J. Zool. Syst. Evol. Research 41 (2003) 127–136 Received on 14 August 2001 2003 Blackwell Verlag, Berlin ISSN 0947–5745

Zoological Museum Copenhagen, Universitetsparken, Copenhagen, Denmark

Kelp flies and concepts – the case of frigida (Fabricius, 1805) and C. nebularum Aldrich, 1929 (Diptera:)

T. R. Laamanen,F.T.Petersen and R. Meier

Abstract The beaches of the North Atlantic and North Pacific are home to flies of the /nebularum complex, which consists of one to three different species depending on whether the two nominal species are accepted and a cryptic species proposed by Remmert is counted. The morphological differences between two populations of C. frigida (Fabricius, 1805) from the North Sea and the Baltic Region and two populations of C. nebularum Aldrich, 1929 from Alaska and Japan are described and discussed for small, medium, and large specimens. Crossing experiments are used to demonstrate that, under laboratoryconditions, no isolation mechanisms between either population exist. Coelopa frigida and C. nebularum are therefore regarded as a single biological species, a conclusion that is congruent with the observation that the genetic distances based on Ef 1-a and 16S rDNA indicate lower levels of differentiation within C. frigida/nebularum than between undisputed Coelopa species. The substantial morphological, breeding and genetic information on the C. frigida/nebularum species complex is then applied to six different species concepts popular in the modern systematic literature. According to the Biological, Hennigian and Recognition Species Concepts, only a single species would be recognized. The EvolutionarySpecies Concept is too vague to be applicable and under two variants of the Phylogenetic Species Concept, C. frigida and C. nebularum would constitute separate species. result confirms that Phylogenetic Species Concepts lead to a higher species number than concepts based on reproductive isolation. Practical and theoretical problems with the various species concepts are briefly discussed.

Keywords: Coelopa – Coelopidae – species concept – hybrid – crossing experiment – allopatric population – sized morphism

Introduction C. nebularum Aldrich, 1929 constitute separate species and The quest for a universallyvalid and applicable species concept (2) whether Remmert’s (1959) claim that, the Baltic and the is as old as biologyand after a period of relative tranquility, North Sea populations of C. frigida are reproductively the interest in the subject is again flaring up, new species isolated, is correct. Most recent authors have considered concepts are being proposed (e.g. van Valen 1976; Rosen 1978; C. frigida and C. nebularum as two species, although Hennig Wiley1978; Eldredge and Cracraft 1980; Mishler and Donog- had proposed a synonomy of C. nebularum with C. frigida in hue 1982; Donoghue 1985; Paterson 1985; Ridley1989; 1937. A precise description of species boundaries is in this case Templeton 1989; Nixon and Wheeler 1990; Vrana and Wheeler not onlywarranted from a taxonomic point of view, but also 1992), and vividlydiscussed as is evident from a large number important for the extensive biological work carried out over of review articles and edited volumes (e.g. Slobodchikoff 1976; the past 20 years on the Coelopidae in general and C. frigida in Willmann 1985; Otte and Endler 1989; Baum 1992; Ereshefsky particular. This has made this species a model for the studyof 1992; O’Hara 1993; Luckow 1995; Goldstein and DeSalle flygenetics and behaviour (summarized in Dayand Gilburn 2000; Wheeler and Meier 2000). This renewed interest is 1997). probablyrelated to a resurgence of systematicsin general and In order to test whether the C. frigida/nebularum species an interest in global estimates of species diversityfor various complex consists of one, two, or three species (counting taxa in particular (Groombridge 1992). Such estimates are Remmert’s cryptic species), crossing experiments were carried vitallydependent on the choice of species concept. For out and the characters that had been proposed for distin- example, by applying a phylogenetic species concept to the guishing the nominal species were studied. Applying the six known ‘biological species’ of the birds-of-paradise (Paradi- different species concepts currentlydebated in systematicsto saeidae), Cracraft (1992) increased the number of recognized the results of the interbreeding experiments and morphological species from 40–42 to approximately90. Such dramatic studies, the species status of the populations according to the changes are unlikelyin groups where much less is known different concepts is discussed. about within-species and population variability, but even here almost each taxon has genera or species-groups where different Natural history and distribution of Coelopa species concepts will yield different species numbers. One example for such a case from Diptera is the Coelopa Coelopidae onlyoccur on beaches with a more or less steady frigida/nebularum species complex. Coelopa frigida (Fabricius, supplyof stranded kelp. Theyare obligatelydependent on 1805) is one of the most common species of Holarctic flies decaying brown algae for breeding and can reach such high living on beaches and breeding in decaying kelp. Despite much population densities that theyare of commercial interest as a study(e.g. Hennig 1937; Remmert 1959), it remains unclear nuisance to beach tourists (Poinar 1977) and citydwellers which populations of kelp flies belong to this species. Much of when the flies disperse inland (Oldroyd 1954; Rebelo 1987; the confusion is due to a bewildering variabilityin bodysize McAlpine 1991). The distribution of the arguablymost (2.5 mm–10 mm), colour, and the number and thickness of abundant coelopid, Coelopa frigida, comprises the beaches of hairs and setae on the bodyand legs. In particular, it remains the North Atlantic. It ranges from Russia (Barents Sea) controversial (1) whether the allopatric C. frigida and through the Baltic region, the North Sea, Faroe Islands, and

