Phylogenetic approaches to nomenclature: a comparison ... - NCBI

Phylogenetic approaches to nomenclature: a comparison based on a nemertean case study Mikael HÌrlin Department of Engineering and Natural Sciences, Section of Biology and Chemistry,VÌxjÎ University, SE-351 95 VÌxjÎ, Sweden ([email protected]) Phylogenetic approaches to biological nomenclature are becoming increasingly common. Here I compare the behaviour of two such approaches, the phylogenetic system of de¢nition and the phylogenetic system of reference, when there is a shift in the preference of phylogenetic hypotheses.The comparison is based on a case study from nemertean systematics and is the ¢rst to compare two di¡erent phylogenetic approaches throughout three stages of change, including two stages of phylogenetic nomenclature. It is concluded that a phylogenetic system of reference in combination with uninomials is superior in conveying phylogenetic information. Keywords: phylogenetic taxonomy; uninomials; contingency; individuality; evolution; cladistics

1. INTRODUCTION

It is well known that phylogenetic biology obtained new life during the 1960s (e.g. Hennig 1950, 1966) and at last faced up to the Darwinian ideology. It is perhaps less well known that the discipline of taxonomy did not quite follow. There are several reasons for this (De Queiroz 1988; HÌrlin 1998a, 1999) but, during the last ten years or so, taxonomy has witnessed a revitalization in terms of phylogenetic theory (e.g. De Queiroz & Gauthier 1990, 1992, 1994; De Queiroz 1992, 1994, 1995, 1996, 1997; Sundberg & Pleijel 1994; Ghiselin 1995, 1997; Schander & Thollesson 1995; Bryant 1996, 1997; Lee 1996; HÌrlin 1998b; HÌrlin & Sundberg 1998; Sereno 1999). Although these new approaches to biological nomenclature emphasize the importance of incorporating phylogenetic theory, they also di¡er in interesting ways (HÌrlin 1998b). The main purpose of this paper is to highlight these di¡erences and discuss them from both a theoretical and a practical point of view. So far, comparisons have only been made between traditional Linnean nomenclature and a de¢nitional approach to phylogenetic nomenclature (e.g. De Queiroz & Gauthier 1992; Cantino et al. 1997; De Queiroz 1997; Moore 1998; but see HÌrlin 1998b). This is the ¢rst study of the di¡erences between the phylogenetic system of de¢nition (PSD) and the phylogenetic system of reference (PSR), as identi¢ed by HÌrlin (1998b), that is based on a real case study. I base my discussion on a case study from nemertean (Nemertea) phylogenetics, which includes two stages of phylogenetic nomenclature as well as one stage of Linnean nomenclature. To set the scene, I begin with a brief review of the theoretical and philosophical aspects of naming (see also HÌrlin 1998b; HÌrlin & Sundberg1998). 2. THE NATURE OF TAXA AND ITS INFLUENCE ON NAMING

Taxa are generally thought of as named clades (De Queiroz & Gauthier 1994). However, what kind of Proc. R. Soc. Lond. B (1999) 266, 2201^2207 Received 1 July 1999 Accepted 6 August 1999

entity is a clade ? Is it a class, an individual or perhaps a natural kind? A class is an entity that is unrestricted in time and space and has instances (identical members) and, thus, de¢ning properties. The idea of taxa as classes (or natural kinds, see below) goes back to antiquity and has dominated both the philosophy of biology as well as theoretical and practical biology ever since (Hull 1965). In the 1960s and 1970s, Ghiselin (1966, 1974) introduced the idea that species are philosophical individuals rather than classes. This idea at ¢rst met strong opposition among philosophers and biologists but has now gained almost general acceptance (Hull 1988; Ghiselin 1997; Baum 1998; HÌrlin 1998c; but see, for example, Kitcher 1984; Mahner & Bunge 1997) in phylogenetic biology. Today taxa at all phylogenetic levels are treated as philosophical individuals. Contrary to a class, an individual is restricted in time and space and lacks instances and de¢ning properties. Instead, an individual is composed of parts that are united as a whole and the paradigmatic individual is a human being. The parts of historical individuals (clades ö including phylogenetic species) share a common ancestry and the composition of the clade is a matter of contingency due to evolutionary processes such as natural selection and genetic drift. Ghiselin (1997) provided an in-depth discussion on the distinction between classes and individuals (see also Baum 1998). The tradition of treating species as special kinds of classes (natural kinds) has recently been emphasized by Mahner & Bunge (1997). The paradigmatic natural kinds are chemical elements such as gold or silver. A natural kind is supposed to have an essence, which is a de¢ning property with a natural cause. For instance, the atom number 79 is the essence and de¢ning property of gold. The natural kind tradition in biology holds that species have essences in terms of morphological structure or genetic make-up. Modern natural kind proponents want to introduce a historical essence (e.g. Gri¤ths 1997, 1999) arguing that a particular common descent is the essence

