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The Compleat cladist : a primer of phylogenetic procedures

1991; Linguagem: Inglês

10.5962/bhl.title.4069

0193-7766

E. O. Wiley,

Mediterranean and Iberian flora and fauna

The core concept of phylogenetic systematics is the use of derived or apomorphic characters to reconstruct common ancestry relationships and the grouping of taxa based on common ancestry.This concept, first formahzed by Hennig ( 1950, 1966), has been slowly, and not so quietly, changing the nature of systematics.Why should we be interested in this approach?What about phylogenetic systematics is different from traditional systematics?The answer is simple: classifications that are not known to be phylogenetic are possibly artificial and are.therefore, useful only for identification and not for asking questions about evolution.There are two other means of making statements of relationship: traditional systematics and phenetics.Traditional systematic methods employ intuition.In practical terms, intuition is character weighting.The scientist studies a group of organisms, selects the character(s) believed to be important (i.e., conservative), and delimits species and groups of species based on these characters.Disagreements usually arise when different scientists think different characters are important.It is difficult to evaluate the evolutionary significance of groups classified by intuition because we do not know why they were created or whether they represent anything real in nature.Because these groups may not be defined at all or may be defined by characters that have no evolutionary significance, such groups may be artificial.Phenetics is an attempt to devise an empirical method for determining taxonomic relationships.In practice, phenetics is no better than traditional systemafics in determining relationships because the various algorithms concentrate on reflecting the total similarity of the organisms in question.Organisms that appear to be more similar are grouped together, ignoring the results of parallel or convergent evolution and again creating possibly artificial groups.Phylogeneticists differ from traditional systematists in that we employ empirical methods to reconstruct phylogenies and strictly evolutionary principles to form classifications rather than relying on intuition or authority.We differ from pheneticists in that our methods seek to find the genealogic relationships among the taxa we study rather than the phenetic or overall similarity relationships.What all this means is that the groups we discover are thought to be natural, or monophyletic.Given any array of taxa, which two are more closely related to each other than either is to any other taxon?We attempt to discover the common ancestry relationships indirectly through finding evidence for common ancestry.Tliis evidence comes in the fonn of shared derived characters (synapomorphies).For example, among Aves (birds), Crocodylia (alligators and crocodiles), and Squamata (lizards, snakes, and amphisbaenians), Aves and Crocodylia are thought to be more closely related because they share a number of synapomorphies thought to have originated in their common ancestor, which appeared after 2 KU MUSEUM OF NATURAL HISTORY.SPECIAL PUBLICATION No. 19 (later than) the common ancestor of all three taxa.This relationship is shown in the form of a phylogenetic tree, a reconstruction of the genealogic relationships.In addition, phylogeneticists view the reconstructed tree (frequently termed a cladogram) as the classification, and when expressing it in a hierarchical scheme, we insist on maintaining monophyletic groups and sister-group relationships.The discovery of monophyletic groups is the basic quest of phylogenetics.Going to all the trouble of finding the groups and then throwing them away does not make sense to us.Ever since the general theory of evolution gained acceptance, systematists have sought the one evolutionary history for organisms and have tried to fit that history into a hierarchical structure.We seek to reflect in our classifications the groups that we find in nature.Because phylogenetic reasoning delimits groups based on common ancestry, we can attempt to reconstruct evolutionary histories and from them develop a hierarchical ranking scheme.Phylogenetic groups are then a reflection of the order in nature.Therefore, our classifications can be used for the study of other characters and for further investigations in biogeography.coevolution, molecular evolution, rates of evolution, ecology, etc.If you wish to use classifications to study evolution, they must reflect the genealogy of the taxa in question.Groups that are potentially artificial cannot be used in such investigations.One of the greatest strengths of the phylogenetic system is that the method and results are transparent, meaning that decisions, whether right or wrong, are based on data that can be examined by any and all persons willing to understand the nature of the data.The phylogenetic system does not depend on some special and mysterious knowledge about organisms that only the "expert" can understand.A critic cannot claim that your idea of the phylogenetic history of a group is wrong just because he has smdied the groups longer than you have.Of course, there are valid disagreements, and there is room for change and improvement.But these disagreements are data based, not opinion based.Phylogenedcs, to put it crudely, is a put-up-or-shut-up scientific discipline.This workbook presents the basics of phylogenetic systematics as we use it today.We also cite references for those interested in following some of the debates currently underway among the proponents of phylogenetic systematics.We hope that this information will stimulate you and illustrate the importance of systematics as the basis of comparative biology.When you have finished this workbook, you should be able to reread this introduction and understand what we are trying to accomplish.As an acid test, go read Hennig (1966); it's the way we got started, and it remains the classic in the field.All new scientific ideas and analytical methods are accompanied by new sets of terms and concepts, which can be unsettling to the tyro and even more unsettling to the experienced systematist who is called upon to abandon the "traditional" meanings of terms and embrace new meanings.The basic rationale for adopting the definitions and concepts presented in this workbook is twofold.First, it is vitally important for systematics and taxonomy to be integrated into in the field of evolutionary theory.Willi Hennig's major motivation for reforming systematics and taxonomy was to bring them in line with the Darwinian Revolution, making the results obtained through phylogenetic systematics directly relevant to studies in other fields of evolutionary research.Second, it is vitally important that the INTRODUCTION, TERMS, AND CONCEPTS 1 1 9. Tree length.-The length of a tree is usually considered the number of evolutionary transformations needed to explain the data given a particular tree topology.You will probably need some time to assimilate all of the definitions presented.A good strategy is to review this chapter and then go to Chapter 2. working your way through the examples.We have found that deeper understanding comes from actual work.Although we cannot pick a real group for you to work on, we have attempted the next best thing, a series of exercises designed to teach basic phylogenetic techniques that we hope will mimic real situations as closely as possible.Quick Quiz -Cliaracters 1 , How would the transformation series in Fig. 1 .4clook if it were polarized and unordered?2. Is character "l" in Fig. 1 .4eapomorphic or plesiomorphic? Chapter Notes and References1.There is no substitute for reading Hennig (1966).We suggest, however, that you become familiar with most of the basics before attempting to read the 1966 text.Hennig (1965) is the most accessible original Hennig.Other classics include Brundin (1966) andCrowson ( 1970).An interesting analysis of Hennig's impact on systematics can be found in Dupuis (1984).A considerable portion of the history of phylogenetic thought (and indeed post-1950 systematics) can be followed in a single journal, Systematic Zoology.We highly recommend that students examine this journal.2. Post-Hennig texts that are suitable for beginners are Eldredge and Cracraft (1980), Wiley (1981a).Ridley (1985), Schoch (1986), Ax (1987), and Sober (1988a).A more difficult text written from the point of view of the transformed cladists is Nelson and Platnick(1981).3. A very readable review of the entire field of systematics is Ridley (1985), whose defense of phylogenetics and criticisms of traditional (evolutionary) taxonomy, phenetics, and transformed cladistics are generally on the mark.Quick Quiz Answers Groups 1 .They are monophyletic because no descendant of their respective common ancestor is left out of the group.2. To make the group 0+A+B monophyletic, you would have to include C. To make the group N+O+A+B+C monophyletic.you could either include M or exclude N.3. Yes; e.g., N+A+B+C contains the monophyletic group ABC.4. You were pretty clever if you answered this one because we haven't covered it yet.The ancestors are represented by intemodes between branches.Obviously they are hypothetical because none of them are named.