? In this way the BSC becomes a clearly testable hypothesis. The Darwinian species is also a testable hypothesis for sympatric taxa. For example, van Oppen et al. (1998) found significant differences in microsatellite allele frequencies between the Lake Malawi cichlids Pseudotropheus zebra and the undescribed P. ‘zebra gold’ (Figure 1). Because the samples were taken from within a small habitat patch, and were thus completely sympatric, such differences are clearly sufficient to designate the taxa as species using Mallet’s genotypic cluster definition of the Darwinian species. A subsequent study showed that the two species would not interbreed in the laboratory when individuals of both sexes of each species were present in the same aquaria (Knight et al., 1998). This indicates, not only that they are biological species, but also that even if they had been totally allopatric in nature, it would have been possible to determine their specific status in the laboratory. Had such an experiment indicated that two allopatric taxa would have crossed in the laboratory, then we could not have determined their status as biological species, because it is possible that they would have remained reproductively isolated in the field as a result of environmental cues which were absent in the

laboratory. However, we would be free to rank them as Darwinain species or not, on the basis of subjective criteria. This accords with current taxonomic practice.

Concluding remarks With the increasing use of molecular techniques, it is now easier than ever to diagnose reproductive isolation. Biological species exist and can sometimes be unambiguously identified. Other vaguer species concepts lack the rigour of a testable hypothesis and so the BSC is likely to continue to be a dominant species definition. It is also clear that many real populations cannot be assigned to biological species. The BSC cannot cope with asexual, introgressing or allopatric populations. There seems to be little point in trying to squeeze all these populations into the biological species concept. In this survey of fish species, it has been clear that the Darwinian concept can resolve most of the difficulties in dealing with problematic cases. This is surprising, as it appears at the outset to be the vaguest and most subjective of all concepts. I suggest that this is an example of ‘fuzzy logic’. Blurred discontinuities exist around the boundaries of species. Concepts based on rigid definitions provide boundaries which often do not coincide with these. There is a mismatch. The Darwinian concept allows us to find these natural discontinuities precisely because it is vague and is simply guiding us to look for any boundary. So long as we keep the BSC in mind, we cannot be tempted to employ the Darwinian concept to split species to levels below that of the interbreeding gene pool. Keeping the BSC as a subset of the Darwinian species (Figure 2) nicely avoids any difficulty with polymorphisms, sex differences, and developmental differences. These genetic consequences are implicit in Mallet’s reformulation of the Darwinian species as a genotypic cluster. Other species concepts simply do not work in many cases. Indeed, I think it is not too presumptuous to state that they are based on hypotheses that have been disproved. The recognition concept is based on incorrect assumptions about animal behaviour. The phylogenetic concept requires the imposition of dichotomous branching on a natural world which is not that simple. The ecological concept makes an unwarranted assumption of the omnipotence of

294

Figure 1. An example of a molecular test for species status. Microsatellite allele frequency distributions (bp, base pairs) for sympatric populations of Pseudotropheus zebra () and the undescribed P. ‘zebra gold’ (), from Nkhata Bay, Lake Malawi. Some degree of genetic differentiation is hinted at by (a) locus UNH002, but we cannot rule out the possibility of between-sample variation (Kolmogorov-Smirnov, D100,188 = 0.107, P > 0.40). However, (b) locus Pzeb 1 indicates a striking and statistically significant difference (Kolmogorov-Smirnov, D100,190 = 0.859, P < 0.001). Using Mallet’s genotypic cluster definition, these two taxa could be designated as species on the basis on Pzeb 1. Data reanalysed from van Oppen et al. (1998).

the competitive exclusion principle. The evolutionary concept requires us to predict the future and fails to cope with reticulate evolution. So the two oldest concepts seem to be the best for fish, and probably for all other taxa too. Darwin was right – taxonomic species are not qualitatively different from varieties. He thought that acceptance of the evolutionary idea had solved the species problem for good. In the last chapter of the Origin of Species, he wrote “we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species”. Darwin underestimated the attractiveness that rigid definitions hold for the human mind.

