The Evolutionist Phylogeneticist Conflict and Classification

Phylogenetic systematic practice requires the conversion of a cladogram into a classification. Hennig (1966) realized that the cladistic system would raise conflicts with other types of classifications. First, branch points in the cladogram are delineators of successively inclusive (increasingly higher ranking) groups as we move toward the root. All subgroups of a group have the same character states that define the more inclusive group, unless recognized reversals have occurred. Any two sister groups are defined by the corresponding two descendant groups found "upstream" of the tree from a bifurcation. This form of classification insists that only monophyletic groups, defined as all of the descendants of a single ancestral species, be recognized. It excludes the recognition of paraphyletic groups - groups that include an ancestor but not all of the descendants. The objection to paraphyletic groups stems from their definition by shared ancestral characters. In many cases, this set of characters fails to define the group under consideration, as other related groups also share the ancestral character states (e.g., Farris 1979).

Paraphyletic groups may preclude our ability to recover the cladogram. Consider the relationships among birds, crocodiles, and other reptiles. The group [crocodiles, other reptiles] is ambiguous, as it is a paraphyletic group bound by symplesiomor-phies, or shared ancestral characters. The states uniting this group are also characteristic of the entire Amniota. The nuculid and solemyid bivalves are united on the basis of an ancestral gill, which would also define a group larger than the class Bivalvia! This hardly gives precision to the defined group Protobranchia. Yet, by describing taxon A as "strongly divergent" and referring it consistently to a clado-gram, phenetic divergence information might be retained. Wiley (1981) discussed various systems, describing degrees of divergence, to annotate classifications.

The objections to the implications of the Hennigian system have mainly come from evolutionary systematists (e.g., Mayr 1969; Simpson 1961, 1975) whose objectives overlap only partially with phylogenetic systematists. Although evolutionary systematics aspires to produce a classification based on monophyletic groupings, the rankings of taxa are not based exclusively on position in a tree. Increasing degree of phenotypic difference from related taxa and numbers of species in a taxon both are used as criteria to raise a taxon to a higher rank, which may create para-phyletic groups, as the group most closely related to the divergent group is defined inevitably by ancestral - not derived - features (see Farris 1975).

Before pressing on to the more arcane aspects of defining classifications by cladis-tic logic, I want to emphasize the main principle that is at stake here with regard to macroevolution. Nearly all current macroevolutionary studies of changes in diversity use the traditional database of systematics, which uses a taxonomic hierarchy composed of a mixture of paraphyletic and monophyletic groups. As we shall discuss further in chapter 7, this taxic approach may obscure some appearances and extinctions of monophyletic groups, which calls into question a practice from which many conclusions about macroevolution derive.

Evolutionary systematists have also objected to the Hennigian classification system, owing to the effect of the addition of newly discovered taxa to an existing classification, which, of course, must add still more branch points. The discovery of new taxa would lead to continual revisions of classifications. The instability thus created becomes more worrisome as the added branch points are closer to the root and therefore define more and more higher taxa. This is particularly true of newly discovered fossils bearing ancestral characters. As Mayr (1974) noted, the "discovery" of the birds immediately defines a synapomorphy with crocodiles, making the other reptiles more distantly related and increasing the overall taxonomic rank of the group [(birds, crocodiles)(other reptiles)]. Using ancestral plus derived states, birds are more divergent phenotypically from [crocodiles, other reptiles] than either of the two reptile groups is to the other. It therefore seems intuitively reasonable to separate the birds off in a rank equal to the crocodiles plus other reptiles (e.g., Michener 1978).

Apparent progressive sequences create the most problems because evolutionary sys-tematists wish to recognize grades of evolution by equal ranks, whereas the Hennigian system seems to require that more "advanced" groups be of lower rank. For example, evolutionary systematists would accept the equal ranking of pelycosaurs, therapsids, and mammals, because it is believed that the mammals, even though derived within the therapsids, are an important new grade of organization, which permitted an extensive evolutionary radiation. Again, as long as the cladal structure is preserved explicitly, one does not necessarily sacrifice any information. This issue lies at the heart of the analysis of fossil data, particularly that of taxonomic survivorship and longevity studies (e.g., Levinton 1974; Raup 1978; Van Valen 1973b). Such analyses would lose important information, if the classification obeyed Hennigian principles, because an analysis of taxonomic longevity will have ecological meaning only if ecologically equivalent groups are contrasted (Van Valen 1984). To make the birds subordinate to the reptiles, for example, masks their possibly equal importance in ecological effects within natural communities, degree of geographic coverage, similar number of species, and so on. Thus, current studies of taxonomic survivorship or diversity at, say, the family level, could benefit from the retention of the evolutionary systematists' frame of reference, as long as it does not obfuscate the genealogy.

