Defining the Genus Category

Genus definitions use four criteria, two lines of evidence (phenotypic and genetic) and fall into two categories. The four criteria are the relationships among taxa, information about their adaptive grade, estimates of the genetic distance that separates them, and the estimated time of divergence. At least one researcher (Dubois, 1988) has suggested that a fifth criterion, evidence of hybridization between species, should be the primary criterion for grouping species into genera. However, even if this suggestion had any merit, it is not clear how evidence of hybridization could be obtained from the fossil record of extinct taxa.

One, or more, of the criteria has been used, or combined, to generate genus definitions that belong to two categories. The first category draws upon phenotypic evidence to make inferences about adaptive grade, and uses both phenotypic and molecular evidence (if the latter is available) to make inferences about relationships. Definitions in the second category rely solely on genetic evidence for either estimates of genetic distance, or for generating estimates about the timing of evolutionary events, which are then converted into criteria for recognizing genera.

Four genus concepts in the first category are reviewed. The first of these, the most widely used genus concept, is the one suggested by Mayr (1950). Mayr proposed that "a genus consists of one species, or a group of species of common ancestry, which differ in a pronounced manner from other groups of species and are separated from them by a decided

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morphological gap" (Mayr, 1950: 110), and he went on to state that the species united in a such a genus must "occupy an ecological situation which is different from that occupied by the species of another genus, or, to use the terminology of Sewall Wright, they occupy a different adaptive plateau" (Mayr, 1950: 110). Thus, according to Mayr, a genus is a group of species of common ancestry (which for Mayr includes both monophyletic and paraphyletic groups) that is adaptively both homogeneous and distinctive. Mayr et al. (1953: 50) suggest that if "unrelated species acquire a superficial similarity owing to parallel adaptations to similar environments" then evidence about relatedness should be given precedence over evidence about adaptation.

The second of the four concepts in the first category was proposed by Willi Hennig in his 1966 volume "Phylogenetic Systematics." Hennig (1966) suggested that only monophyl-etic groups (i.e., all - no more and no less - of the species descended from a recent common ancestor) should be accepted as valid genera, and he promoted the view that, within reason, taxa arising at the same time should be assigned the same taxonomic rank (as will be evident, the use of time as a criterion has been advocated by other researchers since Hennig).

The third of the genus concepts in the first category was proposed by Wood and Collard (1999). These authors suggested that a genus should be a monophylum whose members occupy a single adaptive zone. This definition differs from Mayr's (1950) in that it excludes paraphyletic taxa, but contra the interpretation of Leakey et al. (2001) and Cela-Conde and Altaba (2002), it does not require the adaptive zone of a genus to be unique. Wood and Collard (1999) suggested that two, or more, genera based on different mono-phyletic groups could occupy a similar adaptive zone.

The final genus concept in this first category was outlined by Cela-Conde and Altaba (2002). It is similar to the Wood and Collard (1999) definition in that it requires the component species to be monophyletic, but it differs in that it allows one species in a genus, the species germinalis, to be in a different adaptive zone from the other species in the genus.

The two genus concepts in the second category draw solely on genetic evidence. The first concept focuses on evidence about genetic distance. It suggests that species should be grouped into genera if the genetic distance between them is the same, or less than, the typical genetic distance between pairs of congeneric species in other animal groups (Watson et al., 2001).

The second genus concept in the genetic evidence-only category combines genetic distance and time. With respect to primates it suggests that the species in a primate monophy-lum should be included in the same genus if they originated between 11 and 7 Ma BP (Goodman et al., 1998). Wildman et al. (2003) justified the 11-7 Ma BP date because the majority of genera in other mammalian orders arose in that time period. As Morris Goodman and his colleagues have noted (Goodman et al., 1998, 2001; Wildman et al., 2003), the implication of their interpretation is that the genus Homo would not only include all hominin taxa, but also chimpanzees and bonobos. Subsequently, Colin Groves (one of the authors of the Goodman et al., 1998 study) used a mix of paleontological and genetic evidence to propose an origin time of between 7 and 4 Ma BP as the criterion for delineating extant mammalian genera (Groves, 2001). This led Groves to retain Homo and Pan as separate genera. However, even more recently Groves and a colleague have proposed that primate monophyletic groups should be recognized as genera if they originated between 6 and 4 Ma BP (Cameron and Groves, 2004). If this criterion were to be adopted, the genus Homo would embrace most, if not all, currently accepted hominin taxa.

So, what can we learn from these thumbnail reviews of genus concepts? First, that the sensible strategy is to adopt the adage that "all genera should be clades, but not all clades are genera." Second, most of the researchers who focus their research on the hominin fossil record implicitly, if not explicitly, subscribe to a genus definition that blends information about clades and grades, and this is certainly the case for the way most of these researchers interpret the genus Homo. It is widely assumed, but rarely articulated, that the species included in Homo form a monophyletic group, and it is also widely assumed, but also rarely articulated, that the taxa all share functional characteristics or competencies. In the case of Homo these functional characteristics or competencies have included the ability to use complex language, the ability to make the only type of tool, stone tools, that can be reliably detected in the early archeological record, and the ability to hunt.

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