THEORIES AND METHODS IN DIFFERENT APPROACHES TO PHYLOGENETIC SYSTEMATICS

Cladistics 1 ( 1 ): 1 - 1 1

THEORIES AND METHODS IN DIFFERENT APPROACHES TO PHYLOGENETIC SYSTEMATICS

DANIEL R. BROOKS’ AND E. 0. WILEY2 IDeparfmenf of Zoolou, University of British Columbia, Vancouuer V 6 T I W5 Canada

ZMuseum o j Natural History, University of Kansas, Lawrence K S 66045

Absfrad - Systematic techniques may be viewed as symbolic languages which have both surface structure (measures of descriptive adequacy) and deep structure (measures of explanatory adequacy). Phylogenetic systematics is not theory-neutral because its deep structure embodies evolutionary assumptions. Pattern cladistics stands in relationship to phylogenetic systematics as a part to a whole and is indistinguishable from phylogenetic systematics unless an artificial dichotomy between surface structure and deep structure is maintained. Direct observation of ontogeny as a means of polarizing characters also stands as a part to a whole relative to outgroup comparisons. Direct observation of ontogeny does not resolve any cases that outgroup comparison fails to resolve, and outgroup comparison does resolve some cases where direct observation fails.

Introduction

An interesting difference of opinion characterizes phylogenetic systematics at the moment. A portion of theoretically-minded phylogeneticists, originally united by the belief that contemporary evolutionary theory was inadequate, has split into disparate groups according to their solution to the problem. One group thinks the solution lies in the search for a new, or at least more complete, theoretical framework for biological evolution; that is, one that minimally alleviates the problems which arise from the findings of phylogeneticists. The other group thinks that the problem itself stems from our failure to appreciate the necessity for systematics to be independent of evolutionary theories of any kind. The two groups view systematics as (1) a technique for producing analytical statements about phylogenetic relationships used in testing different hypotheses of evolutionary mechanism, or (2) a technique for producing a general summary statement about natural order which will remain stable despite theorizing about the causal agent(s) responsible for its existence.

Interestingly, both groups have recently focused on the relationship between systematics and ontogeny, one of the great recurring themes of biology: Wiley and Brooks (1982) among the “phylogeneticists;” Nelson and Platnick (1981) and Rosen (1982) among the “pattern cladists.” In this paper, we will examine the logical relationships between these two “schools of thought.” Our discussion will be based in part on the analogy between systematic techniques and languages (Platnick and Cameron, 1977; Brooks, 1981). Then we will investigate some of the implications of each group’s view of the importance of ontogeny to systematics. We will contend that the only logical distinction between the two approaches is degree of extension of findings, but that this has important consequences when the approaches are considered fundamentally opposed.

“Justification” of Systematic Techniques By “justification” we simply mean explication of the reasons behind one’s reliance

on or expectations of an analytical technique. We know that the word justification carries a heavy burden of connotation. We use it only as it has been applied in formal language theory.

Brooks (1981), following a cue from Platnick and Cameron (1977), suggested that systematists might profitably consider systematic techniques as symbolic languages (or perhaps even dialects of the same language) of comparative biology. From the field of formal language theory, we have adopted a well-established set of criteria for justifying

2 CLADISTICS [VOL. 1

symbolic languages (see Chomsky, 1965), including biological systematics (Fig. 1). One may view the operations of formal language theorists in simple terms: first, one

builds one or more models of language according to a set of input rules, and determines how it (they) function(s). Then, one may ask three kinds of questions: (1) does the symbolic language always function according to its input rules?; (2) do those rules have any emergent properties of empirical generality?; and (3) does the symbolic language correspond to any real language? Phylogenetics can be viewed as an empirical method for finding most parsimonious configurations of data based on outgroup comparisons, ontogenetic precedence, or maximum congruence. If it is found empirically that such claims are true, phylogenetics is justified, according to question (1) above. On the other

GENERATIVE G R A M M A R

I DEEP STRUCTURE SURFACE STRUCTURE

SEMANTICS PI-IONETIC S

SYSTEMATIC TECI-INIQUE

TREE

IN T E R IVA L JU STI F I CAT I0 N E X TERNA L J U STI FI CAT 10 N EX PLAN AT0 RY ADEQUACY DESCRIPTIVE ADEQUACY

EMPIRICAL CONSTRUCT

Fig. 1. Relationships among major components of formal language analysis (top) and formal systematic analysis (bottom) adopted in this paper.