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Iceland to the northernmost parts of the North American but theyare bright yellowin large males of both nominal coast (Hennig 1937). However, Hennig (1937) also mentions species. These morphological differences between C. nebularum two male specimens of Chinese C. frigida (Futschau ¼ Fu- and C. frigida are less pronounced in small specimens. Here, chou?) which were unfortunatelydestroyedduring the Second the tergites of C. nebularum are verysimilar to the tergites of World War. The distribution of C. frigida in China thus many C. frigida, because theyare mostlyblack and the setae remains unclear until new specimens become available. The are also largelyrestricted to the lateral and posterior rims. This second nominal species, C. nebularum, has onlybeen recorded variabilityhas not surprisinglycaused much taxonomic from the Pacific coast of North America (north of Vancouver confusion. Much of the variabilityin these characters is Island) through Alaska and the Far East of Russia, the Kuril related to bodysize, which is in turn partiallydependent on Islands to Japan (Steyskal 1965). food supplyfor the larvae and the genetic make-up of the individual (Dayand Gilburn 1997). All populations of C. frigida have a large ab chromosomal inversion which Morphology contains 10% of the fly’s genome. carrying aa are much The main known morphological differences between the larger than bb flies. Strong heterosis maintains this polymorph- nominal species are found in the males and concern tergite ism with the ab heterokaryotypes having a much higher and leg morphology(Hennig 1937). The tergites of a medium viabilityunder crowded larval conditions than both homo- C. nebularum male are shiny, covered with numerous stout karyotypes (Day and Gilburn 1997). There is indirect evidence setae especiallyalong the hind margin of the fourth and fifth from a bimodal size distribution that this chromosome inver- tergite (Figs 7–12). The posterior portions of the tergites are sion system also occurs in C. nebularum (Dunn et al. 1999). bright yellow. The tergites of a typical C. frigida male are matt, the setae are fewer in number and restricted to the lateral and posterior rim (Figs 1–6), and the entire sclerites are brownish- Taxonomic history black with onlya brownish-yellowposterior portion. The legs Coelopa frigida was named in 1805 byFabricius. Probablyin of C. nebularum tend to be darker than the legs of C. frigida, part because of the morphological variability, C. frigida was

