Modern Evolutionary Classification Section 18.2

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

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

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

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

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

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

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

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

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