2. Modern Evolutionary Classification
Section 18.2

3. The tree of life
Darwin’s work lead to the idea that organisms can be classified on the basis of evolutionary relationships, not just on the basis of similarities and differences
This lead to some Linnaean ranks to fall apart
Reptilia class isn’t valid unless birds are reptiles. Dinosaurs also would fall into this class

4. Evolutionary classification
Phylogeny – the evolutionary history of lineages
Phylogenetic systematics – group species into larger catergories that reflect lines of evolutionary descent rather than simply similarities and differences
Common ancestors - Organisms are placed into higher taxa when members are more closely related to one another then to members of any other group
The larger a taxon is, the farther back in time all of it’s members shared a common ancestor
Clades – a group of species that include a single common ancestor and all descendants of that ancestor (living and extinct)
Clades are monophyletic - a single common ancestor for all of its descendants

5. What is a Cladogram?
A cladogram links groups of organisms by showing how evolutionary lines or lineages branch off from common ancestors
Each node represents the last point at which species in lineages abive the node shared a common ancestor
Root of cladogram is common ancestor shared by all

6. Derived Characters
A derived character is a trait that arose o the most recent common ancestor of a particular lineage and was passed along to its descendants
Whether or not a characteristic is derived depends on the level at which you are grouping organisms
Four limbs is a derived character for then entire clade tetrapoda
Hair is a derived character for the entire clade Mammalia
But, for mammals, four limbs is not a derived character, as other organisms besides mammals have this trait
Although carnivores have four limbs and hair, neither are derived characters for this clade
Specialized shearing teeth are however a derived character for this clade

7. Loosing traits
Snakes are reptiles, which form part of the clade tertapoda
Why don’t snakes then have 4 legs?
The ancestors of snakes must have had 4 limbs
Somewhere in the lineage the trait was lost
Distantly related groups can sometimes loose the same character, absence of traits are not used in their analysis

8. How do you interpret a cladogram?
Lowest node represents last common ancestor of all four limbed animals – the clade tetrapoda
Forks in the cladogram show the various groups branched off from the tetropod lineage
In this example, specialized shearing teeth evolved before retractable claws
Each derived character along the main trunk, gives rise to the name of a clade

9. Linnaean classification and cladograms
Some Linnaean classes do correspond well to clades
For example class Mammalia and clade Mammalia
Presence of hair derived character
Traditional groups do not always from clades
Reptiles and Birds share a common ancestor
This was not recognized by Linnaeus, who put birds in heir own class
Class Reptilia without birds is not a clade
Birds form multiple clades
Aves, Dinosauria and Reptilia
An evolutionary biologist would call birds reptiles

10. DNA in classification
Evolutionary biologists don’t just rely on physical characteristics
Genes as derived characters
All organisms carry genetic information passed on from earlier generations
A wide range of organisms share a number of genes and show important homologies that can be used to determine evolutionary relationships
Shared genes contain differences that can be treated as derived characters in cladistic analysis
Similarities and differences in DNA can be used to develop hypotheses about evolutionary relationships
DNA has made evolutionary trees more accurate
Used to supplement data from anatomical analysis

11. Using DNA evidence
DNA evidence can be used when anatomical traits do not provide clear answers
Example – giant and red pandas
Many anatomical similarities with bears and racoons
DNA suggests that Giant pandas share a more recent common ancestor with bears than racoons
DNA evidence also suggests that red pandas don’t form part of the bear clade