As shown by Figure 2.14, progressive sequences yield asymmetrical trees that resemble combs, which emphasize the classification problems mentioned above. Groupings of one taxon with large numbers of others are inevitable, with the sister group criterion. Thus, taxon A would be of equal rank with the taxon grouping [B,C,D,E,F]. Hennig (1966) adhered to this requirement strictly, whereas others complain about redundant taxa; that is, one taxon is monotypic at several rank levels (in Figure 2.14, A is monotypic at five ranks, B at four, etc.). This problem can be solved readily (e.g., Schuh 1976; Wiley 1981). Monophyly is the only essential requirement for a consistent cladistic classification. It should be possible to retrieve the cladogram from the classification; preferably, redundant taxa should be minimized. Schuh (1976) solved this problem for the hemipteran family Miridae. All of the taxa are arrayed in a linear pattern of branching, as in Figure 2.14, which would imply a phyletic evolutionary sequence if synapomorphies along the tree are based on progressive changes of the same characters. All taxa are given equal rank, but the order of the list implies the distance along the main branch toward the taxa with the most derived states. Any listing that can retrieve the cladogram is acceptable; this leads to considerable flexibility in ranking. A proposed cladogram for the mammals (see chapter 6) deals similarly with such cladograms.

The potential problem posed by ranking according to degree of phenotypic divergence, or gaps, can be seen in Figure 2.15. Here, group [C,D] is defined on the basis

Figure 2.14. Two possible classifications that retain the genealogical information of the cladogram. On the left is a classification derived from a cladogram of six taxa. On the right is a classification of the hemipteran family Miridae (after Schuh 1976), with all taxa given the same rank.

Figure 2.15. A cladogram where group [C,D] is phenetically divergent from the closest living relatives [A,B]. Taxa X, Y, and Z are hypothetical newly discovered fossils that span the gap.

of a phenotypic difference between it and [A,B]. But what if fossil intermediates X, Y, and Z are found? The reason for the gap suddenly disappears. The use of gaps thus imposes an instability on the definition of rank. This problem is only exacerbated when examining the fossil record. Although groups notably divergent from their closest relatives are common, other taxa seem to acquire gradually that final complex of characters that gives us the total character set that defines the taxon (chapter 6). Yet we wish to say: "That is a mammal!" This can be recognized implicitly in the accepted taxonomic separation of the ancestral Mesozoic mammals from the therapsids, classed as reptiles. The use of gaps in classifications involves the use of an ecological-evolutionary model as a classificatory criterion. The origin of a highly divergent group is often associated with the movement into a major new habitat and lifestyle (Mayr 1969; Simpson 1953). The assignment of high rank to such a divergent group is a recognition of an ecological-morphological advancement. This criterion, of course, is external to the genealogical structure. This would be well and good if there were one such criterion. But what if there are others, such as mode of development? It would be best to have a system that always refers simply back to the genealogy. Without such a framework, the intent behind classifications will be ambiguous, because rank is used in so many different ways by different investigators.

Because both cladists and evolutionary systematists seek some sort of genealogically based classification, I am sure that the common goal will tend to yield more imaginative solutions to the problem of ranks. Evolutionary systematics recognizes that monophyletic groups will be defined by certain sets of characters, but these characters will vary from group to group and can be discovered only by some sort of character analysis (e.g., Mayr 1969). This conclusion is close to the cladistic approach. We can see more fundamental issues in common between cladists and evolutionary sys-tematists than differences, even though the two camps usually seem to attempt to accentuate the intellectual gaps. The present trend in systematics, designed to reflect phylogeny in classifications, is healthy, no matter what the particular approach. In many respects, it is rather useful that a plurality of phylogenetic approaches be maintained to help sharpen our understanding of evolutionary classification.

Before the era of evolutionary systematics, taxonomists tended to employ restrictive monothetic criteria, where specific characters were used to define differences at a given taxonomic level (e.g., internal characters define only ordinal-level differences in brachiopods). In these idealistic classifications, restrictive monotheticism implied that key characters defined given taxonomic levels (see Mayr 1969 for discussion). This approach, still an integral part of many existing classifications, derives origi nally from Cuvier's notion of subordination of characters, which on the one hand saw organisms as perfectly integrated living functional creatures, but on the other hand saw them as defined inherently by crucial traits that defined the essences of the taxa. This is well illustrated by the brachiopods, where certain characters were believed to define differences among genera, families, and orders (see Williams and Rowell 1965). The monothetic nature of brachiopod classifications of the late nineteenth century (e.g., Beecher 1891; Schuchert 1893) persisted into the middle of the twentieth century (e.g., Cooper 1944). Restrictive monothetic classifications often lead to the spurious uniting of groups with no genealogical significance. Newell (1965, 1969) rejected such simple attempts to classify on the basis of one criterion.

Modern evolutionary systematic approaches have departed from the restrictive monothetic system used, for example, in the brachiopods. Williams and Rowell (1965) recognized that many different characters may define evolutionary change in this group. They concluded that (p. 223)

...all such schemes proposed in the past are incompatible with the evolutionary history of the phylum. Previous monothetic, non-evolutionary classifications were regarded as

"only catalogues ... deliberately arranged for quick identification of stocks."

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