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Classification and Phylogenetic Relationships

Classification and Phylogenetic Relationships. Early natural historians classified organisms based on shared features Now we seek to understand how organisms features are related to phylogenetic relationships, descent from a common ancestor

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Classification and Phylogenetic Relationships

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  1. Classification and Phylogenetic Relationships • Early natural historians classified organisms based on shared features • Now we seek to understand how organisms features are related to phylogenetic relationships, descent from a common ancestor • Classifications and relationships change over time, as we learn more details and test hypotheses

  2. Fig. 11.1 pg 231

  3. Species Concepts • How to recognize species • Morphological species • Biological species • Evolutionary species

  4. Morphological Species • Assemblages of individuals with morphological features in common • Separable from other assemblages by discontinuities in a number of features • Numerical (phenetic) taxonomy • Incorporates statistical tools to identify taxonomic distinctions • High statistical correlation of large number of characters distinguishes groups

  5. From Buffon A sexually interbreeding (or potentially interbreeding) group, separated from other species by reproductive isolation Sibling species Nearly identical but do not normally cross-fertilize Polytipic species Multiple geographic subspecies, can exchange genes if come into contact Difficulties Allopatric populations Subjective scale of success for interbreeding Cannot test fossils Does not apply to plants Horizontal gene transmission between different species Biological Species

  6. Fig 11.2 pg 233 Evolution in the Genus Geum, Evolution 13, 378-388 by W. Gajewski. Reprinted by permission.)

  7. Evolutionary Species • Evolutionary isolation • Not dependent upon sexual isolation • A species concept that incorporates change over time • Stage when group has attained complete separation subjective

  8. Phylogeny • Origin of new species • Phyletic versus Cladogenesis • Phyletic evolution • Changes within a single lineage • Phylogenetic branching, Cladogenesis • Splits and divisions result in multiplication in number of species • The more a group shares common inherited attributes, the more likely their descent from a common ancestor Fig 11.4 pg 235

  9. Phylogeny • Shared attributes can occur by several ways • Homology • Feature is shared because species derived from common ancestor • Homoplasy • Common feature but ancestor was different • Similar adaptation from different starting points • Parallelism • Immediate common ancestor was different • Convergence • Lineage has been separated for considerable time

  10. Homology • A phenotypic feature that is shared due to common ancestry • It is critical to specify the level at which homology is designated • Genetic > Development > Morphology > Function • Serial homology • Similarities among parts of the same organism • E.g. vertebrae • Homologous features, may arise by different developmental pathways

  11. Homology, Partial Homology and Percent Similarity • Homology as applied to DNA sequence similarity • Orthologous genes • Genes duplicated in different species • Paralogous genes • Genes duplicated within a species • Partial Homology, percent similarity • Early molecular workers comparing two genes might have discussed partial homology, percent similarity between to genes • No longer accepted • Homologous or not homologous

  12. Homology and Constructing Phylogenetic Trees • Homology is the basis for constructing phylogenetic trees • Monophyletic groups • Individuals share a common ancestor • Polyphyletic groups • Individuals in do all share a common ancestor • Convergence Fig 11.8 pg 240

  13. Phylogenetic Trees • Phylogeny is something that happened • Classification is an arrangement of its results • Ideal multi-limbed phylogenetic tree would include all ancestral populations • Impossible • Most species are unknown (fossils) • Unable to compute, to complex

  14. Phylogenetic Trees • Taxon (plural taxa) • Unit of classification, whether species, genus, order,… • Groups might not always be monophyletic in different classification schemes Fig 11.9 pg 241

  15. Willi Hennig (1913-1976) Separation of plesiomorphic (ancestral) characters from apomorphic (derived) characters Monophyletic groups Defined by synapomorphic (shared derived) characters Evolutionary steps marked by dichotomous branching of sister taxa Paraphyletic groups Groups that includes some descendents of a common ancestor, but not all Parsimony method Preferred phylogenetic tree has least number of steps Cladistics, Phylogenetic Systematics

  16. Cladistics, Phylogenetic Systematics Fig 11.12 pg 243

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