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7. Evolution 7 (Ch 26): Classification
Transcript of 7. Evolution 7 (Ch 26): Classification
: All descendants of 1 common ancestor.
: Some of the descendants of 1 common ancestor.
: The descendants of >1 common ancesteors.
A major goal of phylogeny is to develop a cladistic system that consists of nested monophyletic groups.
ex. Homo sapiens
Is it a snake or a lizard?
Make Sure You Can
Life's Major Divisions:
Grouping organisms according to the number of shared characteristics they have in common. This is called a "
Can be based on any characteristics.
Ideally, it is based on "
shared derived characteristics
" from a common ancestor
Ex. Reptiles are not a "good" phylogenetic group because they don't traditionally include birds
Making a Cladogram
1. Figure out how many species have a particular characteristic in common.
2. Group species so that the most number of species have the most characteristics in common.
The rule of "
All other things being equal*, the simplest explanation is true. Characteristics are most likely to evolve once
The rule of "
Assuming that mutation rates are equal in different lineages* more closely related organisms will have fewer differences in genetic sequences.
Which tree is the best?
Figure out shared bases
Compute the possibilities, and apply maximum parsimony
Ex. A phylogenetic tree that correlates DNA changes to geological time.
*- these are decidedly big assumptions
Carl Linnaeus (1707-1778)
Hierarchical classification: species are grouped by similarities
: species scientific names have two parts
: itallicized or underlined.
Genus: always capitalized
species: always lowercase
Hierarchy of life:
, the number of possible groups increases.
--based on cell anatomy.
Five (traditional) Kingdoms:
-- based on cell anatomy & nutritional modes. Not scientifically valid anymore.
~2 million described species: Total estimate 10-100 million.
Some Issues With The Old Style
1. The number of kingdoms kept changing.
First two, then three, then five, then six, then eight...
2. A lot of the early work was based on visual observations.
Looks can be decieving (why?)
Described species are heavily macroscopic (why?)
99.9% of all species are microscopic.
The DNA Revolution Comes to Classification
DNA is a reliable indicator of relatedness.
As species diverge, their DNA sequences diverge, too.
This has revolutionized classification.
Types of Groups
The relationship of the 3 domains
How closely related are we?
DNA evidence has identified endosymbiotic ancestors
What is the pattern of life's diversity that has developed during the history of life on earth?
How do we determine evolutionary relationships between organisms?
What are the characteristics of life that are useful for determining evolutionary relationships?
Explain how hierarchical classification of life and binomial nomenclature work, and how they demonstrate evolutionary relationships.
Describe why classical approaches to classification have needed to be overhauled recently due to advances in molecular biology.
Explain the process of creating a cladogram and demonstrate that process if given a list of shared derived characters for a group of organisms by applying the rules of maximum parsimony and/or maximum likelihood.
Identify a taxonomic group as mono-, para-, or polyphyletic
Explain the relationship between classification and phylogeny.
What Happened here?
What Happened here?
: The science of determining evolutionary relationships among organisms
examples of mutations on 2 homologous DNA sequences
The overall rate of mutations in genes acts as a "
" for dating evolution.
Different genes in different organisms have different mutation rates.
Current phylogeny of life
Gene duplications play a large role in evolution
Seriously, we generally use a computer!
If only there were a terrible 80's rap video about the topic....
HIV descended from viruses that infect chimpanzees and other primates
Comparison of HIV smaples throughout the epidemic show clock-like evolution
Difficulties with clocks
doesn't run as smoothly as predicted
natural selection causes irregularities
estimates older than fossil record are very uncertain
But...this is complicated by horizontal transfer... so many researchers suggest a "Ring of Life" rather than a tree
Systematists propose a "PhyloCode"
recognizes only groups that include a common ancestor and all its decendents
includes a branch to represent the last common ancestor of all taxa on the tree
Trees do show patterns of decent
Trees do not indicate when a species evolved or how much genetic change occured
Identify species of whale from whale meat samples
Predict features of an ancestor from features of its decendents
Branch length may reflect number of genetic changes or chronologic time