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(de¢ning property) of a particular clade. The main problem with this approach is that it relies on the notion that there are some de¢ning properties to be used when the name of such a group is de¢ned and, as a corollary, it does not fully acknowledge the contingent nature of historical individuals (HÌrlin 1998b,c, 1999). Evolutionary history is contingent and the result can only be viewed in retrospect and we have no means of fully circumscribing the individual until it has ceased to exist. Therefore, due to its historical fate there are no de¢ning properties and the problem of taxa is rather one of representation than of de¢nition (O'Hara 1993). Furthermore, the inferential nature of phylogenetics leads to the necessity of giving primacy to hypotheses rather than actual individuals when naming (HÌrlin 1998b). Hence, the choice of entity one wishes to name also determines whether or not a name can be de¢ned. De¢nitions are possible for the names of classes and natural kinds, but names of individuals are proper names that cannot be de¢ned (Mill 1875; Ghiselin 1966, 1995, 1997; Kripke 1980; HÌrlin 1998b; HÌrlin & Sundberg 1998). Thus, if one wishes to keep the traditional Linnean approach to nomenclature or adopt a phylogenetic system of de¢nition sensu De Queiroz & Gauthier (1990, 1992, 1994), one is also forced to treat species and other clades as classes or natural kinds rather than individuals. If, on the other hand, one prefers to treat clades as historical individuals then we cannot de¢ne their names since no de¢ning properties are involved (HÌrlin 1998b). A few have tried to reconcile the view between classes and individuals (e.g. De Queiroz 1992) or argued that ostensive ¢xation of names is also possible for natural kinds (Kripke 1980). The rest of the paper is devoted to the problems associated with combining a de¢nitional approach to taxon names and treating taxa as historical individuals. 3. SHIFTING PHYLOGENETIC HYPOTHESES Ð EFFECTS ON NOMENCLATURE UNDER DIFFERENT PHYLOGENETIC APPROACHES

Eureptantic nemerteans are a group of polystiliferous nemerteans (Hoplonemertea: Polystilifera) with worldwide distribution (HÌrlin & Sundberg 1995; HÌrlin 1996). HÌrlin & Sundberg (1995) proposed the ¢rst explicit phylogenetic hypothesis of the group, a study that was later followed by a systematization (HÌrlin 1998a) applying the principles of phylogenetic nomenclature. HÌrlin (2000) has added three recently described species to the phylogenetic hypothesis in HÌrlin & Sundberg (1995) and the results point at slightly di¡erent relationships between eureptantic nemerteans (¢gure 1). (The new phylogenetic hypothesis is based on the same data set as in HÌrlin & Sundberg (1995) but with two of the characters deleted due to too many question marks and with one new character added. The data set was analysed with PAUP* v. 4.0b2 (Swo¡ord 1999) using heuristic search with random addition sequencing (100 replicates) and tree bisection^reconnection swapping. Six equally parsimonious trees were found. For details see HÌrlin (2000).) Here I use these new results to discuss the e¡ects of the PSD and PSR and how they handle changes in tree topology. (The new names introduced should not be Proc. R. Soc. Lond. B (1999)

(a)

(b)