19851 DIFFERENT APPROACHES TO PHYLOGENETICS 3

hand, any systematic technique is equally well justified by the same criterion. This sort of justification is important a priori, because techniques which produce results incon- sistent with their input rules cannot be studied analytically. This is the weakest of all justifications, however, because it does not allow one to choose one method over another as being most appropriate for a given purpose. This form of justification is "neutral" regarding the question of appropriateness.

A stronger form of justification would be to advocate phylogenetics because it provides the most informative (Farris, 1979) or least ambiguous (Brooks, 1981) summation of available data. This allows one to express an empirical preference for a particular method but, just as with the first form of justification, the method is justified on the basis of properties extrinsic to the organisms being analyzed. This is justification according to question (2) listed above. Such justification is called surface structure justijkation, external

just$cation, or measures of descriptive adequacy (Chomsky, 1965; Fig. 1). In formal language theory, surface structure refers to the phonological component of a language - given the symbols, is it possible to pronounce the message correctly? As far as systematic techniques are concerned, surface structure justification refers to any emergent properties of empirical generality discovered by repeated applications of the method. Phylogeneti- cists have so far discovered three major generalities. First, there is no empirical support for taxonomic grades as discrete systematic groups. Second, structural attributes of organisms are better correlated with their species' systematic position than with any functional peculiarities of their habitat. And third, the patterns of systematic relationships among species analyzed thus far exhibit non-random geographic distribution; they are highly correlated with each other and with the geological history of the areas in which they occur.

Methods can also be justified in terms of deep structure or mmures of explanatory adequacy (Chomsky, 1965; Fig. 1). Deep structure justifications relate the structure of the symbols to the meaning of the message. In analysis of languages, this refers to the semantic component; it is justification in terms of the third question listed above. A language theorist might ask if the attributes of a symbolic, formal language conform to any actual language. For systematic biology, we might ask what is the significance of the empirical generalities manifested by the surface structure of phylogenetics. Do they conform to some causal theory? Do they exclude some other causal theories?

In a purely formal sense, a symbolic language which can be shown to be justified in terms of surface structure or of deep structure only is incompletely justified. This does not mean that either is right or wrong. A systematic technique based on a particular theory but having no empirical protocol is incompletely justified, and that casts doubt on the validity of the causal theory. Alternatively, a systematic technique justified only in terms of surface structure has not demonstrated its relevance to studies of attributes of the living world which it classifies. Such methods risk becoming sterile exercises in methodological elegance.

The current debate among phylogeneticists is not the first time these issues have arisen. This suggests that the issues themselves have never been adequately confronted in the past. For example, evolutionary taxonomy seems to be an area of systematic endeavor which draws its deep structure justification from neo-Darwinism (or at least some form of it), but which has not produced an explicit methodology. Numerical taxonomy presented formalized empirical protocols which were avowedly atheoretical (Sokal and Sneath, 1963; Sneath and Sokal, 1973). Debates between evolutionary taxonomists and pheneticists have always been inconclusive and unhelpful because each side judged the other by its own criterion. Thus, Hull (1979) summarized the tone of such inevitably inconclusive debates: "Invariably the advocates of a particular methodology can know what they need to know while their opponents can never hope to know what they need to know."


Phylogenetic systematics was based on the assumption that evolution had occurred, and was responsible for systematic relationships. Phylogenetics was also characterized by an explicit methodology (Hennig, 1966). Phylogenetics would therefore seem to be a fully justified systematic technique, using the formal criteria which we have adopted. This means that phylogeneticists can debate pheneticists on empirical grounds, and can also debate evolutionary taxonomists on evolutionary grounds. As the recent literature attests, they have been busy doing just that.

Why, then, do some phylogeneticists now advocate withdrawing systematics from the arena of discussions of evolutionary theory (e.g., Platnick, 1982; Patterson, 1982; Brady, 1982)? Without attempting to divine personal motivations, we may discuss a possible framework which could result in such a strategy. Let us consider the interface between phylogenetics and evolutionary taxonomy. Productive debate cannot take place over surface structure considerations. Evolutionary taxonomy is immune to purely empirical evaluation because evolutionary taxonomists find their justification in the deep structure of their methods. Evolutionary taxonomists can always avoid becoming phylogeneticists by asserting that phylogenies are more than genealogies; they are also records of adaptive radiations. Thus, phylogenetics could not possibly give an adequate picture of phylogeny, despite its empirical rigor. How, then, could phylogenetics deal with the deep structure issue? One way would be to assert that phylogenetic systematics is the explicit empirical method of neo-Darwinian systematics. In other words, the theoretical principles of neo-Darwinism are better served by phylogenetics than by evolutionary taxonomy. This would seem to be at least the implicit attitude of most working taxonomists who use phylogenetic systematics. Hennig (1966) certainly gave no indication that he thought his method conflicted with current evolutionary theory. This line of argumentation has been denied by a relatively small group of phylogeneticists. For them, the emergent empirical generalities of phylogenetic analyses conflict with at least some aspects, if not the general explanatory framework, of neo-Darwinism.

