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Chapter 27

BIOLOGY 1M03 Chapter 27: Biology Chapter 27.docx

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Jon Stone

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Chapter 27: Phylogenies and the History of Life
Tools for Studying History: Phylogenetic Trees
The evolutionary history of a group of organisms is called its phylogeny
Phylogenies are usually summarized and depicted in the form of a phylogenetic
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, called a node, represents the point in
time when an ancestral group split into two or more descendant groups
A tip, the endpoint of a branch, represents a group living today or one that ended
in extinction
How Researchers Estimate Phylogenies
oPhylogenetic trees are an effective way of summarizing data on the
evolutionary history of a group of organisms
oHowever, the relationships depicted in an evolutionary tree are estimated
from data
oTo infer the historical relationships among species, researchers analyze
the species’ morphological or genetic characteristics or both
oEx. To reconstruct relationships among fossil species of humans,
scientists analyze aspects of tooth, jaw and skull structure
oTo reconstruct relationships among contemporary human populations,
investigators usually compare the sequences of bases in a particular gene
oThe fundamental idea in phylogeny inference is that closely related
species should share many characteristics, while distantly related species
should share fewer characteristics
oTwo general strategies for using data to estimate trees
Phenetic approach to estimating trees is based on computing a
statistic that summarizes the overall similarity among populations,
based on data

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Ex. Researchers might use gene sequences to compute an
overall “genetic distance” between two populations
A genetic distance summarizes the average percentage of
bases in a DNA sequence that differ between two
A computer program then builds a tree that clusters the most
similar populations and places more divergent populations
on more distant branches
Cladistic approach of inferring trees is based on a realization that
relationships among species can be reconstructed by identifying
shared derived characters (synapomorphies) in the species being
A synapomorphy is a trait that certain groups of organisms
have that exists in no others
Synapomorphies allow biologists to recognize
monophyletic groups, also called clades or lineages
Ex. Fur and lactation are synapomorphies that identify
mammals as a monophyletic group
Synapomorphies are characteristics that are shared because
they are derived from traits that existed in their common
How Biologists Distinguish Homology from Homoplasy
oProblems arise from the two different approaches
oThe 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 distantly related groups
oHomology occurs when traits are similar due to shared ancestry
oHomoplasy occurs when traits are similar for reasons other than common
Ex. The aquatic reptiles called ichthyosaurs were very similar to
modern dolphins
Both are large marine animals with streamlined bodies and
large dorsal fins
Both chase down fish and capture them between elongated
jaws filled with dagger-like teeth

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However, no one would argue that both are similar because
the traits they share existed in a common ancestor
Phylogeny has shown that ichthyosaurs are reptiles whereas
dolphins are mammals
Based on these data, it is logical to argue that the similarities
between both result from convergent evolution
Convergent evolution occurs when natural selection favours
similar situations to the problems posed by a similar way of making
a living
But convergent evolution does not occur in the common ancestor of
the similar species (streamlined body and sharp teeth help any
species chase down fish)
Convergent evolution is a common cause of Homoplasy and it
results in what biologists once called analogous traits
oIn many cases, homology and homoplasy are much more difficult to
distinguish than in the previous example
oEx. The Hox genes of insects and vertebrates
Even though insects and vertebrates last shared a common
ancestor 600-700 mya, biologists argue that their Hox genes are
derived from the same ancestral sequences
Several lines of evidence
The genes are organized in a similar way
All of the Hox genes share a 180-base-pair sequence called
the homeobox
oPolypeptide encoded by the homeobox is almost
identical in insects and vertebrates and has a similar
function, it binds to DNA and regulates the expression
of other genes
Products of the Hox genes have similar functions, they
identify the locations of cells in embryos
oThey are also expressed in similar patterns in time
and space
oIn addition, many other animals on lineages that branched off between
insects and mammals have similar genes
oThis is a crucial observation
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