A3.2 on Classification and Cladistics

Classification

Placement of organisms into groups according to traits or evolutionary origins

Classification Benefits (2)

1. Easy information storage and retrieval

2. Identification of unknown species

Taxonomy

Science of classifying organisms into groups

Taxa Levels

Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species

Taxa Levels Mnemonic

King Phillip Came Over For Good S**

Taxon

A classification group of any rank (ex. genus or species)

The more taxa organisms share, the more similar they are

Domain is the most general, species is the most specific

Boundary Paradox (2)

Causes taxonomy to be arbitrary (not 100% reliable)

1. exact time when evolutionary divergence occurs cannot be determined objectively (bc of gradual evolution)

2. Sufficient diversity will divide one genus into two genera and eventually different families

Advantages of Classification (2)

Hypothesis:

If:

1. All organisms from common ancestor is included in the same taxon

2. All species in a taxon have evolved from the same common ancestor

Then:

Organisms in a taxon will share traits from their common ancestor.

Allows for trait prediction with certainty as classification mirrors evolution

Cladogram

Tree diagram showing PROBABLE sequence of evolution

CANNOT BE REGARDED AS PROOF

Clade

A node and all its connected branches

Group of organisms evolved from a common ancestor that SHARE COMMON TRAITS

Includes all currently alive, ancestors, and anything extinct

Most reliable evidence from DNA or AA sequence, but can also use morphological traits

Terminal Branch

End that represents individual clade

Node

branching point, where speciation occurs, giving rise to at least 2 taxa

Fewer nodes = more closely related the two clades are

Root

Initial, common ancestor of all species

Outgroup

a more distantly related group of organisms that serves as a reference group when determining the evolutionary relationships of the ingroup, the set of organisms under study

Molecular Evidence

Since all organisms use DNA/RNA as genome and make proteins, DNA/RNA/AA are used to establish relatedness

Significance of Mutation

As time passes, more mutations accumulate, which can determine how far two species have diverged

*more mutations = more time since diversion = less related*

*similar base sequence = less time = more closely related*

Molecular Clocks

Certain DNA mutates at a constant rate and thus serves as a measurement for time

If rate is reliable, point of divergence can be calculated

Limitation of Molecular Clocks

different proteins/genes mutate at different rates

Mutation Rates Affected By:

1. Generation Time (organism lifetime)

2. Population Size

3. Intensity of Natural Selection

4. Organism Type

Parsimony Analysis

Produces cladograms where sequence variation between clades is accounted with by the smallest number of sequence changes

Suggests order and PROBABLE divergence pattern (not actually how it evolved)

*Simplest is Best*

Need for Reclassification

W/ DNA sequencing easier, we can double check existing classifications

Often, DNA confirmed that traditional classification matched probable evolution pathway

Sometimes, species in a taxon did not share common ancestor, therefore RECLASSIFICATION needed

Figwort Family

Reclassification example, where many figworts too dissimilar to be cohesive family so divided into 5 clades.

Three Domains

Bacteria, Archaea, Eukarya

Categorized by rRNA

Bacteria

prokaryotes, consist of common pathogens (ex. E. coli)

Archaea

prokaryotes, consist of extremophiles (live in extreme places/conditions like undersea vents)

Eukarya

eukaryotes that have a true nucleus (plants/animals/fungi)