Readings 4 notes

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11 Aug 2010
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Readings 4
Pgs 111-119 (refer to pgs. 366-369 and 376-379 after)
-the evolutionary history of a group of species is called its phylogeny and a phylogenetic tree is
a graphical summary of the history
-an evolutionary tree describes the pattern and timing of events that occurred as species
diversified
-synapomorphies are homologous traits that are shared among certain species and are similar
because they were modified in a common ancestor (shared derived traits)
-synapomorphies are useful in estimating phylogenies
-any group that includes an ancestor and all of its descendants is a monophyletic group
-the first idea to understanding why evolutionary relationships can be inferred by analyzing
synapomorphies is that synapomorphies identify evolutionary branch points
-the second idea is that synapomorphies are nested in that each branching event adds one or
more shared derived straits
-phylogeny inference methods that use the two ideas are called cladistic methods
-outgroup analysis is a basic cladistic method in which a relative that clearly branched off
earlier is selected as the outgroup and is compared to a group of interest
-in cases of similar traits that independently evolved in groups of species, species may share
similar traits that were not derived from a common ancestor
-morpohological similarities that have evolved independently in different lineages are due to
convergent evolution
-convergent evolution occurs when natural selection favours similar structures as solutions to
problems posed by similar environments
-evolution at the molecular level entails changes in nucleotide sequence due to mutation
-species can share nucleotide sequences for reasons other than common ancestry (caused by
reversal)
-in DNA sequence data, a change in nucleotide base is termed a reversal
-reversals are common in DNA data because there are only four possible states for each base in
a sequence
-convergence and reversal are homoplasius traits because they are not homologous
-homoplasius traits are not as similar as they initially appear because their underlying
structures are different
-morphological traits that are similar due to shared ancestry should develop in similar ways and
depend on the products of similar alleles
-homoplasius characteristics are troublesome because they suggest that two or more species
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-parsimony is a general logical criterion in which simpler explanations are preferred over more
complex ones
-when parsimony is applied to phylogeny inference, the preferred phylogenetic tree is the one
that minimizes the total amount of evolutionary change that has occurred
-it is valid to assume that convergence and reversal are rare relative to similarity
-in constructing phylogenetic trees, the goal is to measure the characteristics that are least
subject to homoplasy and the most reliable source of synapomorphies
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