Advantages of the Genealogical Approach

Genealogies and character transitions. A framework established from a genealogical algorithm permits a useful analysis of character variation in the context of macro-evolutionary hypotheses. Many macroevolutionary hypotheses attempt to provide mechanisms to explain differential taxon longevity. Claims that taxon longevity depends on biogeographic range (e.g., Boucot 1978; Jackson 1974; Levinton 1974) or that taxon longevity is the result of differential speciation rate or survival of species (e.g., Stanley 1975; Vrba 1983) may depend partially on the nature of character variation within the clades under consideration. In many cases, adaptations of individuals influence the susceptibility to extinction of species and larger taxa. Although speciation rate may ensure survival of a taxon, the possession of certain characters may permit an entire clade to outlast others or might permit descendants of a given clade to invade a new habitat. The testing of such ideas requires a mapping of character transformations on genealogies.

Consider the following hypothesis: Phenotypic evolution occurs because of species selection (Eldredge and Gould 1972; Stanley 1975). Levels below and above the species level are thus irrelevant to the evolutionary trend, which is a net change in character states over time. Take a hypothetical phylogeny of bivalve mollusks (Figure 2.2). A species bears character state A1, representing a compressed elongate shell, and character state B1, representing lack of ornamentation. The clam therefore has a morphology compatible with rapid burrowing in soft substrata (Stanley 1970). Let the ancestral species split into two daughter species. A split of each daughter species results in four taxa. Extant taxa T1 and T2 bear the ancestral character states A1 and B1. Extant taxa T3 and T4 also bear state A1; they, however, have acquired character state B2, representing heavy ornamentation.

From a functional morphological point of view, the ancestral character state A1 interacts with the state of character B, which determines the derived state defining the genealogical groups {T1,T2} and {T3,T4}. Let us call these two taxa "genera." In

Character States AIBI AIBI AIB2 AIB2 Taxa TI T2 T3 T4

Figure 2.2. Hypothetical phylogeny of a lineage of bivalves. See text for explanation.

Ancestor AIBI

our specific example, A1,B1 is a functionally compatible character set, whereas A1,B2 joins two character states that would fail to be functionally harmonious under most circumstances. Squat shells with ornamentation would be preferable in stabilizing the shell on the bottom in swift currents, whereas elongate, compressed shells lacking ornament would be efficient in burrowing. The A2,B1 state is a mixed case, not much good for either function. It would therefore surprise no one if the group {T3,T4} had a higher probability of extinction. Indeed, its evolution probably would have occurred under atypical environmental circumstances.

With this example, we can make several points about the role of functional morphology in predicting relative extinction rates and the basis of extinction. First, the character A defining the {T1,T2,T3,T4} group interacts with the state of the character defining the two included groups. The genus level of response to extinction may be defined by the special set of characters A1,B2, but character state A1 will survive in either of the two taxa: {T1,T2}, {T3,T4}. Thus, taxon mortality at the genus level explains selective loss of the A1,B2 character complex. But this surely is not an emergent phenomenon of the genus level, as we have defined it. The inevitable retention of the A1 character state, moreover, is not readily identified with any taxo-nomic level. Indeed, it is only a matter of coincidence that selection among genera has occurred. Selective mortality can be reckoned from a simple summing of character states. Species become extinct because of the character states they bear; a conclusion that genus-level selection occurs is therefore ambiguous. We can at least, however, identify the taxonomic level at which the crucial combinations of character states result in differing probabilities of extinction.

An improved degree of focus thus emerges from a genealogical approach based on character analysis. At present, a disturbing vagueness plagues the literature. This has been reinforced by the use of taxonomic survivorship curves at many taxonomic levels, with a varied mixture of ecological and evolutionary intents. Levinton (1974), for example, employed the generic level to contrast paleoautecology with taxonomic survivorship among groups of bivalve mollusks. But the generic level was chosen as a matter of convenience, controlled by the available monographic accounts of the Bivalvia. This particular taxonomic level, which did reveal significant differences among bivalve groups, may be irrelevant for the purposes intended, simply because the character complexes involved in autecological aspects of taxon survival were concentrated at another level.

In a similar vein, variance in gastropod form has been found to decrease from the Paleozoic to the present (Cain 1977; Gilinsky 1981). This trend indicates that those taxa deviating from a modal form have tended to become extinct. Is this species selection, as claimed by Gilinsky? Of course species have become extinct. But the selection must be at the taxonomic level corresponding to the acquisition of the set of relatively poorly surviving character states. This may be at a much higher level than that of species and can be properly defined only once a character analysis is done, set against a genealogically based systematic framework.

Genealogical and systematic philosophies. Genealogical investigations may have at least four objectives, which are often intermixed. A character analysis is a study of fea tures of individuals that may be used to construct a classification. The algorithm used to perform the character analysis may be qualitative or quantitative. A genealogy is a network of branchings whose topology reflects the relationships by descent of the taxa under consideration. A classification is an ordering of taxa based on various criteria but usually resulting in a hierarchy of successively inclusive sets (species grouped into genera, which are grouped into families, etc.). The classification may or may not be concordant with the genealogy. Finally, a phylogeny is an inferred genealogical history of a group, hypothesizing ancestor-descendant relationships, biogeography, and so on. The genealogy is only part of the process of producing a phylogeny. Genealogies and phylogenies are hypotheses of relationships and history. To the degree that a classification is meant to reflect a genealogy, it, too, must be regarded as a hypothesis.

Systematics has occupied a central place in the posing and testing of macroevolu-tionary hypotheses. Most of the classic works in the field (Mayr 1942; Mayr 1969; Mayr, Linsley, and Usinger 1953; Simpson 1961) stressed the inherent complexity behind the traditional objectives of systematics. They agreed, however, that a useful classification should account for genealogy and morphological similarity. These two components lay behind evolutionary systematics, an approach that assigns taxo-nomic rank by means of genealogical position in a phylogenetic network and the amount of morphological divergence of a taxon from its ancestral lineage. Phylogenetic reconstruction is mixed with, or follows, classification. The two other major competing systematic philosophies take this mixed strategy to be undesirable.

Phenetics seeks to produce classifications on the basis of overall similarity alone (Sokal and Camin 1965; Sokal and Sneath 1963). Genealogy is not a necessary objective of phenetic classifications, although overall similarity must have some mapping to relationship by descent (Sokal and Sneath 1963). Phylogenetic system-atics seeks to establish a network of genealogically based relationships with no overall similarity criterion employed for classification (e.g., Camin and Sokal 1965; Hennig 1966; Kluge and Farris 1969). Phylogenetic systematists seek to cluster monophyletic groups, or the entire descendant subset of taxa derived from a given ancestor.

Arguments over the preference for any of the three systems usually revolve around several desirable criteria of classifications used by evolutionary biologists:

1. Convenience: The system should yield a classificatory system that is not cumbersome and should be intuitive enough for all to grasp.

2. Congruence: Classifications based on different characters should yield similar results.

3. Genealogy: Most evolutionary biologists desire a classification that reflects evolutionary relationships.

4. Naturalness: Groupings should, in some readily understood sense, reflect directly the character states used to determine the classification (Gilmour 1961).

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