But Mayr was right, too. Biological species do exist. He always knew there were exceptions, and intended only to apply the BSC to extant sexual forms. Mayr underestimated just how many exceptions there would turn out to be. All species are Darwinian species, but we have a testable hypothesis that a particular species may or may not be a biological species. This may be expressed formally in set theory. Biological species are a subset of Darwinian species. One final plea. ‘Biological species’ is an unfortunate term, a ‘persuasive definition’ intended to push the message that Mayr’s is the uniquely true species definition (Ruse, 1998). It is clear that there are species other

295

Figure 2. Venn diagram of relationship between Darwinian and Mayrian species, indicating the types of populations contained in each. Allopatric refers to the geographical relationship between the population in question and all other closely related populations with which introgression might be possible. Mayrian species are defined as populations that can be unambiguously demonstrated (solid boundary line) to be ‘biological’ species, either through coexistence in sympatry with sister taxa without introgression occurring, or else through laboratory studies indicating that introgression with closely related taxa is not possible. These are then a subset of Darwinian species, which are defined in sympatry by Mallet’s genotypic clustering method, and subjectively in allopatry (broken line). Populations laying outside the boundary of the Darwinian species are subjectively ranked as infraspecific.

than biological species, so why not drop Mayr’s term? All species are biological entities, but not all are ‘biological species’. Mallet has called the subjective concept the ‘Darwinian species’. Why not have call the objectively definable species the ‘Mayrian species’? Acknowledgements I thank Gary Carvalho, Joe Nelson and an anonymous referee for helpful comments on an earlier version of this manuscript. References Avise, J.C. (1994) Molecular Markers, Natural History and Evolution. Chapman and Hall, London. 511 pp. Avise, J.C. and Ayala, F.J. (1976) Genetic differentiation is speciose versus depauperate phylads: Evidence from California minnows. Evolution 30, 46–58.

Avise, J.C. and Smith, M.H. (1977) Gene frequency comparisons of sunfish (Centrarchidae) populations at various stages of evolutionary divergence. Syst. Zool. 26, 319–325. Barton, N. (1996) Natural selection and random genetic drift as causes of evolution on islands. Phil. Trans. R. Soc. Lond. B 351, 785–795. Basolo, A.L. (1990) Female preference predates the evolution of the sword in the swordtail fish. Science 250, 808–810. Behnke, R.J. (1972) The systematics of salmonid fishes of recently glaciated lakes. J. Fish. Res. Bd Can. 29, 639–671. Bernatchez, L., Glemet, H., Wilson, C.C. and Danzmann, R.G. (1995) Introgression and fixation of Arctic char (Salvelinus alpinus) mitochondrial genome in an allopatric population of brook trout (Salvelinus fontinalis). Can. J. Fish. Aquat. Sci. 52, 179–185. Carvalho, G.R. and Hauser, L. (1994) Molecular genetics and the stock concept in fisheries. Rev. Fish Biol. Fish. 4, 326–350. Carvalho, G.R. and Hauser, L. (1995) Genetic impacts of fish introductions in African Lakes. In: Pitcher, T.J. and Hart, P.J.B. eds. The Impact of Species Changes in African Lakes. Chapman and Hall, London, pp. 457–493. Carvalho, G.R., Shaw, P.W. and Hauser, L. (1996) Artificial introductions, evolutionary change and population differentiation in Trinidadian guppies (Poecilia reticulata: Poeciliidae). Biol. J. Linn. Soc. 57, 219–234. Cole, K.A. and Noakes, D.L.G. (1997) Gonadal development and sexual allocation in mangrove killifish, Rivulus marmoratus (Pisces: Atherinomorpha). Copeia 1997, 596–600. Coyne, J.A. (1994) Ernst Mayr and the origin of species. Evolution 48, 19–30. Cracraft, J. (1989) Speciation and its ontology: The empirical consequences of alternative species concepts for understanding patterns and processes of differentiation. In: Otte, D. and Endler, J.A. eds. Speciation and Its Consequences. Sinauer, Sunderland, MA, pp. 25–59. Crapon de Caprona, M.-D. and Fritzsch, B. (1984) Interspecific fertile hybrids of haplochromine Cichlidae (Teleostei) and their possible importance for speciation. Neth. J. Zool. 34, 503–538. Darwin, C. (1859) On the Origin of Species by Means of Natural Selection. John Murray, London. 458 pp. Davis, W.P., Taylor, D.S. and Turner, B.J. (1990) Field observations of the ecology and habits of mangrove Rivulus (Rivulus marmoratus) in Belize and Florida. Ichthyol. Explor. Freshwaters 1, 123–134. Dennett, D. (1995) Darwin’s Dangerous Idea. Penguin Press, London. 586 pp. Dobzhansky, T. (1937) Genetics and the Origin of Species. Columbia University Press, New York. 364 pp. Dover, G. (1995) A species definition: A functional approach. Trends Ecol. Evol. 10, 489–490. Dowling, T.E. and DeMarais, B.D. (1993) Evolutionary significance of introgressive hybridization in cyprinid fishes. Nature 362, 444–446. Dowling, T.E. and Hoeh, W.R. (1991) The extent of introgression outside the contact zone between Notropsis cornutus and N. chrysocephalus (Teleostei, Cyprinidae). Evolution 45, 944–956. Eccles, D.H. and Trewavas, E. (1989) Malawian Cichlid Fishes. Lake Fish Movies, Herten. 334 pp. Echelle, A.A. and Mosier, D.T. (1981) All-female fish: Cryptic species of Menidia (Atherinidae). Science 212, 1411–1413. Endler, J.A. (1977) Geographic Variation, Speciation and Clines. Princeton University Press, Princeton, NJ. 320 pp.

