Lecture 9: Evolution & Classification

Because of how evolution occurs: Hierarchical, nested classification is natural

There is ONE TRUE PHYLOGENY

• Based on interrelationships

• Life started at one point & diverged

Origin

Speciation

Study of Evolutionary History

• Taxonomy: classification (naming)

• Systematics: describes evol’nry relationships

Assume: similarity in heritable characters signifies closeness of relationship

Use characters to deduce relationships & classify

Types of Taxonomy

Phenetic: • Groups species by phenotypic similarity• May use physical, immunological, or genetic

traitsPhylogenetic:• Use evolutionary relationships• How recently shared common ancestorPhenetic & phylogenetic taxonomy often give

similar resultsMore on this next class

Terminology

Evolution occurs in two ways:

1. Anagenesis: directional change in a lineage

2. Cladogenesis: branching by speciation

Rate & pattern of anagenesis + branching pattern

True Phylogeny

Reconstructing Phylogenies

Use:

1. Ancestral Character (Plesiomorph):• Primitive • Inherited with little or no change from

ancestor

2. Derived Character (Apomorph)• Recently changed

Only CHARACTERS are PRIMITIVE, not SPECIES

Reconstructing Phylogenies

Use Shared Characters:

• Because of PARSIMONY

(smallest number of changes in phylogeny)

• Change takes time

• Change is unlikely

• Shared characters usually indicate close relationships

Shared Characters

1. Ancestral Homologies• Character found in both taxa• Character found in common ancestor• Character not in all descendants

2. Derived Homologies• Character found in both taxa• Character found in common ancestor• Character in all descendants of common

ancestor

3. Analogies:• Characters have no common history • Characters are not in common ancestor• Characters developed independently• CONVERGENCE• May be evolutionary reversals to ancestral state

- cause loss of info about relationships

A A A A

AA

A

A

A A A A

AA

A

A A A A

AA

A

Derived Homology Ancestral Homology Analogy

- character A - character A - character A

More realistic example

abcdef abcdef abcdef abcdef abcdef

abcdef

a

b

c

def

a= ancestral

a= derived

Phylogenetic Groupings1. Monophyletic

• Shared derived homologies• Contains all the descendants of a common ancestor• e.g. all birds

2. Paraphyletic• Shared ancestral homologies• Species with derived characters not included• Some but not all descendants of a common ancestor• e.g. fish; reptiles – missing birds, mammals

3. Polyphyletic• Analogies• Common ancestor not in group• Shared characters evolved independently• e.g. vultures

Groupings

A A A A

AA

A

A A A A

AA

A

A A A A

AA

A

Monophyletic Paraphyletic Polyphyletic

Phylogenetic Reconstruction: Whales

• Sea-dwelling ~ 53.5 mya• Descendants of Artiodactyla (even-

toed ungulates)• Rudimentary & vestigial characters

common to land mammals (pelvic girdle, diaphragm, sensory structures)

• Intermediary fossils: Ambulocetus – the walking whale (47 mya)

Whale Phylogeny continued

• Paraxonic foot symmetry: characteristic of artiodactyla (axis passes b/w 3rd/4th digits)

• Molecular studies: closest to artiodactyla out of 48 mammals

• Not just related to artiodactyls they ARE artiodactyls

• Geochemical studies: move from FW to SW in tooth oxygen ratios

• Recent evidence: hippos more closely related to whales than other artiodactyla

Previous viewpoint

Recent viewpoint