Figs 1–12. Morphologyof fourth to fifth male abdominal tergites for three size classes and four popula- tions of the Coelopa frigida/nebu- larum species group. 1–3: C. frigida DK; 1: small; 2: medium; 3: large; 4–6: C. frigida UK; 4: small; 5: medium; 6: large; 7–9: C. nebularum USA; 7: small; 8: medium; 9: large; 10–12: C. nebularum Japan; 10: small; 11: medium; 12: large. Bro- ken line indicates shift in colour; scale bars ¼ 0.5 mm Coelopa frigida and species concepts 129 described five more times within the next 50 years. Based on requirement for potential interbreeding to arrive at the concept the Irish fauna Haliday(1833) contributed three new names: that species are ‘groups of interbreeding natural populations C. gravis (Haliday, 1833), C. simplex (Haliday, 1833), and that are reproductivelyisolated from other such groups’ (Mayr C. parvula (Haliday, 1833), one each for different size classes. 2000, p. 17). In its newer form, the Biological Species Concept The Scandinavian fauna was treated byZetterstedt (1847) who is onlyapplicable to sympatricand parapatric populations, proposed the name C. nitidula (Zetterstedt, 1847) for small while the earlier and more popular definition is also applicable specimens, and Stenhammar (1854) who described the large to allopatric populations. It is this earlier version that has been Scandinavian representatives as C. eximia (Stenhammar, used here, not the least because the populations of the Coelopa 1854). As part of Aldrich’s revision (1929) of the North frigida/nebularum complex are not sympatric. The populations American Coelopa, he proposed C. nebularum and doubted are regarded as belonging to different species if theycan be that C. gravis and C. parvula were different species. Hennig bred separatelyin the laboratory,but are not able to produce (1937) was the first to revise the entire Palaearctic fauna of fertile hybrid offspring in mixed cultures. If they do interbreed Coelopa and recognized that C. gravis, C. simplex, C. parvula, under laboratoryconditions and the hybridscan be success- C. nitidula, and C. eximia are all synonymous with C. frigida. fullybackcrossed with the parental populations, the results Hennig’s hypothesis was later supported by Remmert (1959) indicate that theybelong to one biological species. who was able to generate all morphological variants from the same culture byvaryinggrowing conditions. Based on morphology, Hennig (1937) also proposed that C. nebularum Hennigian Species Concept is onlya subspecies of C. frigida. However, his synonym has With regard to the spatial dimension of species taxa, this never gained acceptance and was neither followed in the most concept is similar to a Biological Species Concept in that it uses recent catalogues of the Palaearctic (Gorodkov 1984) and strict reproductive isolation to determine species boundaries Nearctic (Steyskal 1965; Vockeroth 1987) faunas, nor in the (Meier and Willmann 2000). Strict isolation has not been remaining biological literature (e.g. Dunn et al. 1999; Crean achieved until all gene flow is prevented byintrinsic isolation et al. 2000; Weall and Gilburn 2000). mechanisms. This contrasts with geographic ‘isolation’ (separ- We here tested Hennig’s hypothesis by adding new infor- ation) where intrinsic isolation mechanisms are often lacking. mation on the reproductive isolation of C. frigida and Meier and Willmann (2000) define that species are ‘reproduc- C. nebularum and byproviding a more complete surveyover tivelyisolated natural populations or groups of natural the critical morphological features. Remmert’s (1959) claim populations. Theyoriginate via the dissolution of the stem that the Baltic populations of C. frigida constitute a cryptic species in a speciation event and cease to exist either through biological species reproductivelyisolated from the North Sea extinction or speciation’ (Meier and Willmann 2000, p. 31). In C. frigida is also tested. our study, we will only discuss, species boundaries in the spatial dimension, and then the Biological and the Hennigian Species Concept will yield the same results, because the main Species concepts differences between the concepts involve the temporal dimen- The goal of the studyis to clarifythe species boundaries within sion of species. the C. frigida/nebularum complex. Yet what is a species and how can we decide which populations should be regarded as separate species? In evolutionarybiologyand systematics Recognition Species Concept species are considered a basic and essential unit, but many This concept is also closelyrelated to the Biological Species different species concepts have been proposed and recently Concept. Recognition species are the ‘most inclusive popula- discussed (see references above). We will illustrate the differ- tion of individual biparental organisms which share a common ences and problems with the most popular concepts byusing fertilization system’ (Paterson 1985, p. 25). According to this the case of C. frigida/nebularum, for which much biological, species concept, the C. frigida and C. nebularum would belong morphological, and genetic data is available. We chose the to one species if individuals of each sex having been taken from following commonly-used species concepts for our study, and the different populations copulate, and thus recognize each after a brief introduction, we will discuss the kind of data that other as mates, regardless of whether or not fertile offspring is would be required byeach for deciding on the specific status of produced (see Paterson 1985). C. frigida and C. nebularum. With the exception of the The remaining species concepts that will be discussed are Biological and Recognition Species Concepts we will be using, closelyrelated to the rise of numerical cladistics. Manycladists those definitions of the concepts that were published in a were verydissatisfied with old species definitions based on recent volume on species concepts (Wheeler and Meier 2000). reproductive isolation and thus proposed new concepts inten- For the Biological Species Concept we will discuss, a previous ded to more adequatelyclassifythe variabilityof populations version which is in widespread use, and the Recognition in nature. Species Concept was not covered in Wheeler and Meier’s (2000) volume. Evolutionary Species Concept Wileyand Mayden(2000a, p. 73) are currentlythe main Biological Species Concept proponents of the evolutionaryspecies concept: ‘An evolu- Mayr proposed two different definitions for this most- tionaryspecies is an entitycomposed of organisms which commonlyused species concept. The original one defines maintains its identityfrom other such entities through time and species as ‘groups of actually(or potentially)interbreeding over space, and which has its own independent evolutionary natural populations that are reproductivelyisolated from other fate and historical tendencies.’ In the present case, it is argued such groups’ (Mayr 1953, p. 25). Mayr later dropped the that if the two populations cannot successfullyinterbreed, then 130 Laamanen, Petersen and Meier theydo have different ‘historical tendencies’ and ‘evolutionary In order to obtain virgin females for the crossing experiments, single fates’ and should be regarded as two different evolutionary puparia were either picked or floated with water from the breeding species. If the two populations do successfullyinterbreed, the substrate. Each puparium was placed into a glass vial (50 · 16 mm), closed with cotton, and provisioned with moist cotton and some sugar- evolutionaryspecies definition is difficult to apply,because a water. After eclosion 2–10 females and 4–10 males were placed into a 1-l certain amount of gene flow between evolutionaryspecies is glass jar stocked with algae. When larvae appeared in the hybrid cultures, permitted (Wileyand Mayden2000b). those parental flies that were still alive and accessible were removed and killed, and offspring, puparia were again harvested as described above. For backcrossing, new cultures were established which were stocked Phylogenetic Species Concepts with virgin-hybrids and virgins from the parental populations. Two other concepts, both called ‘Phylogenetic Species Con- The hybridization experiments yielded multiple hybrid combinations (see Table 1). For C. frigida UK and C. nebularum USA all hybrid cepts’ were recentlydiscussed byWheeler and Platnick (2000) combinations were backcrossed with both parental populations. When and Mishler and Theriot (2000). Phylogenetic species as all backcrosses for these two populations proved to be successful, the defined byWheeler and Platnick (2000, p. 56) are ‘the smallest remaining crosses were simplified by(1) setting up onlya single aggregation of (sexual) populations or (asexual) lineages hybridization experiment with a larger number of virgins from both diagnosable bya unique combination of character states’. parental populations and (2) not trying all possible backcrossing We propose to studythe morphologyof C. frigida and combinations (see Table 1). It was also economized bycrossing C. frigida DK onlywith C. nebularum Japan, but not with C. nebularum C. nebularum which can both be considered to constitute USA, as C. frigida DK produces fertile offspring with C. frigida UK, different populations or ‘aggregations of populations’. If the whereas the latter produces fertile offspring with C. nebularum USA. populations differ bya unique combination of character states, Flies from both the parental populations and hybrid cultures were theywould be two different phylogeneticspecies sensu Wheeler taken continuouslyfrom the culture during the experiment. Prior to and Platnick. examination, males were killed in 70% alcohol after feeding on sugar Mishler and Theriot’s (2000) phylogenetic species concept is water for approximately1 h in order to expand the abdomen for morphological study. more complicated in that it requires a phylogenetic analysis For the morphological studies C. frigida, C. nebularum, and their prior to species description: ‘A species is the least inclusive hybrids were divided in to three size classes based on hind tibial length taxon recognized in a formal phylogenetic classification. As (small: 1.2–1.3 mm; medium: 1.4–1.5 mm; large: 1.7–1.8 mm). Digital with all hierarchical levels of taxa in such a classification, images of the tergites and legs were taken using a Leica Q600S camera organisms are grouped into species because of evidence of system . In order to study potential differences in the morphology of monophyly. Taxa are ranked as species rather than at some the male genitalia, male postabdomens were prepared for SEM study higher level because theyare the smallest monophyleticgroups byextending the aedeagus and critical-point-dryingthe specimens. The male genitalia were then studied under a Jeol JSM-840 scanning deemed worthyof formal recognition, because of the amount electron microscope. of support for their monophyly and/or their importance in For two populations of C. frigida (UK and DK), one population of biological processes operating on the lineage in question’ C. nebularum (USA), and four additional species of Coelopa, DNA (Mishler and Theriot 2000, p. 46–47). Recentlya cladistic sequence information was available for Elongation factor 1)a and 16S analysis of the Coelopidae based on morphological and rDNA (Meier and Wiegmann, in press). For these taxa the absolute paup molecular data was completed in which two populations of and mean character differences were computed using * (Swofford 2001) in order to compare the genetic differentiation between the C. frigida and one population of C. nebularum were included as populations of the C. frigida/nebularum complex with those of separate terminal taxa (Meier and Wiegmann, in press). If the undisputed species within Coelopa. cladistic analysis and/or the morphological study reveals one good autapomorphyeach for C. frigida and C. nebularum, then theycould be regarded as at least two different phylogenetic Results species sensu Mishler and Theriot. At least some replicates of all hybridization experiments between the populations of C. frigida and C. nebularum were Materials and methods successful (Table 1). The same was true for the backcrosses of the hybrids with the parental populations (Tables 2–4). The Two cultures of C. frigida [Longhoughton, Northumberland, UK; morphological variabilitywithin C. frigida proved to be much Hornbæk, Zealand, Denmark (DK)] and two of C. nebularum (Road’s smaller than in C. nebularum (Table 5). C. frigida has fewer End, Kodiak Island, AK, USA; Kyushu Island, Japan) were established in the laboratory. The flies were bred in 5.5-l glass jars, (six setae) and finer setae on the posterior margins of the approximatelyhalf-filled with kelp from different species of . The fourth and fifth tergites than C. nebularum males (>12 setae; Fucus was either harvested fresh from the Baltic Sea or collected on Figs 1–12). However, there appears to be little variation within beaches. In the latter case the algae were frozen for a minimum of 24 h and between the two nominal species with regard to the male at )35C before being used. In one case dried and rehydrated Fucus genitalic morphology(Figs 13–16). The onlyobserved discrete was used. difference concerns the presence/absence of microtrichia on the