296 Ferguson, A. (1989) Genetic differences among brown trout, Salmo trutta, stocks and their importance for conservation and management of the species. Freshwat. Biol. 21, 35–46. Ferguson, A. and Taggart, J.B. (1991) Genetic differentiation among the sympatric brown trout (Salmo trutta) populations of Lough Melvin, Ireland. Biol. J. Linn. Soc. 43, 221–237. Giuffra, E., Guyomard, R. and Forneris, G. (1996) Phylogenetic relationships and introgression patterns between incipient parapatric species of Italian brown trout (Salmo trutta L. complex). Mol. Ecol. 5, 207–220. Goddard, K.A. and Dawley, R.M. (1990) Clonal inheritance of a diploid nulcear genome by a hybrid freshwater minnow (Phoxinus eos-neogaeus, Pisces, Cyprinidae). Evolution 34, 1052– 1065. Harrington, R.W., Jr (1961) Oviparous hermaphroditic fish with internal fertilization. Science 135, 1749–1750. Hert, E. (1989) The function of egg-spots in an African mouthbrooding cichlid fish. Anim. Behav. 37, 726–732. Hillis, D.M., Mable, B.K. and Moritz, C. (eds.) (1996) Molecular Systematics, 2nd edn. Sinauer, Sunderland, MA. Hubbs, C.L. (1955) Hybridization between fish species in nature. Syst. Zool. 4, 1–20. Jansson, H., Holmgren, I., Wedin, K. and Andersson, T. (1991) High frequency of natural hybrids between Atlantic salmon (Salmo salar L.) and brown trout (S. trutta) in a Swedish River. J. Fish Biol. 39 (Suppl. A), 343–348. Kallman, K.D. (1962) Gynogenesis in the teleost Mollienesia formsoa Girard, with a discussion on the detection of parthenogenesis in vertebrates by tissue transplantation. J. Genet. 58, 7–24. Kallman, K.D. and Harrington, R.W., Jr. (1964) Evidence for the existence of homozygous clones in the self-fertilising hermaphroditic teleost Rivulus marmoratus. Biol. Bull. 126, 101–114. King, M. (1993) Species Evolution: The Role of Chromosome Change. Cambridge University Press, Cambridge. 336 pp. Knight, M.E. Turner, G.F., Rico, C., van Oppen, M.J.H. and Hewitt, G.M. (1998) Microsatellite paternity analysis on captive Lake Malawi cichlids supports reproductive isolation by direct mate choice. Mol. Ecol. 7, 1605–1610. Kornfield, I. (1978) Evidence for rapid speciation in African cichlid fishes. Experientia 34, 335–336. Lynch, J.D. (1989) The gauge of speciation: On the frequencies of modes of speciation. In: Otte, D. and Endler, J.A. eds. Speciation and Its Consequences. Sinauer, Sunderland, MA, pp. 527–553. Magurran, A.E. (1993) Individual differences and alternative behaviours In: Pitcher, T.J. ed. The Behaviour of Teleost Fishes, 2nd edn. Chapman and Hall, London, pp. 441–478. Mallet, J. (1995) A species definition for the modern synthesis. Trends Ecol. Evol. 10, 294–299. Mayden, R.L. and Wood, R.M. (1995) Systematics, species concepts and the evolutionarily significant unit in biodiversity and conservation biology. Am. Fish. Soc. Symp. 17, 58–113. Mayr, E. (1942) Systematics and the Origin of Species. Harvard University Press, Cambridge, MA. 334 pp. Mayr, E. (1963) Animal Species and Evolution. Harvard University Press, Cambridge, MA. 797 pp. Mayr, E. (1982) The Growth of Biological Thought: Diversity, Evolution and Inheritance. Harvard University Press, Cambridge, MA. 974 pp. McElroy, D.M. and Kornfield, I. (1993) Novel jaw morphology in hybrids between Pseudotropheus zebra and Labeotropheus fulleborni (Teleostei: Cichlidae) from Lake Malawi. Copeia 1993, 933–945.

