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The Impact of the Recognizing Evolution on Systematics 1

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The Impact of the Recognizing Evolution on Systematics 1 The impact of the recognizing evolution on systematics 1. Genealogical relationships between species could serve as the basis for taxonomy 2. Two sources of similarity: (a) similarity from descent (b) similarity caused by convergence (driven by natural selection for the same function). Phylogeny as the basis of Taxonomy Before the acceptance of evolutionary theory, \related" and \naturalness" where used with a variety of meanings. After Darwin \genealogically related" when we say \related" and we could define \naturalness" of taxa by whether or not they recognize clades. clade { a branch of a phylogenetic tree including an ancestral species and all of its descendants. monophyletic { the adjective form (from the Greek words \mono" for one and \phylon" for race, class or tribe). A clade is a monophyletic group. Darwin's largest contributions to systematics 1. provided a theoretical base for understanding the existence of the Linnean hierarchy and \relatedness" among organisms. 2. provided the expectation for a historical continuity among organisms { led to an emphasis on phylogeny reconstruction that underpins current systematics. The impact of the recognizing evolution on systematics 1. Genealogical relationships between species could serve as the basis for taxonomy 2. Two sources of similarity: (a) similarity from descent (b) similarity caused by convergence (driven by natural selection for the same function). Phylogeny as the basis of Taxonomy clade { a branch of a phylogenetic tree including an ancestral species and all of its descendants. monophyletic { the adjective form (from the Greek words \mono" for one and \phylon" for race, class or tribe). A clade is a monophyletic group. Pisces Amphibia Mammalia Testudines Lepidosauria Crocodylia Aves Archosauria Diapsida Reptilia Amniota Tetrapoda Vertebrata Monophyly image from http://en.wikipedia.org/wiki/Monophyly The impact of the recognizing evolution on systematics 1. Genealogical relationships between species could serve as the basis for taxonomy 2. Two sources of similarity: (a) similarity from descent (b) similarity caused by convergence (driven by natural selection for the same function). Similarities from common descent { \homologous characters" • may exhibit anatomical correspondences coupled with functional difference { co-opting of existing structures. • similarity in seemingly arbitrary features { \frozen accidents" Convergent (\analogous") characters tend to: • have similar function, and similar in form on a gross level { differ in details. • present problems when we try to imagine a continuum of descent (final structure made by different parts, or significant devolopmental differences). • have obvious fitness implications. These \rules of thumb" too vague to provide an error-proof means of distinguishing from homology, but they capture a key insight of evolutionary thinking. Taxonomy after Darwin A burst of interest in phylogeny reconstruction, e.g., tree like constructions of Haeckel(1860 - 1890's). But in the late 1800's and early 1900's there was a decline in systematics: 1. uncertainty about the reliability of phylogeny reconstruction and how to separate this from classification (conceptual problems) 2. disappointment in failure to resolve higher level phylogeny. 3. practical procedure for inferring phylogenies were lacking { 4. growing competition from other emerging branches of biology (embryology, cytology, Mendelian genetics, physiology, biochemistry, etc.) 5. Development of the codes of nomenclature became a focus of some researchers 6. Rise of population thinking became a focus of systematists. With the growth of the field of genetics and an understanding of the structure of populations, a new direction was forged for systematics. International codes of nomenclature Zoology (1901) Botany (1930) Bacteriology (1947) The codes provided for: 1. rules for choosing among competing names 2. rules for how names must be proposed to be valid. \The New Systematics" book of that title by Huxley, J. (1940) gave its name to the movement { blended into the Modern Synthesis of evolutionary biology. • a merger of \evolutionary taxonomy", genetics, and theory of populations • Concentrated on `microtaxonomy' { species, subspecies and populations. Phylogenetics before the 1960's 1. Many systematists conceded that phylogeny should be the basis of taxonomy but were very pessimistic about the prospects of inferring phylogenies. 2. Phylogeny estimates were the results of ad hoc, inscrutable analyses by experts rather than clear protocols. 3. There was debate on whether or not phylogenetic information should be the only information affecting taxonomy. Three schools of Systematics Evolutionary Phenetics Phylogenetic Systematics Systematics We can estimate ? No Yes phyologenies for most groups? Taxonomic procedures ? Yes Yes must be standardized? Taxonomy should reflect No No Yes phylogeny only? Evolutionary Systematics Different types of evolutionary change 1. cladogenesis - speciation, splitting of a lineage into 2 or more descendants 2. anagenesis - change within a lineage. \Evolutionary" systematists felt that both types of changes must be reflected in classification { so that classification reflected both major components of evolution. Pisces Amphibia Mammalia Testudines Lepidosauria Crocodylia Aves Archosauria Diapsida Reptilia Amniota Tetrapoda Vertebrata Paraphyly image from http://en.wikipedia.org/wiki/Monophyly Pisces Amphibia Mammalia Testudines Lepidosauria Crocodylia Aves Archosauria Diapsida Reptilia Amniota Tetrapoda Vertebrata Polyphyletic image from http://en.wikipedia.org/wiki/Monophyly Criteria for Delimitation and Ranking of a group Quoted (or paraphrased) from page 267 Mayr and Ashlock (1991) 1. Distinctness (size of gap between groups) 2. Degree of difference (within a group - tight clusters argue for ranking). 3. Evolutionary role (uniqueness of adaptive zone) 4. Grade characteristics. grades are { \similar in general level of organization" (Simpson, 1961). E.g. prokaryotes. 5. Size of taxon 6. Equivalence of ranking in related taxa (balance) 7. Stability Classic examples of the evolutionary systematics approach 1. Aves and Reptilia as classes { despite the fact that some \Reptiles" (e.g. crocodylomporhs) are more closely related to birds than they are to lizards. 2. 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