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Analogous - number reflecting the degree of similarity...

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Analogous/homologous problem revolves around the distinction of the similarity of characters with adaptive or genetic bases. This leads to the distinction between grade and clade. Grade = level of adaptation; organisms of similar grade due to similar adaptations due to convergence (e.g., the "dog" grade or the "anteater" grade that goes across marsupial/placental distinction). Clade = a group descended from one common ancestor; a genetic lineage (e.g., the placental clade vs. the marsupial clade). There are different schools of systematics : different schools place different emphasis on the goals of systematics. Some will emphasize classification over phylogeny (grade over clade); another emphasizes phylogeny over classification (clade over grade). Phenetics (Numerical taxonomy) classification based on overall similarity of organisms. Treat all characters of equal weight and amass as many character as you can. Enter the characters into a computer that runs an algorithm that gives you a
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Unformatted text preview: number reflecting the degree of similarity between different taxa. Assumes : homologous and analogous characters will be in there together, but rate of character change is roughly proportional to evolutionary distance and the homologous characters will carry the day. Results plotted in a Phenogram showing evolutionary relationships. See fig. 14.1, pg. 373, 14.4, pg. 378. Cladistics (Phylogenetic systematics) Clade is everything. Define a hierarchical series of dichotomous branching events reflecting ancestor-descendant relationships. Seeks to identify monophyletic groups that, by definition, are derived from a single common ancestor. Defines these groups as taxa sharing derived characters ( synapomorphies ). Assumes that speciation is dichotomous producing two sister taxa and that the ancestral taxon disappears at the speciation event. See handout for examples and terminology; see fig. 14.1, 14.2, 14.6, pg. 373, 376, 382....
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