McKaye, K.R., Kocher, T., Reinthal, P. and Kornfield, I. (1982) A sympatric sibling species complex of Petrotilapia Trewavas from Lake Malawi analysed by enzyme electrophoresis. Zool. J. Linn. Soc. 76, 91–96. McKaye, K.R., Kocher, T., Reinthal, P., Harrison, R. and Kornfield, I. (1984) Genetic evidence for allopatric and sympatric differentiation among color morphs of a Lake Malawi cichlid fish. Evolution 38, 215–219. Meyer, A., Kocher, T.D., Basasibwaki, P. and Wilson, A.C. (1990) Monophyletic origin of Lake Victoria cichlid fishes suggested by mitochondrial DNA sequences. Nature 347, 550–553. Mitton, J.B. (1997) Selection in Natural Populations. Oxford University Press, New York. 240 pp. Moran, P. and Kornfield, I. (1993) Retention of an ancestral polymorphism in the mbuna species flock (Teleostei: Cichlidae) of Lake Malawi. Mol. Biol. Evol. 10, 1015–1029. Musyl, M.K. and Keenan, C.P. (1996) Evidence for cryptic speciation in Australian freshwater eel-tailed catfish, Tandanus tandanus (Teleostei: Plotosidae). Copeia 1996, 526–534. Nei, M. (1972) Genetic distance between populations. Am. Nat. 106, 283–292. Ohara, R.J. (1993) Systematic generalization, historical fate, and the species problem. Syst. Biol. 42, 231–246. Paterson, H.E.H. (1980) A comment on ‘mate recognition systems’. Evolution 34, 330–331. Paterson, H.E.H. (1993) Animal species and sexual selection. In: Lees, D.R. and Edmunds, D. eds. Evolutionary Patterns and Processes. Academic Press, London, pp. 209–228. Payne, R.H., Child, A.R. and Forrest, A. (1972) The existence of natural hybrids between the European trout and the Atlantic salmon. J. Fish Biol. 4, 223–236. Philipp, D.P., Childers, W.F. and Whitt, G.S. (1983) A biochemical genetic evaluation of the northern and Florida subspecies of largemouth bass. Trans. Am. Fish Soc. 112, 1–20. Purdom, C.E. (1993) Genetics and Fish Breeding. Chapman & Hall, London. 227 pp. Ribbink, A.J. (1986) The species concept, sibling species and speciation. Ann. Mus. Roy. Afr. Centr. Sc. Zoo. 251, 109–116. Ribbink, A.J. (1994) Alternative perspectives on some controversial aspects of cichlid fish speciation. Adv. Limnol. 44, 101–125. Ribbink, A.J., Marsh, B.A., Marsh, A.C. Ribbink, A.C. and Sharp, B.J. (1983) A preliminary survey of the cichlid fishes of rocky habitats in Lake Malawi. S. Afr. J. Zool. 18, 149–310. Richardson, B.J., Baverstock, P.R. and Adams, M. (1986) Allozyme Electrophoresis: A Handbook for Animal and Population Studies Academic Press, London. Ruse, M. (1998) All my love is towards individuals. Evolution 52, 283–288. Russell, B. (1945) A History of Western Philosophy. Simon and Schuster, New York. Ryan, M.J. (1990) Signals, species and sexual selection. Am. Scient. 78, 46–52. Ryan, M.J. and Rand, A.S. (1993) Species recognition and sexual selection as a unitary problem in animal communication. Evolution 47, 647–657. Ryan, M.J. and Wagner, W. (1987) Asymmetries in mating preferences between species: female swordtails prefer heterospecific males. Science 236, 595–597. Ryman, N. (1991) Conservation genetics considerations in fishery management J. Fish Biol. 39(Suppl. A), 211–224. Schliewen, U.K., Tautz, D. and Pääbo, S. (1994) Sympatric speciation suggested by monophyly of crater lake cichlids. Nature 368, 629–632.