This group of phylogeneticists exhibits divergent behavior based on their solutions to the problem of this perceived incompatibility between neo-Darwinism and phylogenetics. Beatty (1982) called one group “phylogenetic cladists” and the other group “pattern cladists.” The first group thinks that the incompatibility results from the incompleteness or incorrectness of neo-Darwinism, and that a new theoretical framework is needed. The second group insists that the problem stems from a basic misunderstanding about the role of systematics in general. They stress that a stable systematic framework is a necessary prerequisite for any theorizing about evolutionary mechanisms; this is true enough. From this, they conclude that it is vital for systematics to be independent of any causal theories. The issue of deep structure justification is inappropriate in this latter view.

Divergent opinions and behavior on an issue do not always indicate that there is a valid dichotomy at stake. In this case, the impression of a dichotomy stems from the belief that there can be theory-free science. In terms of our analogy with language theory, it is not considered valid to assert that surface structure and deep structure exist inde- pendently. There is a semantic component implied by any syntactical structure, whether one chooses to investigate it or not. Conversely, expression of meaningful information is not possible without syntactical rules. If there is a systematic technique with an empirical protocol, there is at least one causal theory which provides deep structure justification for it, or the technique is irrelevant for biological study. Whether one is interested in investigating the question of deep structure or not is a matter of personal interest (Patterson, 1982). The only difference in the views is one of degree of complete- ness. For one group, the emphasis is on surface structure; for the other, emphasis is on what surface structure can tell us about deep structure.


For example, Platnick (1979) wrote,

The refusal to accept incongruence (i.e., randomness) as a feature of the real world leads us back to what was suggested as the first principle of cladistics: that nature is ordered in a single specifiable pattern. Admittedly, that’s not a scientific theory. . .[but] such a metaphysical statement, when translated into a methodological rule, is a necessary underpinning of all science.

Compare that statement with the following one by Papentin (1980),

Elucidation of a pattern based on analysis of natural complexity, without any information other than the pattern itself, is only a hypothesis concerning the reality of the pattern itself. It says nothing about the natural laws underlying its existence.

The two statements seem to us to be in fundamental agreement, diverging only at the point of probing the causal explanations for empirical findings, and then only slightly. Platnick‘s statement is neutral about whether or not one would use systematics to investigate causal mechanisms, whereas Papentin assumed that such investigations would be the rationale for seeking such patterns. Platnick (1982) and Patterson (1982) have stressed the empirical agreement between pattern cladists and phylogeneticists. This close agreement between the two views can be obscured if one insists on a strict dichotomy between deep structure and surface structure, or between process and pattern in this case. For example, Wiley (1979) advocated using phylogenetic analysis to help improve our under- standing of speciation. Platnick (1979) stated: “these arguments reflect a concern with the mechanism of speciation, as if the purpose of taxonomy were to investigate the process of speciation (be it dichotomous or otherwise), come to some conclusions about it, and then construct classifications in such a way as to insure their compatibility with those particular conclusions about how the evolutionary process has functioned (Wiley, 1979, argues in favor of this position).” But this conclusion is valid only if the false dichotomy is maintained; that is, if one concludes that there are two different approaches to phylogenetic systematics, rather than a single one which can be extended (e.g., Wiley, 1979) or not (e.g., Platnick, 1979) according to the interests of the investigator. The positions are internested, not dichotomous.

Our position is that pattern cladistics is indistinguishable from phylogenetic systematics which stops short of probing for causality. It thus offers nothing which is not provided by phylogenetics. We may investigate this assertion further by examining the consequences of different methods for polarizing characters.

