Classification, Systematics, and Phylogeny

UNCORRECTED PAGE PROOFS Classification, Systematics, 2 and Phylogeny Our classifications will come to be, as far as they can be so made, genealogies. fpo Charles Darwin, The Origin of Species, 1859 And you see that every time I made a further division, up came more boxes based on these divisions until I had a huge pyramid of boxes. Finally you see that while I was splitting the cycle up into finer and finer pieces, I was also building a structure. This structure of concepts is formally called a hierarchy and since ancient times has been a basic structure for all Western knowledge. Robert M. Pirsig, Zen and the Art of Motorcycle Maintenance, 1974 his book deals with the field of comparative biology, or what may be called the science of the diversity of life. To understand invertebrate zoolo- gy, one must understand comparative biology, the tasks of which are to de- Tscribe the characteristics and patterns of living systems and to explain those patterns by the scientific method. When those patterns have resulted from evolutionary processes, they illuminate the history of life on Earth. Biologists have been undertaking comparative studies of anatomy, morphology, embryolo- gy, physiology, and behavior for over 150 years. Many biologists, particularly sys- tematists, do so with the specific intent of recovering the history of life. Because we cannot directly observe that history, we must rely on the strength of the scientific method to reconstruct it, or infer it. This chapter provides an overview of this process. Comparative biology, then, in its attempt to understand diversity in the living world, deals with three distinguishable elements: (1) descriptions of organisms, particularly in terms of similarities and differences in their characteristics; (2) the phylogenetic history of organisms through time; and (3) the distributional history of organisms in space. The field of biological systematics has experienced a revolution in its theory and application in the past 30 years, especially with regard to phylogenetic re- construction. Some philosophical aspects and operating principles of this exciting field are described in this chapter. It is essential that biology students have a basic grasp of how classifications are developed and phylogenetic relationships in- ferred, and we urge you to reflect carefully on the ideas presented below. 24 CHAPTER TWOUNCORRECTED PAGE PROOFS Biological Classification of ancestry and descent depicted in phylogenetic trees. So, we see that a biological classification scheme is re- The term biological classification has two meanings. ally a set of hypotheses defined and summarized by a First, it means the process of classifying, which consists of phylogenetic tree. Thus, classifications, like other hy- delimiting, ordering, and ranking organisms in groups. potheses and theories in science, have a third function, Second, it means the product of this process, or the classi- that of prediction. The more precise and less ambiguous ficatory scheme itself. The living world has an objective the classification, the greater its predictive value. Pre- structure that can be empirically documented and de- dictability is another way of saying testability, and it is scribed. One goal of biology is to discover and describe testability that places an endeavor in the realm of sci- this structure, and classification is one way of doing this. ence rather than in the realm of art, faith, or rhetoric. Carrying out the process of biological classification con- Like other theories, classifications are always subject to stitutes one of the principal tasks of the systematist (or refutation, refinement, and growth as new data become taxonomist). available. These new data may be in the form of newly The construction of a classification may at first ap- discovered species or characteristics of organisms, new pear straightforward; basically, the process consists of tools for the analysis of characters, or new ideas regard- analyzing patterns in the distribution of characters ing how characteristics are evaluated. Changes in classi- among organisms. On the basis of such analyses, speci- fications reflect changes in our view and understanding mens are grouped into species (the word “species” is of the natural world. both singular and plural); related species are grouped into genera (singular, genus); related genera are grouped to form families; and so forth. The grouping Nomenclature process creates a system of subordinated, or nested, taxa (singular, taxon) arranged in a hierarchical fashion fol- The names employed within classifications are gov- lowing basic set theory. If the taxa are properly grouped erned by rules and recommendations that are analogous according to their degree of shared similarity, the hierar- to the rules of grammar that govern the use of the Eng- chy will reflect patterns of evolutionary descent—the lish language. The primary goals of biological nomen- “descent with modification” of Darwin. clature are the creation of classifications in which (1) any The concept of similarity is fundamental to taxono- single kind of organism has one and only one correct my, the classificatory process, and comparative biology name, and (2) no two kinds of organisms bear the same as a whole. Similarity, evaluated on the basis of charac- name. Nomenclature is an important tool of biologists teristics shared among organisms, is generally accepted that facilitates communication and stability* by biologists to be a measure of biological (evolution- Prior to the mid-1700s, animal and plant names con- ary) relatedness among taxa. The concept of related- sisted of one to several words or often simply a descrip- ness, or genealogical kinship, is also fundamental to tive phrase. In 1758 the great Swedish naturalist Carl von systematics and evolutionary biology. Patterns of relat- Linné (Carolus Linnaeus, in the Latinized form he pre- edness are usually displayed by biologists in branching ferred) established a system of naming organisms now diagrams called trees (e.g., phylogenetic, genealogical, referred to as binomial nomenclature. Linnaeus’s system or evolutionary trees). Once constructed, such trees can required that every organism have a two-part scientific then be converted into classification schemes, which are name—a binomen. The two parts of a binomen are the a dynamic way of representing our understanding of generic, or genus, name and the specific epithet (= trivial the history of life on Earth. Thus, trees and classifica- name). For example, the scientific name for one of the tions are actually hypotheses of the evolution of life and common Pacific Coast sea stars is Pisaster giganteus. These the natural order it has created. two names together constitute the binomen; Pisaster is the Classifications are necessary for several reasons, not animal’s generic (genus) name, and giganteus is its specif- the least of which is to efficiently catalog the enormous ic epithet. The specific epithet is never used alone, but number of species of organisms on Earth. Over 1.7 mil- must be preceded by the generic name, and the animal’s lion different species of prokaryotes and eukaryotes have “species name” is thus the complete binomen. Use of the been named and described. The insects alone comprise first letter of a genus name preceding the specific epithet nearly a million named species, and over 350,000 of these is also acceptable once the name has appeared spelled out are beetles! Classifications provide a detailed system for on the page or in a short article (e.g., P. giganteus). storage and retrieval of names. Second, and most important to evolutionary biologists, classifications serve a de- scriptive function. This function is served not only by the *We generally avoid using common, or vernacular, names in this book, simply because they are frequently misleading. Most inver- descriptions that define each taxon, but also, as noted tebrates have no specific common name, and those that do typi- above, by the detailed hypotheses of evolutionary rela- cally have more than one name. For example, several dozen dif- tionships among the organisms that inhabit Earth. In ferent species of sea slugs are known as “Spanish dancers.”All manner of creatures are called “bugs,” most of which are not true other words, classifications are (or should be) constructed bugs (Hemiptera) at all (e.g., “ladybugs,” “sowbugs,” “potato from evolutionary relationships; that is, from the patterns bugs.”) UNCORRECTEDCLASSIFICATION, PAGE SYSTEMATICS, PROOFS AND PHYLOGENY 25 The 1758 version of Linnaeus’s system is actually the part of its range, which is designated as a subspecies, tenth edition of his famous Systema Naturae, in which he Pisaster giganteus capitatus. listed all animals known to him at that time and included All codes of biological nomenclature share the fol- critical guidelines for classifying organisms. Linnaeus lowing six basic principles: distinguished and named over 4,400 species of animals, 1. Botanical and zoological codes are independent of including Homo sapiens. Linnaeus’s Species Plantarum (in each other. It is therefore permissible, although not rec- which he named over 8,000 species) had done the same ommended, for a plant genus and an animal genus to for the plants in 1753. Linnaeus was one of the first natu- bear the same name (e.g., the name Cannabis is used ralists to emphasize the use of similarities among species for both a plant genus and a bird genus). or other taxa in constructing a classification, rather than 2. A taxon can bear one and only one correct name. using differences among them. In doing so, he unknow- 3. No two genera within a given code can bear the same ingly began classifying organisms by virtue of their ge- name (i.e., generic names are unique); and no two netic, and hence evolutionary, relatedness. Linnaeus pro- species within one genus can bear the same name (i.e., duced his Systema Naturae 100 years prior to the binomens are unique). appearance of Darwin and Wallace’s theory of evolution 4.

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