Drepanogigas albolineatus Drepanophorella sebae Drepanophorella pajungae Drepanophorella waingapuensis Drepanophorella rosea Drepanophorella lifuensis Paradrepanophorus corallinicola Paradrepanophorus nisidensis Paradrepanophorus crassus Paradrepanophorus obiensis Drepanophorina argus Drepanophorina guineensis Drepanophorina lata Drepanophorina savuensis Gerardanemertes willeyana Punnettia lankesteri Punnettia pilorhynchus Punnettia tiurensis Punnettia rhamphocephalus Punnettia natans Punnettia splendida Punnettia micrommata Fumionemertes maldivensis Wijnhoffia timorensis Bergia cerina Gibsonnemertes spectabilis Crandallia hubrechti Drepanophorus rubrostriatus Drepanophorus modestus Brinkmannia mediterranea Drepanobanda trilineata Uniporus borealis Uniporus acutocaudatus Uniporus hyalinus Drepanogigas albolineatus Drepanophorella sebae Drepanophorella pajungae Drepanophorella waingapuensis Drepanophorella rosea Drepanophorella lifuensis Paradrepanophorus corallinicola Paradrepanophorus obiensis Drepanophorina argus Drepanophorina guineensis Drepanophorina lata Drepanophorina savuensis Gerardanemertes willeyana Punnettia splendida Punnettia micrommata Punnettia lankesteri Punnettia pilorhynchus Punnettia tiurensis Punnettia rhamphocephalus Punnettia natans Kamegrinenemertes parmiornatus Drepanophoriella histriania Paradrepanophorus nisidensis Paradrepanophorus crassus Polyschista curacaoensis Fumionemertes maldivensis Wijnhoffia timorensis Bergia cerina Gibsonnemertes spectabilis Crandallia hubrechti Drepanophorus rubrostriatus Drepanophorus modestus Brinkmannia mediterranea Drepanobanda trilineata Uniporus borealis Uniporus acutocaudatus Uniporus hyalinus

Figure 1. Two phylogenetic hypotheses of the eureptantic nemerteans. (a) From HÌrlin & Sundberg (1995). (b) New hypothesis (HÌrlin 2000), including three species (Kamegrinenemertes parmiornatus, Drepanophoriella histriana and Polyschista curacaoensis) not present in (a).

treated as validly published. They only serve as discussion material for phylogenetic nomenclature. The phylogeny and phylogenetic nomenclature of Eureptantia will be revisited in another paper (HÌrlin 2000). No italics are used for taxon names since it is argued that all levels are phylogenetically equal ö only inclusiveness di¡ers.) The discussion is based on the names introduced by HÌrlin (1998a), which are shown in table 1. In the PSD discussion the texts accompanying the names should be


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Table 1. Eureptantic taxon names and how they refer (The texts accompanying the names follow the form suggested by Schander & Thollesson (1995) for reasons discussed by HÌrlin (1998a,b). The table is modi¢ed from HÌrlin (1998a).) Eureptantia ö the least-inclusive clade comprising Uniporus and Aequifurcata Uniporus ö the least-inclusive clade comprising Uniporus hyalinus and Uniporus borealis Aequifurcata ö the least-inclusive clade comprising Drepanobanda trilineata and Inaequifurcata Drepanobanda ö the most-inclusive clade comprising Drepanobanda trilineata but not Brinkmannia Brinkmannia ö the most-inclusive clade comprising Brinkmannia mediterranea but not Drepanophorus Drepanophorus ö the least-inclusive clade comprising Drepanophorus modestus and Drepanophorus rubrostriatus Punnettia ö the least-inclusive clade comprising Punnettia lankesteri and Punnettia micrommata Drepanophorina ö the least-inclusive clade comprising Drepanophorina savuensis and Drepanophorina argus Inaequifurcata ö the least-inclusive clade comprising Paradrepanophorus and Drepanogigas albolineatus Drepanogigas ö the most-inclusive clade comprising Drepanogigas albolineatus but not Drepanophorella Gibsonnemertesöthe most-inclusive clade comprising Gibsonnemertes spectabilis but not Bergia Fumionemertes ö the most-inclusive clade comprising Fumionemertes maldivensis but not Punnettia Wijnho¤aöthe most-inclusive clade comprising Wijnho¤a timorensis but not Fumionemertes Bergia ö the most-inclusive clade comprising Bergia cerina but not Wijnho¤a Crandalliaöthe most-inclusive clade comprising Crandallia hubrechti but not Gibsonnemertes Gerardanemertes ö the most-inclusive clade comprising Gerardanemertes willeyana but not Drepanophorella Drepanophorellaöthe least-inclusive clade comprising Drepanophorella sebae and Drepanophorella lifuensis Paradrepanophorus ö the least-inclusive clade comprising Paradrepanophorus corallinicola and Paradrepanophorus obiensis

treated as de¢nitions, while in the PSR case they should be treated just as a means of reference. (a) The phylogenetic system of de¢nition