Outgroup Comparisons and Direct Observation of Ontogeny as Methods for Polarizing Characters

Outgroup comparisons (Lundberg, 1972; Stevens, 1980; Watrous and Wheeler, 1981 ; Wiley, 1981) refers to the method of polarizing characters in which that character of a homologous series which is found among members of the group being classified and in relatives (“outgroups”) of that group is designated the plesiomorphic character. Direct observation of ontogeny (Nelson and Platnick, 1981) refers to the method of polarizing characters in which characters appearing in the ontogenies of both of a pair of species are designated plesiomorphic whereas characters appearing in the ontogeny of only one are designated apomorphic. Both of these techniques can be applied mechanically, and will produce a cladogram. Thus, we can apply each criterion to a given example, and determine whether or not the criteria produce divergent results.

Figure 2 depicts a cladogram construction using outgroup comparisons to polarize characters from different developmental stages, or semaphoronts (Hennig, 1966). In Figure 3, we may see that outgroup comparisons and direct observation of ontogeny need not produce the same answer. One might view the results in Figure 3 in two differ-


1 A D C D 2 A B C D E 3 A B C D E F 1 A B C D E F G

1 2 3 4

Fig. 2 . Phylogenetic analysis based on polarizing characters from different developmental stages by outgroup comparisons. Species 1 is the outgroup. The presence of stages A, B, C, and D is synapomorphic for species 1, 2, 3, and 4 (2a). The presence of stage E is synapomorphic for species 2, 3, and 4 (2b). The presence of stage F is synapomorphic for species 3 and 4 (2c). The presence of stage G is autapomorphic for species 4 (2d). The resulting cladogram is shown in 2e.

1 ABCD 1

2 ABCDE 3 ABCDEF 4 ABCDEFG

No G

Fig. 3. Polarizing characters based on direct observation of ontogeny and outgroup comparisons. Direct observation of ontogeny for species 1,2,3, and 4 produces the left-hand cladogram regardless of the outgroup. If species 1 is the outgroup, the same cladogram results. If species 4 is the outgroup, indicating evolution by means of terminal deletion, the cladogram on the right is produced.

ent ways. On one hand, it is true that the change in outgroup is responsible for the divergent cladograms, and one might therefore assume that outgroup comparisons represent a potential source of systematic instability which direct observation of ontogeny avoids. On the other hand, it can be shown that cladograms based on outgroup comparisons will diverge from those based on direct observation of ontogeny under predictable circumstances.

There are three general classes of changes in ontogenetic programs: (1) addition to a pre-existing program: (2) substitution of part of a pre-existing program; or (3) deletion of part of a pre-existing program. Outgroup comparisons and direct observation of ontogeny give the same, correct, answer for examples of class (1) changes. For examples of class (2) and class (3) changes, outgroup comparisons provide the correct answer and direct observation of ontogeny does not (Figs. 3-6). This holds true for homoplasious


and non-homoplasious examples (Table 1). We may see empirically that the “stability” gained from direct observation of ontogeny is actually a manifestation of a lack of sensitivity in the approach as a testing protocol. In addition, we observe that the classes of ontogenetic changes for which direct observation provides the correct answer do not include any classes not also resolved correctly by outgroup comparisons. In other words, direct observation of ontogeny does not provide us with information, unavailable by outgroup comparisons, which might compensate for its lack of sensitivity. Thus, we view direct observation of ontogeny as an incomplete method of character polarization relative to outgroup comparisons, just as pattern cladistics is incomplete relative to phylogenetic systematics.

There is a theoretical question involved in this discussion as well as an empirical one. Is phylogenetics theory-neutral? If it could be shown that the method for constructing cladograms carried no theoretical load, or if it could be shown that phylogenetics is equally applicable to any evolutionary theory, the theory-neutral epithet could be applied. We have already expressed our opinion that such a state of affairs is not necessarily desirable, but that is another question.

-i X A B C D E 1 A B C D E F K 2 A B C D F G 3 A B C D F t l

no E

Fig. 4. Non-terminal deletion and character polarity. Direct observation of ontogeny for species 1, 2, and 3 produces the cladogram on the left. Polarizing characters based on the out group"^" produces the cladogram on the right.

1 X A B C D E

2 A B C D F 1 A B C D E

3 A B C D F

Fig. 5. Terminal substitution and character polarity. Direct observation of ontogeny provides the dadogram on the left for species 1, 2, and 3. Polarizing characters using the outgroup “x” produces the cladogram on the right.

