Chapter 25:Phylogeny and

Systematics

“Taxonomy is the division of organisms into categories based on… similarities and

differences.” p. 495, Campbell & Reece (2005)

Shown is a phylogenetic tree

• Fossils are preserved remnants of organisms that lived in the past

• Fossils form in sedimentary rock, the oldest fossils in the lower strata and younger on the upper strata

• Fossil record is incomplete and favors larger organisms, those with hard bodies or parts, those that were abundant and widespread.

Fossil Record

Foss

ils

• Homology- similarity due to common ancestry (same structure different function): wing of bat, arm of human, flipper of whale

• Analogy- similarity due to convergent evolution ( same function different structure) :two kinds of moles

Morphological Evidence

Analogies

These “moles”

only look similar

Placental

Marsupial

Computers are used to analyze DNA sequences for homologous nucleic bases.

Molecular Evidence

Taxo

nom

ic C

ateg

orie

s Did (Domain)

Karen’s (Kingdom)

Pups (Phylum)

Chew (Class)

On (Order)

Fuzzy (Family)

Grey (Genus)

Squirrels (Species)

Binomial Nomenclature

Example: Escherichia coli, E. coli, Escherichia spp., The genus name (Escherichia) is always capitalized

and comes first The species name (coli) is never capitalized The species name is never used without the genus

name The genus name may be used without the specific

epithet Both genus and species names are always

underlined or italicized Genera may be abbreviated, e.g., the E. in E. coli,

Cladistics

Cladistics is a technique by which organisms are assigned to different (monophyletic) taxa

Cladistics works by grouping together organisms such that within taxa individuals share more homologies than they do with individuals found in different taxa

Cladistics also rejects the inclusion of similarities...• …that result from convergent evolution (i.e.,

analogies)• …that are homologies that are shared with other

taxa (i.e., shared primitive characteristics)

Classifying Clades

Each deeper branch point represents

greater divergence

Classifying Proper Clades: Passing the Test

A clade consists of an ancestral species plus all

descendant species

Here is a missed ancestor species

Mon

ophy

letic

A correct grouping

A clade

Ancestor

All descendants

Par

aphy

letic

This is missing descendant

species

Pol

yphy

letic

Convergent evolution?

Polyphylies happen when species are included in

“clades” that they don’t belong in

Convergent Evolution

Polyphyletic taxa occur as a consequence of mistaking analogies for homologies

Analogies result from convergent evolution: the two species do similar things in similar environments so evolve similarly

The key difference between an analogy and a homology is:

• The common ancestor between the two species will lack the common structure

Ana

logi

es

Shared Derived Characters SDC

Cladogram based on Shared Derived Characteristics.

An outgroup is a species that is related to the species being

studied (ingroup), but less closely related

Molecular ClocksThe molecular clock is a yardstick for

measuring absolute time of evolutionary change based on the observation that some genes and other regions of genomes seem to evolve at constant rates

Neutral theory states that much evolutionary change in genes and proteins has no effect on fitness

It states that the rate of molecular change in these genes and proteins should be regular like a clock

• Phylogenetic analysis shows that HIV is descended from viruses that infect chimpanzees and other primates

• Comparison of HIV samples throughout the epidemic shows that the virus evolved in a very clocklike way

Applying a Molecular Clock- The Origin of HIV

• The tree of life is divided into three great clades called domains: Bacteria, Archaea, and Eukarya

• 1- last common ancestor of all living things

• 2. ancestor of eukaryotic cells

• 3. symbiosis of mitochondrial ancestor

• 4. symbiosis of chloroplast ancestor

Universal Tree of Life

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