The idea with a PSD is that, once a name is de¢ned, the de¢nition will help in identifying a monophyletic group that ¢ts the de¢nition in all possible hypotheses where the reference taxa are included (but see Schander & Thollesson 1995). Furthermore, it is often argued that the same monophyletic group is identi¢ed, albeit with di¡erent content. The latter argument for a de¢nitional approach faces several problems as discussed by HÌrlin (1998b). Now, consider ¢gure 2a. The left-hand tree represents the phylogenetic nomenclature introduced by HÌrlin (1998a) and the right-hand tree the possible changes in inclusivity under the new hypothesis. De¢nitions (both stem and node based) of the names are given in table 1. The bold lines indicate inclusiveness and the taxon names above the tree (Inaequifurcata and Aequifurcata) are both node based sensu Schander & Thollesson (1995). Three new taxa (Kameginemertes parmiornatus Iwata 1998, Drepanophoriella histriana Senz 1993 and Polyschista curacaoensis Stiasny-Wijnho¡ 1925) are introduced in the new tree and the internal relationship between the included taxa is slightly di¡erent. These three taxa were described as new genera by the authors without an explicit phylogenetic hypothesis, which is not good practice since it tends to promote monotypic taxa and, thus, minimizes the phylogenetic information content in the taxonomy. When incorporated into a phylogenetic analysis, such monotypic taxa may either end up ¢tting under a more inclusive binomial taxon name (i.e. Fumionemertes) or outside such de¢nitions (i.e. Kameginemertes and Drepanophoriella). Because of the de¢nition `the most-inclusive clade comprising Fumionemertes maldivensis but not Punnettia', Fumionemertes switches from referring to F. maldivensis alone to a more inclusive group comprising Fumionemertes nisidensis, Fumionemertes crassus, Fumionemertes curacaoensis and Fumionemertes maldivensis. Proc. R. Soc. Lond. B (1999)

The taxon name Punnettia was de¢ned as `the leastinclusive clade comprising Punnettia lankesteri and Punnettia micrommata' in the left-hand tree (¢gure 2a). This de¢nition leaves us with two options in the new right-hand tree. We could either, as illustrated in the ¢gure, make Punnettia a more inclusive taxon name keeping the taxon names Drepanogigas, Drepanophorella, Paradrepanophorus and Drepanophorina as names of subtaxa to Punnettia or we could use Punnettia as the ¢rst part of a binomial for all species in the least-inclusive clade comprising Kameginemertes parmiornatus and Drepanogigas albolineatus. In the ¢rst case we have to introduce, at a minimum, three new names in order to capture the phylogenetic information and keep the binomials referring to monophyletic groups. One name for the least-inclusive clade comprising Newname lankesteri and Drepanophorella histriana, one name for Newname splendida and one name for Newname micrommata. The latter case, on the other hand, means that the clades previously referred to by the names Drepanophorella, Paradrepanophorus and Drepanophorina as well as the least or most-inclusive clade comprising Kameginemertes parmiornatus and Newname lankesteri need new names so that the phylogenetic taxonomy does not lose hierarchical information. Either way, several new names have to be introduced and the previous de¢nitions dictate the possible outcomes. (b) The phylogenetic system of reference

Contrary to the PSD, a PSR does not use de¢nitions of taxon names. Instead, the names only refer under a given hypothesis and, should we later chose an alternative hypothesis (as in ¢gure 2b), we also face a new naming situation (HÌrlin 1998b). This approach provides the taxonomist with a higher degree of freedom, always gives primacy to the tree rather than the de¢nition and still refers explicitly to monophyletic groups. Consider ¢gure 2b which is identical to ¢gure 2a with regard to the tree topologies and texts accompanying the taxon names. However, several di¡erences follow. First, the names are