2 A B F D 3 A B F D E

Fig. 6. Non-terminal substitution and character polarity. Direct observation of ontogeny provides the dadogram on the left for species 1 , 2 , and 3. Polarizing characters using the outgroup “x” produces the dadogram on the right.


Table 1

Classes of ontogenetic changes and the ability of outgroup comparisons ( 1 ) and direct observation of ontogeny (2) to resolve correct polarity. + = correct polarity resolved; - = correct polarity not resolved; + # = correct polarity resolved if correct outgroup used. All classes of homoplasious changes may be correctly resolved by reference to more than one outgroup, but there is no secondary check for direct observation of ontogeny.

Classes of Ontogenetic Changes Polarizing Methods

Non-Homoplasious Terminal addition Terminal substitution Terminal deletion Non-terminal addition Non-terminal substitution Non-terminal deletion

Homoplasious Terminal addition (Parallelism I) Terminal substitution (Parallelism 11) Terminal deletion (Reversal I) Non-terminal addition (Parallelism 111)

1

+ # + # + # + #

2

Non-terminal substitution (Parallelism IV) i# - Non-terminal deletion (Reversal 11) + # -

Nelson and Platnick (1981) presented direct observation of ontogeny as a theory- neutral method of polarizing characters. There are two influential views concerning the significance of similarities in ontogenies of two species. The first view, developed by von Baer (1828-1837), was based in the tradition of Aristotelian essentialism. The developing organism was seen as striving to attain its eidos, an idealized or typological and immutable final form. The more similar the eidos of two different species, the more similar would be their ontogenies. Von Baer assumed that every organism began life in the same state, steadily diverging along the lines of the eidos of its species. The second view is the evolutionary view. This view traditionally asserted that the developmental stages represent a sequence of new adult forms brought forth by natural selection and added to the pre-existing ontogeny. In this view, phylogeny creates ontogeny; this is the view which is associated with Haeckel’s Law, “ontogeny recapitulates phylogeny.” Contemporary modifications of this view suggest that natural selection can bring forth adaptive changes in any portion of the developmental program, and when this happens the phylogenetic relationships of the species involved can become obscured.

We have seen that the cases for which direct observation of ontogeny provides the correct resolution of characters are those involving addition of characters to a pre-existing developmental program, or recapitulation. If one is to reject evolutionary considerations, then the justification for this approach must come from a non-evolutionary source. Nelson and Platnick cite as their justification von Baer, an essentialist. We do not wish to debate whether essentialism is theory-neutral or theory-antagonistic relative to evolutionary theory. What is important is that this essentialistic view is hardly theory-neutral; it is non-evolutionary, but that is a different matter. In addition, because direct observation of ontogeny fails under conditions of deletion or substitution in developmental programs, advocates of that approach must be able to claim that deletions and substitu- tions do not occur. That is hardly a theory-neutral position, because it amounts to a claim of prior knowledge (see Kluge, this issue).

We have suggested that direct observation of ontogeny represents a restricted subset of outcomes provided by outgroup comparisons. The justification for using outgroup


comparisons has always been explicitly evolutionary. But is it a prescription which is so general that it would be consistent with any evolutionary theory? If so, then phylogenetics would be effectively theory-neutral without having to invoke non-evolutionary justifications.

It is our belief that the methodology of phylogenetic systematics embodies an evolutionary world view which excludes some possible mechanisms. Hennig‘s (1950a) chief preoccupation was with German idealized morphology, which stressed the search for archetypal forms. Hennig wished to show that all species are mosaics of plesiomorphic and apomorphic traits, so there is never any such thing as an archetype in reality. Hennig also wanted to stress that the systematic orderliness found in nature could be explained without resorting to archetypes. We may briefly state our impression of Hennig‘s view:

1) Any irreversible process operating on discrete units produces a hierarchy. Thus, species produced by evolution should be related to each other hierarchically (see also Goldschmidt, 1952; Riedl, 1978).

2) All species are combinations of plesiomorphic and apomorphic traits - archetypes do not exist.

3) The phylogeny of a group is its species’ genealogy, which can be detected by deter- mining the sequences of plesiomorphic and apomorphic characters, and finding the hierarchical pattern they predict. Homologous traits will all co-vary, and homoplasious characters will be randomly distributed among species. In simpler terms, the most parsimonious arrangement of plesiomorphic and apomorphic characters will be the phylogeny.

4) Cladograms based on characters from different semaphoronts (equivalent developmental stages) will predict the same phylogeny.

