BIO153H5 Chapter Notes - Chapter 27: Cenozoic, Paleozoic, Doushantuo Formation

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29 Jan 2013
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CHAPTER 27: PHYLOGENIES AND THE HISTORY OF LIFE
Key Concepts
Phylogenies and the fossil record are the major tools that biologists use to study the
history of life.
The Cambrian explosion was the rapid morphological and ecological diversification
of animals that occurred during the Cambrian period.
Adaptive radiations are a major pattern in the history of life. They are instances of
rapid diversification associated with new ecological opportunities and new
morphological innovations.
Mass extinctions have occurred repeatedly throughout the history of life. They
rapidly eliminate most of the species alive in a more or less random manner.
27.1 Tools for Studying History: Phylogenetic Trees
Phylogeny the evolutionary history of a groups of organisms
o Are usually depicted in the form of a phylogenetic tree
Phylogenetic Tree shows the ancestor-descendant relationships among
populations or species, ad clarifies who is related to whom
Branch represents a population through time
Node the point where two branches diverge (also known as a fork)
o Represents the point in time when an ancestral group split into two or more
descendant groups
Tip the endpoint of a branch
o Represents a group (a species or larger taxon) living today or one that ended
in extinction
How Do Researchers Estimate Phylogenies?
Phylogenetic trees are an extremely effective way of summarizing data on the
evolutionary history of a group of organisms
The fundamental idea in phylogeny inference is that closely related species should
share many of their characteristics, while distantly related species should share
fewer characteristics
There are two strategies used to estimate trees: phenetic and the cladistic approach
Phenetic Approach estimating trees based on the overall similarity among
populations
A tree is built that clusters the most similar populations and places more divergent
populations on more distant branches
Cladistic Approach inferring trees focuses on synapomorphies, the shared
derived characters of the species under study
A synapomorphy is a trait that certain groups of organisms have that exists in o
others
Synapomorphies allow biologists to recognize monophyletic groups (also called
clades or lineages)
How Can Biologist Distinguish Homology from Homoplasy?
The issue is that traits can be similar in two species not because those traits were
present in a common ancestor, but because similar traits evolved independently in
two distinctly related groups
Homoplasy occurs when traits are similar for reasons other than common
ancestry
Homology occurs when traits are similar due to shared ancestry
Convergent Evolution occurs when natural selection favors similar solutions to
the problems posed by a similar way of making a living
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