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Aequifurcata

Aequifurcata

Inaequifurcata

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Inaequifurcata

Punnettia Aequifurcata

Aequifurcata

Drepanogigas albolineatus Drepanophorella sebae Drepanophorella pajungae Drepanophorella waingapuensis Drepanophorella rosea Drepanophorella lifuensis Paradrepanophorus corallinicola Paradrepanophorus nisidensis Paradrepanophorus crassus Paradrepanophorus obiensis Drepanophorina argus Drepanophorina guineensis Drepanophorina lata Drepanophorina savuensis Gerardanemertes willeyana Punnettia lankesteri Punnettia pilorhynchus Punnettia tiurensis Punnettia rhamphocephalus Punnettia natans Punnettia splendida Punnettia micrommata Fumionemertes maldivensis Wijnhoffia timorensis Bergia cerina Gibsonnemertes spectabilis Crandallia hubrechti Drepanophorus rubrostriatus Drepanophorus modestus Brinkmannia mediterranea Drepanobanda trilineata Uniporus borealis Uniporus acutocaudatus Uniporus hyalinus Drepanogigas albolineatus Drepanophorella sebae Drepanophorella pajungae Drepanophorella waingapuensis Drepanophorella rosea Drepanophorella lifuensis Paradrepanophorus corallinicola Paradrepanophorus obiensis Drepanophorina argus Drepanophorina guineensis Drepanophorina lata Drepanophorina savuensis Gerardanemertes willeyana Newname splendida Newname micrommata Newname lankesteri Newname pilorhynchus Newname tiurensis Newname rhamphocephalus Newname natans Kamegrinenemertes parmiornatus Drepanophoriella histriana Fumionemertes nisidensis Fumionemertes crassus Fumionemertes curacaoensis Fumionemertes maldivensis Wijnhoffia timorensis Bergia cerina Gibsonnemertes spectabilis Crandallia hubrechti Drepanophorus rubrostriatus Drepanophorus modestus Brinkmannia mediterranea Drepanobanda trilineata Uniporus borealis Uniporus acutocaudatus Uniporus hyalinus

Inaequifurcata

(b)

Inaequifurcata

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clades. In the example, I have chosen to discard the referrals of taxon names referring to monotypic taxa (e.g. Crandallia hubrechti). Other names have been expanded from node to stem based (e.g. Drepanophorus) in order to

Figure 2. Two approaches to phylogenetic nomenclature. The left-hand trees represent the phylogenetic nomenclature introduced by HÌrlin (1998a). Bold lines indicate inclusiveness. The way the taxon names refer are listed in table 1. (a) Shifts in nomenclature following a PSD. (b) Shifts in nomenclature following a PSR.

not de¢ned, which means that the way the names refer may be altered when the new hypothesis is at hand and, thus, primacy is given to the tree topology rather than de¢nitions. This way we avoid names referring to unintended

Proc. R. Soc. Lond. B (1999)

Drepanogigas albolineatus Drepanophorella sebae Drepanophorella pajungae Drepanophorella waingapuensis Drepanophorella rosea Drepanophorella lifuensis Paradrepanophorus corallinicola Paradrepanophorus nisidensis Paradrepanophorus crassus Paradrepanophorus obiensis Drepanophorina argus Drepanophorina guineensis Drepanophorina lata Drepanophorina savuensis Gerardanemertes willeyana Punnettia lankesteri Punnettia pilorhynchus Punnettia tiurensis Punnettia rhamphocephalus Punnettia natans Punnettia splendida Punnettia micrommata Fumionemertes maldivensis Wijnhoffia timorensis Bergia cerina Gibsonnemertes spectabilis Crandallia hubrechti Drepanophorus rubrostriatus Drepanophorus modestus Brinkmannia mediterranea Drepanobanda trilineata Uniporus borealis Uniporus acutocaudatus Uniporus hyalinus Drepanogigas albolineatus Drepanophorella sebae Drepanophorella pajungae Drepanophorella waingapuensis Drepanophorella rosea Drepanophorella lifuensis Paradrepanophorus corallinicola Paradrepanophorus obiensis Drepanophorina argus Drepanophorina guineensis Drepanophorina lata Drepanophorina savuensis Gerardanemertes willeyana Newname splendida Newname micrommata Xenonemertes lankesteri Xenonemertes pilorhynchus Xenonemertes tiurensis Xenonemertes rhamphocephalus Xenonemertes natans Xenonemertes parmiornatus Xenonemertes histriana Polyschista nisidensis Polyschista crassus Polyschista curacaoensis Polyschista maldivensis Wijnhoffia timorensis Bergia cerina Gibsonnemertes spectabilis Crandallia hubrechti Drepanophorus rubrostriatus Drepanophorus modestus Brinkmannia mediterranea Drepanobanda trilineata Uniporus borealis Uniporus acutocaudatus Uniporus hyalinus