Table 1. Crosses between Coelopa C. frigida UK $ C. frigida DK $ C. nebularum USA $ C. nebularum Japan $ frigida and C. nebularum C. frigida UK # par. 1 (1) 3 (5) not crossed C. frigida DK # 1 (1) par. not crossed 1 (1) C. nebularum USA # 3 (5) not crossed par. 1 (1) C. nebularum Japan # not crossed 1 (1) 1 (1) par. par. ¼ parental. The first figure is number of successful crosses. The one in brackets is number of replicates. Coelopa frigida and species concepts 131

Table 2. Backcrosses of UK · $: frig $ · neb #$: frig. # · neb $#: frig $ · neb ##: frig. # · neb $ USA Coelopa frigida ⁄ C. nebularum hybrids with parental populations C. frigida # 2 (2) 2 (2) n.a. n.a. C. frigida $ n.a. n.a. 2 (2) 3 (3) C. nebularum # 2 (2) 2 (2) n.a. n.a. C. nebularum $ n.a. n.a. 2 (2) 1 (1)

n.a. ¼ not applicable. The first figure is number of successful crosses. The one in brackets is number of replicates.

Table 3. Backcrosses of Coelopa frigida (DK/UK) hybrids with willing to mate. There is thus no obvious prezygotic mating parental populations element in Coelopa behaviour (e.g. elaborate courtship) that $: frig DK $ · frig UK ##: frig DK $ · frig UK # would likelyonlyfail under laboratoryconditions. Further- more, the densities of flies in the laboratorycultures were not C. frigida DK # 1(1) n.a. higher than observed under field conditions. There is thus no C. frigida DK $ n.a. 1 (1) reason to doubt that C. frigida and C. nebularum would C. frigida UK # 1(1) n.a. C. frigida UK $ n.a. n.c. also interbreed under natural conditions. Note that this conclusion is not a prediction about the future of C. frigida n.a. ¼ not applicable; n.c. ¼ not carried out. and C. nebularum. It is onlyconcluded that at the present time The first figure is number of successful crosses. The one in brackets is there is no evidence for reproductive isolation between these number of replicates. nominal species. Morphologyfails to provide characters for discriminating Table 4. Backcrosses of Coelopa frigida DK · C. nebularum Japan between the two C. frigida populations (see also Remmert hybrids with parental populations 1959), but there are characters that allow the separation of C. frigida and C. nebularum males. The most reliable ones are $: frig DK $ · neb Jap ##: frig DK $ · neb Jap # rather inconspicuous (number of setae on the hind margin of C. frigida DK # 1(1) n.a. the fourth and fifth tergites; presence/absence of microtrichia C. frigida DK $ n.a. 1 (1) on epandrium), while the much more conspicuous features onlyallow the separation of large specimens (based on surface n.a. ¼ not applicable; n.c. ¼ not carried out. texture and colour of the tergites, setal morphologyand The first figure is number of successes. The one in brackets is number abundance on the disc Figs 1–12; Table 5). The morphologyof of replicates. hybrid flies is consistent with the expectations given that the characters are polygenic. Most hybrid specimens are recogniz- epandrium. Theyare present in both C. frigida populations, able as such, because theyhave intermediate morphologies. with the specimens of the DK population (Fig. 13) having However, some specimens are indistinguishable from the flies considerablymore microtrichia than C. frigida UK (Fig. 14). in the parental populations. The six different species concepts Coelopa nebularum males lack these microtrichia altogether discussed in the ‘introduction’ are now applied. (Figs 15–16). The genetic distances between the C. frigida and C. nebularum populations are relativelysmall compared with Species Concepts undisputed Coelopa species (Table 6). Both, the Biological and the Hennigian Species Concepts rely on reproductive isolation as the criterion for delimiting Discussion contemporaryspecies. Therefore, in this studyboth yield Under laboratoryconditions the allopatric C. frigida and the same conclusion regarding the present-daynumber of C. nebularum can hybridize to produce fertile offspring. There species in the C. frigida/nebularum complex. Onlyone species is also no evidence for reproductive isolation between neither should be recognized and this result confirms Hennig’s the Baltic and North Sea populations of C. frigida (contraryto synonymization of C. nebularum with C. frigida (Hennig Remmert, 1959), nor the Alaskan and Japanese populations of 1937). This conclusion is also compatible with the genetic C. nebularum. To what extent these results would hold under data (Table 6). The mean character difference between field conditions remains necessarilyspeculative as is the case the populations is smaller than the character difference for anybiological investigation under laboratoryconditions. between ‘good’ Coelopa species. If the total number of However, the present experiments suggest that there is no characters is used, the same conclusions generallyhold, postzygotic isolation. In order to judge whether the laboratory although the genetic difference between C. alluaudi Se´ guy, conditions are so unnatural that existing prezygotic mech- 1941 and C. ursina (Wiedemann, 1824) is equal to the amount anisms mayhave broken down, it is instructive to consult the of differentiation between the least similar populations of the literature on Coelopa mating behaviour (summarized in Day C. frigida/nebularum species complex (C. frigida DK versus and Gilburn 1997). Courtship is missing in C. frigida and C. nebularum USA). males will mount anyother Coelopa individual in its path, Paterson’s Recognition Species Concept yields the same although there is some evidence for limited short-distance result as the Biological and the Hennigian Species Concepts. attraction between opposite sexes (Dayand Gilburn 1997). Coelopa frigida and C. nebularum are onlyone species, because Females have specific male rejection responses that need to be flies of opposite sex from C. frigida and C. nebularum recognize overcome bythe males before genital contact can be estab- each other as mates. However, the Recognition Species lished. Occasionally, males will also voluntarily reject females Concept would probablyrecognize an even larger assemblage 132 Laamanen, Petersen and Meier

