Name:Date:
Classification: How Is Life Organized?
Big Ideas:
Big Idea
Chapters
Illustrative Examples
1. The process of evolution drives the diversity and unit of
life.
1.b.2: Phylogenetic trees and cladograms are graphical
representations (models) of evolutionary histories can be
tested.
26.1, 26.2, 26.3
• Number of heart chambers in animals
• Opposable thumbs
• Absence of legs in some sea mammals
1.d.2 Scientific evidence from many different disciplines
supports models of the origin of life.
26.6
Cells as a Basis for Classification
At a very basic level, organisms can be divided based on whether
they have cells or not. This will determine whether they are
classified as “living” or “non-living”.
-Viruses are not considered to be non-living as they are not
made up of a cell(s). Viruses are just protein capsules containing
genetic material (RNA or DNA).
-Cells are only found in living organisms and they represent the
lowest level of structure capable of performing all the activities
required for life (e.g. Able to respond to their environment,
metabolism)
Living organisms are divided into two large groups depending on
what type of cell(s) they have. Name and describe the differences
between the two:
a) Prokaryotic:
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b) Eukaryotic:
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Label the main structures highlighting the simularities and
differences between these cell types.
Phylogeny and the Tree of Life
Phylogenies Reflect Evolutionary Histories
A phylogeny is the evolutionary history of a species or group of
species.
What type of evidence can be used to discover a species
phylogeny? For example, how would you tell if the organism in the
image above is a legless lizard or a snake?
a) ________________________________________________________
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b) ______________________________________________________
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Systematics Connects Classification with Phylogeny
Systematics is the branch of biology that classifies organisms
and determines their evolutionary relationships. This is also
called “taxonomy”
Systematics groups organisms into categories which range from
very inclusive to very specific. Examine the diagram of a house
cat’s classification:
a) Which category (taxon) is most inclusive?
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b) Which taxon is least inclusive?
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c) What do each taxon represent?
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d) What is represented by the scientific name Felis catus?
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e) What is any easy way to remember the correct order of the
taxonomic categories?
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Phylogeny and classification are combined through the creation
of a phylogenetic tree. Again, this is hierarchical and becomes
more inclusive as you move through it.
Phylogenetic Trees
Homologous and Analogous Traits
Reconstructing a species’ evolutionary history involves sorting
homologous from analogous features and then infering phylogeny.
This sorting is often not as clear cut as it sounds because of:
a) Phenotypical similarities that are products of common
ancestory can be difficult to distinguish from analogous
traits.
*It is important to separate phenotype and genotype when
construting phylogenetic trees.
b) Molecular sequences are affected by mutations, masking
homologous segments. Additionally, they can also contain analogous
segments!
The diagram right illustrates how computer programs can find
homologous DNA segments in separate species that would not be
obvious to scientists.
Statistical tools have been developed that can determine if
molecular segments are analogous.
In general, very similar molecular segments are most likely
homologies. In distantly related organisms identicle bases are most
likely coincidental.
Molecular systematics uses data from DNA and other moelcules to
determine evolutionary relationships.
The tree right illustrates the characteristics of a general
phylogenetic tree:
What are sister taxa?
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What does a polytomy represent?
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What does Taxon G represent?
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Is the tree “rooted”?
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*Three important points about phylogenetic trees:
1. Phylogenetic trees show patterns of descent and not
phenotypic similarity. Related species may not resemble each other
if they evovled under different environmental pressures or at
different rates.
2. Branching represents patterns of descent and not the ages of
species.
3. Taxon did not evolve from the taxa next to it, but from a
common ancestor that both share.
A practical application of molecular systematics is determining
the origins of unknown whale meat samples. Read the investigation
on page 539 and answer the questions below.
1. Where did the unknown samples of meat come from?
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2. What evidence was used to construct the phylogenetic
tree?
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3. Which of the unknown samples were found to be illegal whale
meat?
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Cladistics
Cladistics uses common ancestry to group species into clades,
which include the ancesteral species and all of its
descendents.
Which of the following represents a clade?
Characteristics shared amongst members of a clade can be
catagorized as:
a) Shared Ancestral Character
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b) Shared Derived Character
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Shared derived characters can be organized into a Character
Table that allows for a Phylogenetic Tree to be created. Two
groupings of species are created in the process:
a) Outgroup:
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b) Ingroup:
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Phylogenetic Trees and Time
Phylogenetic trees can be arranged so that their branch lengths
are proportional to the amount of genetic change.
Which organism in the vertebrate lineage has undergone the most
genetic change?
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Phylogenetic trees can also reveal the amount of time since an
event occurred.
How long ago did mice and humans diverge?
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Phylogenetic Trees and Parsimony/Likelihood
Phylogenetic trees are constructed using two principles:
a) Maximum Parsimony (Occam’s Razor): The simplist explanation
consistent with the facts if likely the correct one.
b) Maximum Likelihood: The most likely sequence of evolutionary
events.
Apply these ideas to the data and trees to the right.
Which tree best fits the data?
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For these three closely related bird species which of the three
trees illustrated below is correct?
The Character Table below illustrates the four nucleotide DNA
sequence for each species and demonstrates how they could have
evolved. Focus specifically on Site 1 to answer the question.
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Phylogenetic Trees as Hypothesis
Interestingly, we can test the strength of phylogenetic trees by
testing hypothesis against them.
For example, based on the tree arrangement below would it be
safe to say that any characteristics found in both crocodiles and
birds should have also been present in all dinosaurs?
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Birds and crocodiles do have a number of characteristics in
common (four-chambered heart, territorial/sexual selection singing,
nest building, brooding) and there is some evidence that dinosaurs
also had some of these:
Molecular Systematics and Evolutionary History
Molecular systematics is useful in recreating evolutionary
histories as it:
a) Can provide a timeline of evolution to support or refute the
timeline created from fossil evidence.
b) Useful where no fossil records exist.
c) Provides evidence of simularity between organisms that no
longer posses homologous traits.
d) Can be used to look at short (mtDNA, evolved relatively
recently) or long (rRNA, changes relatively slowely) time
periods.
Molecular Clocks
Because the rate of mutation in specific genes tends to remain
constant, we can use the number of mutations as indicators of the
length of time since species have diverged and how long ago gene
duplication occurred. Molecular clocks can then act to measure
evolutionary time. Below is a mammalian molecular clock based on a
code for seven different proteins:
Green dots represent primate species whose proteins have evolved
more slowly than other mammals.
If two mammals have a 40 nucleotide difference for these
proteins, how long ago did they diverge?
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Below is a molecular clock constructed from a gene in HIV.
Black dots represent DNA sequences iof an HIV gene in patient
blood samples.
When did the first Aids virus emerge in the human
population?
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Constructing the Tree of Life
Domain is the largest classification taxa. What is evidence is
used to determine to which domain organisms belong?
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Below is an image of our current understanding of the phylogeny
of the three domains of life.
Next to each of the images below, name and briefly describe the
domain it represents.
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*See the video for an overview of the kingdoms:
http://www.youtube.com/watch?v=BQ1JaYxBH40&feature=fvw
Take your own notes to explain how the kingdoms of the Domain
Eukarya are divided:
Kingdom
Uni/Multi Cellular?
Nutrition
Other Info
Protista
Kingdom
Uni/Multi Cellular?
Nutrition
Other Info
Fungi
Kingdom
Uni/Multi Cellular?
Nutrition
Other Info
Plantae
Kingdom
Uni/Multi Cellular?
Nutrition
Other Info
Animalia
Evidence from the analysis of DNA that codes for rRNA has
allowed scientists to construct the following possible tree of
life:
Biology 11: ClassificationPage 9