BIOL 140 Lecture Notes - Phylogenetic Tree, Cladistics, Ichthyosaur

37 views3 pages
Published on 27 Jun 2012
School
UBC
Department
Biology
Course
BIOL 140
Professor
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
life on earth? (2)
1) Construct phylogenies
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
phylogenies?
-make inferences by looking at morphological or genetic characteristics or both:
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
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Document Summary

Species divergence is the outcome of selection and evolution. How do biologists study the history of life on earth? (2) Homology vs. homoplasy: construct phylogenies, fossil record. Is the evolutionary history of a group of organisms. Phylogenies are usually summarized and depicted in the form of a 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.

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