Table 5. Morphological description of leg and tergite characters of males in the Coelopa frigida/nebularum species complex (the latter illustrated in Figs 1–12)

Number and position of Distribution of colour and Leg colour bristles on tergites surface texture on tergites

DK: C. frigida small males Brown Few, relativelylong bristles Mostlyblack, matt, with at along posterior and lateral margins; most a narrow, light-brown numerous spinules on disc posterior stripe on last tergite DK: C. frigida medium males Light-brown As in small specimens Mostlyblack, matt, sometimes with a narrow, light-brown posterior stripe on last two tergites DK: C. frigida large males Dark-yellow to As in small specimens Mostlyblack, matt, always light-brown with a narrow, light-brown posterior stripe on last two tergites UK: C. frigida small males Dull-yellow Few, relatively long bristles Mostlydark-brown, matt, along posterior and with at most a narrow, yellow lateral margins; numerous posterior stripe on last tergite spinules on disc UK C. frigida medium males Dull-yellow As in small specimens Also with yellow sides and a yellow posterior stripe on the penultimate tergite UK C. frigida large males Dull-yellow As in small specimens Narrow yellow posterior stripe on the last tergites less common USA: C. nebularum small males Black Many, relatively long bristles Black, shiny, with yellow sides on lateral sides of disc and and a narrow, yellow posterior along posterior and lateral margins; stripe on at least the last tergite few spinules on disc USA: C. nebularum Brown Bristles short and stout Yellow posterior stripe on medium males on larger area of disc than at least the last tergite, stripe in small specimens usuallywider USA: C. nebularum large males Bright-yellow Bristles even shorter and more Less shiny, stripe usually widespread on disc than in wider than in medium specimens medium specimens JAPAN: C. nebularum small males Light-brown Many, relatively long bristles Black, shiny, with yellow sides on lateral sides of disc and and a broad, yellow posterior along posterior and lateral margins; stripe on last two tergites few spinules on disc JAPAN: C. nebularum Light-brown Many, bristles of variable size Yellow part even wider, only medium males covering most of the disc and a small brown part left especiallyalong posterior of the black area and lateral margins; few spinules on disc JAPAN: C. nebularum large males Brown Many, bristles of variable size Last two tergites almost entirely covering the entire disc yellow, sometimes with a small brown stripe at the anterior part of Coelopa nominal species as a single recognition species, tions constitute one or two different evolutionaryspecies, we because males of manyspecies will at least attempt copulation would have to determine whether theyare likelyto have their with females of other nominal species. Paterson (1985) only own ‘independent evolutionaryfate(s) and historical tenden- accepts premating isolation mechanisms as valid for separating cies’ (Wileyand Mayden2000a). We are thus asked to make species, because he speculates that selection will eventually predictions about the future which is inherentlyproblematic in favour the origin of premating isolation mechanisms in those evolutionarybiology.Furthermore, Wiley(1981) argues that a populations currentlyonlyisolated bypostmating mech- certain amount of gene flow is acceptable between evolution- anisms. As a consequence, Recognition species will at least aryspecies. Such vague statements should be avoided, because occasionallycomprise multiple reproductivelyisolated popu- theyrender the concept subjective. How much gene flow is lations and thus underestimate the number of units with acceptable? Different systematists will disagree and the disag- permanentlyseparated gene pools. One could think that one reement cannot be settled bycollecting data. Instead the advantage of the Recognition Species Concept might be that it species boundarywill remain a matter of opinion. It is thus is easier to applythan the Biological Species Concept. concluded that despite the large amount of genetic and However, in manyorganisms with difficult-to-studybehaviour, morphological data for the C. frigida/nebularum complex, the it is at least as complicated to observe mating behaviour as it is EvolutionarySpecies Concept cannot be unambiguously to test for reproductive isolation. applied. The EvolutionarySpecies Concept is verydifficult to apply The Phylogenetic Species Concept sensu Wheeler and to the C. frigida/nebularum complex. There is potential for Platnick would likelyconsider C. frigida and C. nebularum as gene flow between the populations, but the populations are separate species. Davis and Nixon (1992) have described a currentlyallopatric. In order to decide whether the popula- method (‘populations aggregation analysis’) for recognizing Coelopa frigida and species concepts 133

