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Biol 121- 2010.03.26- Evolution- Phylogeny, History of Life on Earth (Ch. 27).docx

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BIOL 121
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Biol 121 225 Freeman 543-556 Mar 26, 10 Evolution: Phylogeny and History of Life -variation is the key to natural selection -species divergence is the outcome of selection and evolution How do biologists study the history of 1) Construct phylogenies life on earth? (2) 2) Fossil record Phylogeny -is the evolutionary history of a group of organisms -phylogenies are usually summarized and depicted in the form of a phylogenetic tree Phylogenetic tree -shows the ancestor-descendant relationships among populations or species, and clarifies who is related to whom -in a phylogenetic tree, a branch represents a population through time; the point where two branches diverge is a node that represents the time when an ancestral group split into 2+ descent groups -the endpoint of a branch represents a group (a species or larger taxon) living today, or one that went extinct (dead end) -phylogenetic trees are an extremely effective way of summarizing data on the evolutionary history of a group of organisms -however, the genealogical relationships among species cannot be known with absolute certainty but are instead estimated from given data How do researchers estimate -make inferences by looking at morphological or genetic characteristics or both: phylogenies? closely related spp should share many of their characteristics, distantly related species should share fewer characteristics -e.g. morphology of fossils compared to contemporary populations to see what process has occurred -e.g. analysis of base sequence -two methods of using data to estimate trees: 1) Phenetic -compute a statistic that summarizes overall relatedness among populations based on some data -for example, use base sequence in a gene to compute a genetic distance (relatedness to different groups) -computer program then builds a tree that clusters the most similar populations and places more-divergent populations on more-distant branches 2) Cladistic -based on the realization that relationships among spp can be reconstructed by identifying shared derived characters, or synapomorphies – traits in one group that exists in no other groups -can be molecular or morphological -allows recognition of monophyletic groups (aka clades or lineages) Homology vs. homoplasy -homology – when traits are similar due to shared ancestry -homoplasy – when traits are similar for reasons other than common ancestry -issue with phonetic and cladistic analyses is that traits can be similar in two species not because those traits were present in a common ancestor, but Biol 121 225 Freeman 543-556 Mar 26, 10 because similar traits evolved independently in two distantly related groups -this is known as convergent evolution – when natural selection favours similar solutions (or similar traits) to the problems posed by a similar way of making a living e.g. common dolphin and the ichthyosaur are two species that are far apart on the evolutionary tree, suggesting that they are not closely related -however, they both have streamlined bodies, long jaws with teeth, and fins + flippers Parsimony -parsimony is when the most likely explanation or pattern is the one that implies the least amount of change -to reduce the chance that homoplasy will lead to erroneous conclusion about which species are most closely related, biologists who use cladistic approach will also use parsimony -e.g. biologist compares branching patterns that are theoretically possible and count the number of changes in DNA sequences that would be required to produce each pattern -convergent evol and other causes of homoplasy should be rare compared with similarity due to shared descent, so the tree that implies the fewest overall evolutionary changes should be the one that most accurately reflects what really happened during evolution
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