o Distance methods minimize the total of all the percentage differences among

O distance methods minimize the total of all the

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o“Distance” methods minimize the total of all the percentage differences among all the sequences. oMore complex “character-state” methods minimize the total number of base changes or search for the most likely pattern of base changes among all the sequences. Although we can never be certain precisely which tree truly reflects phylogeny, if the trees are based on a large amount of accurate data, the various methods usually yield similar trees. Phylogenetic trees are hypotheses. Any phylogenetic tree represents a hypothesis about how the organisms in the tree are related. oThe best hypothesis is the one that best fits all the available data. A hypothesis may be modified when new evidence compels systematists to revise their trees. oMany older phylogenetic hypotheses have been changed or rejected since the introduction of molecular methods for comparing species and tracing phylogeny. Scientists can make and test predictions based on the assumption that a phylogeny is correct.
For example, in an approach known as phylogenetic bracketing, scientists can predict by parsimony that features shared by two groups of closely related ancestors are present in their common ancestor and all of its descendents. Evidence suggests that birds descended from a group of bipedal Saurischian theropod dinosaurs. Birds and crocodiles share four-chambered hearts. Both “sing” to defend territories and attract mates. Both brood their eggs. Based on these observations, biologists predict that dinosaurs had four-chambered hearts, sang, and brooded eggs in nests. The fossil record does not provide evidence of dinosaur heart structure or singing behavior. However, fossilized dinosaur nests have been found with fossilized adults crouched over the eggs in a brooding posture. This evidence provides independent data supporting the hypothesis that birds descended from dinosaurs. Concept 26.4 An organism’s evolutionary history is documented in its genome.Molecular systematics is a valuable tool for tracing an organism’s evolutionary history. The molecular approach helps scientists understand phylogenetic relationships that cannot be measured by comparative anatomy and other nonmolecular methods. oFor example, molecular systematics helps biologists uncover evolutionary relationships between groups that have no grounds for morphological comparison, such as mammals and bacteria. oThis approach allows biologists to construct phylogenies among groups of microbes with no fossil record. Molecular systematics enables scientists to compare genetic divergence within a species. Molecular biology has helped to extend systematics to evolutionary relationships far above and below the species level.

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