Figs 13–16. Morphologyof the male genitalia of four populations of the Coelopa frigida/nebularum species complex. 13: C. frigida DK; 14: C. frigida UK; 15: C. nebularum USA; 16: C. nebularum Japan. Arrows indicate position of microtrichia. Scale bars are 100 lm phylogenetic species. In Brower’s (1999) more formal descrip- and (4) the North Sea and Baltic populations of C. frigida are tion it consists of five steps: (1) identification of individual not distinguished bycharacters and thus constitute a single organisms as representatives of local populations; (2) identi- phylogenetic species. The same is true for the Alaskan and fication of attributes useful for comparing populations; (3) Japanese populations of C. nebularum. However, C. frigida tabulation of attributes for a local population to develop a and C. nebularum each have unique combinations of character population’s attribute profile; (4) comparison of profiles states and are thus two phylogenetic species sensu Wheeler among populations to distinguish between characters (fixed and Platnick. The unique combinations of character states for differences within populations) and traits (not fixed) and (5) C. frigida are: (1) less than six setae on hind margin of the aggregation of those populations into a single species which fourth and fifth tergites; (2) microtrichia on epandrium are not distinguished bycharacters. present and for C. nebularum, and (1) >12 setae on hind In applying this recipe to our case, (1) each of the four margin of the fouth and fifth tergites; (2) microtrichia on sampled populations could be regarded as a ‘local popula- epandrium absent. tion’; (2) the attributes of interest are listed in Table 5; (3) the We believe that classifying C. frigida and C. nebularum as four population profiles can also be taken from the same table two species is nevertheless problematic.

Table 6. Total character (below diagonal) and mean character differences (above diagonal) between Coelopa species based on the gene sequence of Ef1-a and 16S rDNA

C. frigida UK C. frigida DK C. nebularum USA C. alluaudi C. ursina C. vanduzeei C. pilipes

C. frigida UK 0.00978 0.01761 0.03213 0.03066 0.03131 0.05284 C. frigida DK 15 0.02218 0.03748 0.03588 0.03849 0.06654 C. nebularum USA 27 34 0.03146 0.02935 0.03001 0.05936 C. alluaudi 48 56 47 0.02276 0.03882 0.05622 C. ursina 47 55 45 34 0.03588 0.05414 C. vanduzeei 48 59 46 58 55 0.05088 C. pilipes 81 102 91 84 83 78

Distances between populations of the C. frigida/nebularum species complex are given in bold. 134 Laamanen, Petersen and Meier