297 Schluter, D. (1996) Ecological speciation in postglacial fishes. Phil. Trans. R. Soc. Lond. B. 351, 807–814. Seehausen, O. (1996) Lake Victoria Rock Cichlids. Verduijn Cichlids, Zevenhuizen, Netherlands. 304 pp. Seehausen, O. and van Alphen, J.J.M. (1998) The effect of coloration on female mate choice in closely-related Lake Victoria cichlids (Haplochromis nyererei complex). Behav. Ecol. Sociobiol. 42, 1–8. Seehausen, O., van Alphen, J.J.M. and Witte, F. (1997) Cichlid fish diversity threatened by eutrophication that curbs sexual selection. Science 277, 1808–1811. Seehausen, O., Lippitsch, E., Bouton, N. and Zwennes, H. (1998) Mbipi, the rock-dwelling cichlids of Lake Victoria: Description of three new genera and fifteen new species (Teleostei). Ichthyol. Explor, Freshwaters 9, 129–228. Simpson, G.G. (1961) Principles of Animal Taxonomy. Columbia University Press, New York. 247 pp. Smith, G.R (1993) Introgression in fishes: Significance for paleontology, cladistics and evolutionary rates. Syst. Biol. 41, 41–57. Spencer, H.G., Lambert, D.M. and McArdle, B.H. (1987) Reinforcement, species and speciation: a reply to Butlin. Am. Nat. 130, 958–962. Svedäng, H. (1992) Observations on interbreeding between dwarf and normal arctic charr. Salvelinus alpinus, from Stora Rösjön, central Sweden. Env. Biol. Fishes 33, 293–298. Taggart, J.B. and Ferguson, A. (1991) Minisatellite DNA fingerprints of salmonid fishes. Anim. Genet. 21, 377–389. Taylor, E.B. and Bentzen, P. (1993) Molecular genetic evidence for reproductive isolation between sympatric populations of smelt Osmerus in Lake Utopia, south-western New Brunswick, Canada. Mol. Ecol. 2, 345–357. Templeton, A.R. (1989) The meaning of species and speciation: A genetic perspective. In: Otte, D. and Endler, J.A. eds. Speciation and Its Consequences. Sinauer, Sunderland, MA, pp. 3–27. Thompson, C.E., Taylor, E.B. and McPhail, J.D. (1997) Parallel evolution of lake-stream pairs of threespine sticklebacks (Gasterosteus) inferred from mitochondrial DNA variation. Evolution 51, 1955–1965.

Thorpe, J.P. (1983) The molecular clock hypothesis, biochemical evolution, genetic differentiation and systematics. Ann. Rev. Ecol. Syst. 13, 139–168. Turner, B.J., Elder, J.F. Jr, Laughlin, T.F., Davis, W.P. and Taylor, D.S. (1992) Extreme clonal diversity and divergence in populations of a selfing hermaphroditic fish. Proc. Natl Acad. Sci. USA 89, 10643–10647. Turner, G.F. (1993) Teleost mating behaviour. In: Pitcher, T.J. ed. The Behaviour of Teleost Fishes, 2nd edn. Chapman and Hall, London, pp. 307–332. Turner, G.F. (1994) Speciation in Lake Malawi cichlids: A critical review. Adv. Limnol. 44, 139–160. Turner, G.F. and Burrows, M.T. (1995) A model of sympatric speciation by sexual selection. Proc. R. Soc. Lond. B 260, 287–292. van Oppen, M.J.H., Turner, G.F., Rico, C., Deutsch, J.C., Robinson, R.L., Genner, M.J. and Hewitt, G.M. (1998) Assortative mating among rock-dwelling cichlids supports high estimates of species richness from Lake Malawi. Mol. Ecol. 7, 991–1001. Van Valen, L. (1976) Ecological species, multispecies and oaks. Taxon 25, 233–239. Verheyen, E. and Van Rompaey, J. (1986) Genetics and speciation in African lacustrine cichlids. Ann. Mus. Roy. Afr. Centr. Sc. Zool. 251, 95–101. Verspoor, E. and Hammar, J. (1991) Introgressive hybridization in fishes: The biochemical evidence. J. Fish Biol. 39(Suppl. A), 309–334. Vrijenhoek, R.C. (1989) Genotypic diversity and co-existence among sexual and clonal lineages of Peociliopsis. In: Otte, D. and Endler, J.A. eds. Speciation and its Consequences. Sinauer, Sunderland, MA, pp. 386–400. Vrijenhoek, R.C. (1944) Unisexual fish: Models for studying ecology and evolution. Ann. Rev. Ecol. Syst. 25, 71–96. Wiley, E.O. (1978) The evolutionary species concept reconsidered. Syst. Zool. 27, 17–26.