Goodwin (1982) suggested that a “rational taxonomy” for reflecting his theory of evolutionary mechanism would be a periodic table of morphogenetic fields. The assertion that species will be related to each other hierarchically rather than in a periodic table excludes this theory from the realm of theories allowed by phylogenetics. We also note, parenthetically, that any systematic technique which chooses a hierarchy of internested sets is based on the assumption that the most informative view of the world is through internested sets.

It has been apparent to evolutionary taxonomists for a long time that Hennig‘s defini- tion of phylogeny differed from theirs (e.g., Mayr, 1974). Hennig equated phylogeny with genealogy; a cladogram is a phylogeny. Evolutionary taxonomists consider phylogeny to consist of genealogyplus adaptive radiations; to them, a cladogram is an incomplete phylogeny. Phylogenetic systematics thus excludes any evolutionary theory which includes evolutionary processes above the species level.

Finally, phylogenetic systematics excludes theories of evolution which assert that different stages in ontogeny may be better correlated with secondary adaptive changes than with phylogeny. Characters drawn from different semaphoronts will predict the same phylogeny no matter what selective regime they live under at different periods in their lives. We might look at this point in a different way by asking: (1) can we expect taxonomic congruence of characters from different semaphoronts? and (2) can we expect this congruence even in the absence of recapitulation? The first question is answered “no” by those who feel that adaptive changes in, say, larval form can result in larval morphology which is not indicative of phylogeny (e.g., de Beer, 1958; Charlesworth, Lande, and Slatkin, 1982). It is answered “yes” by phylogeneticists. The second question is answered “no” by those who feel that all non-recapitulatory changes are adaptive changes (this is a subset of question 1). Phylogeneticists assert that such congruence will occur whether or not recapitulation occurs. For example, larval and adult digenetic trematodes, and some larval and adult insects, are subjected to widely varying environ- ments, and the larval stages are not recapitulatory. Nonetheless, phylogenetic studies have shown that there is a high degree of congruence between larval and adult characters

10 CLADISTICS (VOL. 1

(OGrady, next issue; Brooks, OGrady, and Glen, in press; Hennig, 1950b, 1953, 1969; Howden, 1982). Thus, phylogenetics rules out evolutionary theories which (1) allow adaptive changes in ontogeny to obscure phylogenetic relationships or (2) exclude non- recapitulatory changes.

Summary

Phylogenetic systematics is not theory-neutral. The range of evolutionary theories excluded by phylogenetics may not seem very extensive to some, but this is because systematic analysis by itself is not enough to establish a unified theory of evolutionary mechanism. On the other hand, the patterns of evolutionary change are something with which any theory of evolutionary mechanism must come to grips.

Pattern cladistics stands in relationship to phylogenetic systematics as a part to a whole. Pattern cladistics is a more conservative approach than phylogenetics, but does not provide information or insights which are not also included in the phylogenetics research program. The two approaches should be viewed as expressions of personal preferences towards extending the findings of one’s research (Patterson, 1982) and need not be seen as fundamentally different scientific philosophies.

Direct observation of ontogeny as a method for polarizing characters stands in relationship to outgroup comparisons also as a part to a whole. Both can be implemented empirically. Direct observation of ontogeny does not resolve any cases of evolutionary change in ontogeny which outgroup comparisons fail to resolve, and outgroup comparisons resolve some cases which direct observation fails to resolve. Consistent with the conservative approach represented by pattern cladistics, direct observation of ontogeny is a conservative approach to polarizing characters. The conservative nature of this approach, however, renders it insensitive to some evolutionarily important classes of ontogenetic changes. Therefore, it can be justified only by invoking theoretical admoni- tions proscribing the existence of those classes of changes, and advocating a non- evolutionary view of ontogeny . Outgroup comparison derives its justification from evolutionary theory in general, but implementation of the phylogeneticist program using outgroup comparisons excludes some evolutionary theories and supports others.

Acknowledgments We express our appreciation to William L. Fink, Arnold G. Kluge, Jack Maze, and

Richard T. O’Grady for much fruitful discussion and thought. Ms. Maggie Hampong prepared the illustrations. This study was supported by Natural Sciences and Engineering Council (NSERC) operating grant A-7697 to DRB and National Science Foundation (NSF) grant DEB-8103532 to EOW. We thank those organizations for support.

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THEORIES AND METHODS IN DIFFERENT APPROACHES TO PHYLOGENETIC SYSTEMATICS