(1) There is strong experimental evidence that if C. frigida phylogenetic species sensu Mishler and Theriot. The treatment and C. nebularum populations were to become parapatric or of C. nebularum is less obvious. This taxon is not supported by sympatric, hybrids would be formed. The interbreeding anyautapomorphyand thus cannot be recognized as a experiments suggest that hybridization breaks down the two separate species sensu Mishler and Theriot. Again, under this ‘unique combination(s) of character states’ of the two phylo- variant of the Phylogenetic Species Concept, systematists will genetic species, thus creating a new hybrid taxon. This new face orphan specimens not belonging to anyspecies. In the taxon cannot be regarded as a new phylogenetic species, past, such ‘plesiomorphic’ restgroups not supported as mono- because all its attributes would also be found in the two phyletic have been called ‘metaspecies’ (Donoghue 1985; parental species, and it thus lacks the ‘unique combination of Mishler and Brandon 1987). However, metaspecies are not character states’ required for recognition as a phylogenetic proper species according to Mishler and Theriot’s species species. It is also not identical to either of the parental species, concept and the metaspecies concept is at best a patch covering because once the parental species hybridize the characters a serious flaw in the original species definition. distinguishing the species turn into traits because theyare now Furthermore, from the authors’ viewpoint, the Phylogenetic variable within the new taxon. For example, presence/absence Species Concept sensu Mishler and Theriot is too subjective of microtrichia on the epandrium was a character for C. frigida (cf. discussion of EvolutionarySpecies Concept). The defini- and C. nebularum, but it is a trait for the hybrid taxon. One tion demands that the systematists decide which monophyletic might think that a synonymization of the parental species with groups are ‘deemed worthyof formal recognition’. Different their hybrid offspring species would adequately deal with the systematists will clearly disagree in many cases and there is no problem. However, such action is not permitted under the objective criterion bywhich their disagreement could be definition of the concept, because the parental species fulfilled resolved. all species requirements at the time of description. Hybridiza- tion between phylogenetic species thus creates a taxon consisting of orphan specimens that do not belong to any Conclusion species-level taxon. This is regarded as a major drawback of None of the populations of the C. frigida/nebularum complex is the Phylogenetic Species Concept sensu Wheeler and Platnick. reproductivelyisolated bypostzygoticisolation mechanisms (2) Furthermore, hybrid taxa also introduce serious com- and behavioural work on Coelopa suggests that there is little plications to cladistic analyses. In the present case, hybridiza- chance for prezygotic isolation. According to the Biological tion will add a hybrid taxon to the two valid phylogenetic and Hennigian Species Concepts, the C. frigida/nebularum species. Anycladistic analysisusing these three taxa would complex should therefore be treated as a single, extremely either place two as sistergroups to each other or yield an widespread Northern Hemisphere species with a veryvariable unresolved trichotomy. Both results would be incorrect morphologywithin and between populations. Using a species because the reticulate origin of the hybrid terminal would be concept based on reproductive isolation to delimit species may disguised. Ironically, the application of the ‘phylogenetic’ require an extensive amount of information, but should lead to species concept can thus yield terminals that cause problems the recognition of taxa as species that are incapable of forming with accuratelyreconstructing phylogeneticrelationships. the kind of hybrid taxa that are problematic for carrying out One might argue that hybridization of phylogenetic species phylogenetic analyses. It is believed that the alternative species will be so rare that these problems can be ignored. However, concepts suffer from serious flaws. The EvolutionarySpecies Cracraft recognizes 90 phylogenetic species in the birds-of- Concept and the Phylogenetic Species Concept sensu Mishler paradise where the application of the Biological Species and Theriot leave too much room for subjective decisions to Concept onlyfound 40–42. This implies that manyphylo- yield objective species boundaries. The Recognition Species genetic species are not reproductivelyisolated. Allopatric Concept fails to recognize taxa as species whose gene pools are population can quicklybecome parapatric or sympatric, permanentlyisolated and thus form independent units in especiallywith today’snumerous faunal introductions and evolution. Both Phylogenetic Species Concepts have their own changes in species-ranges due to climatic changes. New hybrid problems with taxa that cannot be attributed to anyspecies. In taxa between phylogenetic species will certainly originate Wheeler and Platnick’s concept hybrids do not belong to any within the next decades and even within the C. frigida/ species, because theylack a ‘unique combination of character nebularum complex global warming should lead to pronounced states’. Mishler and Theriot’s Phylogenetic Species Concept distributional changes. creates ‘metaspecies’ which cannot be formallyrecognized as Under the Phylogenetic Species Concept sensu Mishler and species, because theyare not ‘monophyletic’.This study Theriot (2000) one would probablyconclude that C. frigida documents that the number of recognized species depends on and C. nebularum are two taxa. The phylogenetic analysis the choice of species concept and suggests that new species based on morphological and molecular characters byMeier descriptions should always specify which species concept was and Wiegmann (in press) finds a clade consisting of the Baltic used. and the North Sea population of C. frigida with C. nebularum (USA) being its sistergroup. Both relationship hypotheses are insensitive to analysis conditions and have very high bootstrap Acknowledgement and jackknife support. The sistergroup to C. frigida/nebularum The authors would like to thank Drs Andrew Gilburn and Derek is a clade composed of C. alluaudi and C. ursina. Both species Dunn from the Universityof Manchester for supplyingus with have a large number of hairs on the hind margins of the Coelopa cultures from Longhoughton and Kodiak Island and Dr Brad Sinclair from the Alexander-Koenig-Museum in Bonn for providing abdominal tergites (RM, pers. observ.). 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