Why No Subspecies in Spiders?

303 European Arachnology 2000 (S. Toft & N. Scharff eds.), pp. 303-314. © Aarhus University Press, Aarhus, 2002. ISBN 87 7934 001 6 (Proceedings of the 19th European Colloquium of Arachnology, Århus 17-22 July 2000) Why no subspecies in spiders? OTTO KRAUS Zoologisches Institut und Zoologisches Museum, Universität Hamburg, Martin-Luther-King-Platz 3, D-20146 Hamburg, Germany ([email protected]) Abstract The several tens of thousands of spider species that have been named so far have almost exclusively proved to be morphospecies; some of them have been tested and proved to be biospecies, but subspecies have only been recognized in a few exceptional instances. This phenomenon cannot be explained by regular speciation mechanisms alone: obviously, these are no different from those ocurring in other groups of the animal kingdom. However, the species groups that have been sufficiently inves- tigated in spiders indicate that separation fairly quickly produces superspecies composed of allospecies (‘semispecies’). The origin of allospecies patterns can be explained by a combination of the regular effects of separation with functional needs to safeguard sperm transfer between sexual partners. Selection pressure towards optimal co-adaptation between male and female copulatory organs may shorten transitory phases at a subspecies level. This could explain the origin of superspecies composed of similar but clearly distinguishable biospecies (allospecies); primarily, they are unable to coex- ist sympatrically or even syntopically. Key words: Arachnida, Araneae, speciation, genital coadaptation, superspecies, allospecies INTRODUCTION only been named in exceptional instances. It It was Ted Locket who raised the critical ques- may be questionable as to whether such nominal tion of whether some linyphiid spiders from taxa really form subspecies, i.e. subunits of remote localities in Europe really represented polytypic species. The aim of the present contri- true species, as there were only slight morpho- bution is to stimulate discussion by investigating logical character differences on which to base this comparatively unusual situation in more this decision. This was almost 25 years ago on detail. Of course, tradition has played and is the occasion of the 7th International Congress of continuing to play a predominant role, i.e. the Arachnology held in 1977 in Exeter. His prelimi- stabilizing effect of long-established taxonomic nary remarks did not form part of the congress practice. However, the real biological situation volume (see Merrett 1978), but the problem was may not be reflected by practicing conventional of course not forgotten by systematicists, who procedures of this kind. There is accordingly a were interested in the general aspects of speci- real need to try to explain why there is practi- ation and evolutionary biology. However, I am cally no problem in distinguishing morphospe- not aware of any sound discussion later. Not cies from morphospecies without any intermedi- even a hypothesis has been published. So far, ate forms. This aspect is directly linked to modes spider taxonomists have stamped tens of thou- of speciation in spiders. sands of morphospecies, but subspecies have 304 European Arachnology 2000 GENERAL REMARKS ON SPECIES AND ally been distinguished by different authors. SPECIATION Polytypic species seem to occur almost univer- The investigation and subsequent attempts to sally (Rensch 1929). explain this situation will first need some gen- The counterpart - monotypic species - seems eral remarks. to be exceptional. This term designates species without subspecies. They may be geographically Allopatric versus sympatric speciation widely distributed. I refer to the holarctic distri- Allopatric speciation is the predominant mode bution of the araneid Araneus diadematus Clerck, of speciation in animals. Separation by barriers 1757 (Fig. 1) and similar distribution patterns of causes interruption of genetic exchange between various species of the Erigonidae. Could it be groups of populations; peripheral isolates espe- that species of this kind consist of just one single cially play a major role. Accordingly, interrup- panmictic population? If so, they may be barred tion of the coherent function of gene flow per- from geographic speciation. The same seems to mits divergence (see Mayr 1942, 1963). Sympat- be true for freshwater Bryozoa and various spe- ric speciation, i.e. speciation without geographic cies of the Tardigrada, e.g., Macrobiotus hufelandi isolation, cannot be definitively excluded, but C.A.S. Schultze, 1834 but there is evidence now there is no reason to believe that mechanisms of that this tardigrad species is composite this kind could be of major importance. (Bertolani & Rebecchi 1993). These