Phylogenetic approaches to nomenclature accommodate some newly preferred evolutionary hypothesis better. Furthermore, I have chosen to discard the name Punnettia since it carries too many unwanted connotations to various clades (a messy name). Instead, I have reintroduced the name Xenonemertes for the `most-inclusive clade comprising Xenonemertes pilorhynchus and Xenonemertes rhamphocephalus'. The reference taxa are chosen so that one of them represents the species originally described as Xenonemertes (i.e. X. rhamphocephalus). Two of the new species also fall under this name. Furthermore, I have chosen to discard the name Fumionemertes to avoid confusion with inclusivity compared with the previous hypothesis. Instead I have used the name Polyschista for the `least-inclusive clade comprising Polyschista curacaoensis and Polyschista maldivensis'. 4. DISCUSSION

(a) PSD versus PSR

This is the ¢rst study to compare the PSD and PSR in a real phylogenetic case. Basically, the results from this case study support the conclusions reached in the theoretical paper by HÌrlin (1998b). The di¡erence in practical outcome between these two approaches is minimized if uninomials are also used at the species level (see below) and such an approach is therefore encouraged. This study also reveals that it is not necessarily the case that a PSD provides more stable names since drastic changes may occur (i.e. the Punnettia case above). One may of course argue, as Schander & Thollesson (1995) and Cantino et al. (1997) did, that e¡ects like the Punnettia case are due to poorly chosen reference taxa in the ¢rst place. However, the point is that, in most cases, it is impossible to predict the behaviour of reference taxa (or other taxa) in future analyses. Hence, it is important to rerun the phylogenetic analysis when new taxa are encountered ö it is a huge fallacy to try and ¢t them into a cladogram based on diagnostic or apomorphic characters identi¢ed in an analysis where these taxa did not participate. Such an approach does not give full primacy to the congruence between all available information and, as a corollary, does not acknowledge the primacy of the tree (HÌrlin 1999). In this paper I have shown that some of the supposed practical advantages of a PSD, such as more stable names and fewer new names, are overstated. A PSR may even provide a system where fewer new names need to be introduced (¢gure 2). Furthermore, a PSR gives true primacy to tree topology and introduces names that refer to these topologies without dragging old names and their various connotations around. Thus, for both theoretical and practical reasons, I argue that a phylogenetic system of reference is superior in handling phylogenetic information throughout changing phylogenetic hypotheses.

(b) Uninomials

Cantino (1998) argued in favour of hyphenated uninomials where the ¢rst part (the traditional genus name) does not necessarily indicate a relationship. This was basically also the idea behind the phylogenetic nomenclature of the eureptantic nemerteans proposed by HÌrlin (1998a) and an idea also supported by Gri¤ths (1976) and Sundberg & Pleijel (1994). The advantage is


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that the species names are kept intact but at the cost of a potentially high degree of confusion. Using pure uninomials, as suggested by Schander & Thollesson (1995) and HÌrlin (1999), is a better way of communicating phylogenetic information throughout changes of hypotheses since it avoids such confusion and uniqueness is guaranteed due to the nested hierarchy of the names (e.g. Sebae, Drepanophorella, Inaequifurcata, Aequifurcata). This is a powerful way to avoid problems with mandatory names. Many of the problems above can be avoided if one uses uninomials instead of binomials for traditional species taxa. For instance, far fewer new names need to be introduced in the PSD case, since not all branches need to go under a more inclusive name than their uninomial. However, the problem of de¢nitions dictating the naming procedure for more inclusive taxon names remains for the PSD approach. Similar bene¢ts are valid for the PSR approach and, since no de¢nitional rules obey, a PSR is very e¡ective in handling the taxonomists desire to name parts of trees throughout changing choices of hypotheses. Thus, regardless of whether one's preferences lie with a PSD or a PSR, uninomials are preferable since they are much better suited than binomials to handling changes in phylogenetic hypotheses. Therefore, uninomials should be used at all levels in phylogenetic taxonomy. (c) Naming ö a prologue

