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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?

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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