and other Until now, no sound argument has been put cases have in common the trait that the species forward to contradict this general view. concerned share extraordinary dispersal abili- ties. I refer to ballooning in spiders, to blepharo- Polytypic and monotypic species blasts in freshwater bryozoans and to the dessi- In most sufficiently studied major groups of the cated cryptobiotic states in tardigrades. animal kingdom, many biospecies1 comprise two or more subspecies. As defined by Mayr ANALYSIS (1963), subspecies form an ‘aggregate of local The situation in spiders is heterogeneous. There populations of a species inhabiting a geographic are sedentary species, and in the other extreme, subdivision of the range of the species and dif- others are world champions in ballooning like fering taxonomically from other populations of the Tetragnathidae in the Northern Hemisphere. the species’. Numerous examples are well-docu- Species may have extremely wide holarctic or mented from mammals to birds and from liz- pantropic distribution patterns, such as Neoscona ards to salamanders, etc. This is also true for in- nauticus (L. Koch, 1875). Others are confined to vertebrates such as carabid beetles (Mossakow- relatively small areas (see, for example, Thaler's ski & Weber 1976), and pulmonate gastropods work on alpine species (1994; Thaler et al. 1994)). (Knipper 1939; Mayr & Rosen 1956). See Mayr Almost all these species have in common the (1963) for details and further documentation. fact that they are distinct morphospecies. There According to this definition, the subdivision is practically no variation. This is especially true of polytypic species into subspecies is based on for the taxonomically decisive genital structures. typology. This explains why, in a given species, Examples of the narrow limits for variation of different numbers of subspecies have occasion- such details are presented in Figs. 9-10; they are partly correlated with the occurrence of sepa- 1The present author continues to accept the bio- rated populations (see Kraus & Kraus 1988). In- species concept sensu Mayr (1963) as it is biologi- termediate populations or at least single inter- cal. What is called the phylogenetic species concept mediate specimens between different mor- does not seem to be applicable in the present dis- phospecies are almost unknown in nature2 — cussion: it is based on individuals, not classes, and with the exception of extremely rare teratologi- sets of characters (Goldstein & DeSalle 2000), i.e. cal individuals. There is usually no difficulty in close to typology. Kraus: No subspecies in spiders? 305 Fig. 1. Holarctic distribution of Araneus diadematus and limited distribution of its presumed sister species, A. pallidus (from Grasshoff 1968). assigning even single specimens to morphospe- of the genus Heptathela and closely related forms cies A, B, or C. (Ryuthela) studied by Haupt (1983) indicates a It is highly improbable that the modes of correlation with their separation on different speciation in spiders could be principally differ- islands (Fig. 2). With respect to only slight different from those in other animals or at least in ences between material of different origins, other groups of terrestrial arthropods. For better Haupt divided H. kimurai (Kishida, 1920) into understanding, three aspects should be consid- the nominate and three further subspecies, and ered: 1. Arguments in favor of regular allopatric speciation mechanisms Various cases are wellknown enough to provide information on speciation events caused by geo- graphical separation. Some selected examples are: Mesothelae Species differentiation in island representatives 2Hybrids between Tegenaria gigantea Chamberlin & Ivie, 1935 and T. saeva Blackwall, 1844 were found in Yorkshire, England, where both species occur sympatrically (Oxford & Smith 1987; Oxford & Plowman 1991). Only 3% of the sampled male specimens were identified as intermediates. They occur occasionally; no hybrid populations were found. Hence, the specific status of the two Tegenaria spe- Fig. 2. Distribution pattern of island representatives cies is not invalidated. There is evidence that two of the genera Heptathela and Ryuthela on southern closely related synanthropic species are expanding Japanese islands. Further differentation into pre- their ranges, perhaps caused by human interference sumed subspecies (H. k. kimurai, k. amamiensis, k. (see Mayr 1963). This case does not seem to be of higoensis, k. yanbaruensis and R. n. nishirai, n. ishigakien- any relevance for the present discussion. sis) not indicated (from Haupt 1983). 306 European Arachnology 2000 Fig. 3. Phylogenetic biogeography of a clade of Hawaiian Tetragnatha species (from Gillespie 1993);

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