BIO152H5 Chapter Notes - Chapter 27: Shocked Quartz, Genetic Drift, Cyanobacteria

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29 Jan 2013
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CHAPTER 27: PHYLOGENIES AND THE HISTORY OF LIFE (PGS. 571-595)
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
Introduction
- In biology we must consider profound changes in the nature of life on Earth over immense
periods of time.
- Lets first look at two major analytical tools that biologists use to reconstruct the history of life:
phylogenetic trees and the fossil record.
27.1 Tools for Studying History: Phylogenetic Trees
- Phylogeny the evolutionary history of a group of organisms
- Phylogenetic tree shows ancestor-descendant relationships among populations or species
- An ancestor and all its descendants form a monophyletic group (also called a clade or lineage)
Reading a Phylogenetic Tree
- Branches represent populations through time. Adjacent branches are sister taxa (a taxon is any
named group of organisms).
- Tips are the tree’s endpoints and represent living groups or a group’s end in extinction
- The names at the tips can represent species or larger groups
- Nodes occur where an ancestral group split into two or more descendant groups
- A polytomy is a node where more than two descendant groups branch off
- In rooted phylogenies the most ancient node of the tree is shown at the bottom
- The location of this node is determined using an outgroup, a taxonomic group that diverged
before the rest of the taxa being studied
How Do Researchers Estimate Phylogenies?
- Phylogenetic trees are an extremely effective way of summarizing data on the evolutionary
history of a group of organisms
- Researchers analyze morphological and/or genetic characteristics to infer phylogenetic
relationships among species.
- There are two general strategies for using data to estimate trees: the phenetic and the cladistic
approaches.
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Notes From Reading
CHAPTER 27: PHYLOGENIES AND THE HISTORY OF LIFE (PGS. 571-595)
- The phenetic approach is based on computing a statistic that summarizes the overall similarity
among populations.
- A computer program then compares the statistics for different populations and builds a tree
that clusters the most similar populations and places more divergent populations on more
distant branches.
- The cladistic approach to 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 no others
- Said another way, a synapomorphy is a trait that certain groups of organisms have that exists in
no others.
- When many such traits have been measured, traits unique to each monophyletic group are
identified and the groups are placed on a tree in the appropriate relationship to one another.
Synapomorphies Identify Monophyletic Groups
Distinguishing Homology from Homoplasy
- Problems can arise with both phenetic and cladistic analyses because similar traits can evolve
independently in two distant species rather than from a trait present in a common ancestor.
- The issue is that traits can be similar in two species not because those traits were present in a
common ancestor, but because similar traited evolved independently intwo distantly related
groups
- Homoplasy occurs when traits are similar for reasons other than common ancestry. Figure 27.2a
shows an example comparing the similar traits of dolphins and extinct marine reptiles called
ichthyosaurs.
- Homology occurs when traits are similar due to shared ancestry. Figure 27.2b shows an example
using the Hox genes.
- Convergent evolution occurs when natural selection favors similar solutions to the problems
posed by a similar way of life, as shown by the dolphin and ichthyosaur.
- Convergent evolution is a common cause of homoplasy.
- If similar traits found in distantly related lineages are indeed similar due to common ancestry,
then similar traits should be found in many intervening lineages on the tree of life.
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