Phylogenetic biology deals with historical entities (clades) that may span over million of years. These clades need to be inferred from data on extant and fossil taxa. The assumption that these clades, in a philosophical sense, are individuals, together with their inferential nature, comes with the corollary of what should count as the same individual under di¡erent phylogenetic hypotheses (HÌrlin 1998b; HÌrlin & Sundberg 1998; Thollesson 1998), i.e. what happens with the way a name refers when we chose another hypothesis than the one used when introducing the name. The basic idea is that a name keeps referring to the same taxon through changes of phylogenetic hypotheses (De Queiroz & Gauthier 1992, 1994). In traditional Linnean taxonomy a name is tagged to a particular type specimen and associated with a particular rank in the Linnean hierarchy. The type specimen is deposited in a museum as a reference for future research. In the same sense as the type specimen is the name bearer for the species, so is the species for the genus, the genus for the family and so forth. In phylogenetic nomenclature, two or more taxa are chosen as nomenclatural `types' so that the names are always tied to monophyletic groups. To put these issues in a broader perspective, consider the following analogies. (The analogies should be interpreted under the assumption that primacy is given to hypotheses rather than actual individuals. The reason is that reconstructions and hypotheses are the currency in phylogenetic biology rather than the individuals per se and, therefore, e¡ects such as horizontal switches are commonplace when new hypotheses are accepted.) My name is Mikael. Assume that the heart in my body de¢nes my name, which is to say that the heart in my body is of the type of the name `Mikael'. In other words, the body with that particular heart goes under the name

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Mikael. Now assume that I have a severe accident and am left brain dead. Simultaneously, Charles su¡ers from heart problems and is waiting for a new heart and a heart transplant. The doctors decide to use my heart to save Charles. So what happens when Charles wakes up after surgery? Well, if we apply the rules of modern biological nomenclature (whether Linnean or phylogenetic does not really matteröit is just a matter of number of types), then Charles no longer goes under the name Charles. Instead, his new name is Mikael since the type (my heart) for that name is now in his body. In phylogenetic taxonomy (PSD) one uses extant species to de¢ne a taxon name such as `the least-inclusive clade comprising A and B'. Assume that the name Philadelphia Flyers is de¢ned as the hockey team containing Mikael Rhenberg and Eric Lindros. That is, just as taxa A and B are used as types in the phylogenetic example, so are Mikael Rhenberg and Eric Lindros used as types to de¢ne the name of a particular National Hockey League hockey team. Later Mikael Rhenberg was traded to Tampa Bay in Florida and, thus, no longer plays for the Philadelphia Flyers. So what happens to the name Philadelphia Flyers? Does the name Philadelphia Flyers now refer to the Philadelphia Flyers and Tampa Bay collectively as one team? This of course also depends on the way the name Tampa Bay is de¢ned. To make things even more complicated, today Mikael Rhenberg is back in the Philadelphia Flyers and once again the name Philadelphia Flyers refers to the `same' team as at the beginning of this exercise. Similar changes are, as we have seen above, commonplace in phylogenetic systematics with names de¢ned phylogenetically and intentionally. The result is potential confusion since priority is given to nomenclatural rules and de¢nitions rather than decisions based on phylogenetic hypotheses. Both these examples are from the world outside biological nomenclature, but I think that they are good illustrators of the problem of de¢nition. Naming individuals in the non-scienti¢c community does not contain a de¢nitional component. For instance, before I got married my surname was Svensson. My wife and I could either chose to keep our own names, Svensson and HÌrlin, respectively or to use one of them to refer to us being married. The important point is that this is a choice we had to make in order to best re£ect the new situation. That is, when faced with new situations we must reconsider how we want a name to refer. If nomenclatural rules similar to the ones used in biological nomenclature are obeyed then a great deal of £exibility in terms of reference would be lost. Taxon names are important labels for communicating hypotheses of evolutionary relationships. The PSR that I proposed in an earlier paper (HÌrlin 1998b) is thus more in line with a common-sense approach to naming since it always puts the named item in focus rather than nomenclatural rules. I thank Mats Envall, Michael T. Ghiselin and Fredrik Pleijel for comments on various drafts of this manuscript. Mikael Thollesson and one anonymous referee are gratefully acknowledged for providing constructive comments on the manuscript. I am also grateful to Helmut Sattmann (Natural History Museum in Vienna), Jacob Van der Land (National Museum of Proc. R. Soc. Lond. B (1999)

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