What is a species? Species are evolutionarily independent units evolutionary independence – gene flow is low enough

1. What is a species? • Species are evolutionarily independent units • evolutionary independence – gene flow is low enough • that species can diverge • How to distinguish species? • (1) morphologically distinct ? ? ? doesn’t always work, but sometimes it’s all that’s possible

2. What is a species? • Species are evolutionarily independent units • evolutionary independence – gene flow is low enough • that species can diverge • How to distinguish species? • (1) morphologically distinct • (2) reproductive isolation • – failure to hybridize or produce fertile or viable hybrid offspring •  confirms lack of gene flow can’t always determine whether groups are reproductively isolated sometimes groups may stay distinct even with some hybridization Crow hybrid zone

3. What is a species? • Species are evolutionarily independent units • evolutionary independence – gene flow is low enough • that species can diverge • How to distinguish species? • (1) morphologically distinct • (2) reproductive isolation • – failure to hybridize or produce fertile or viable hybrid offspring •  confirms lack of gene flow • (3) monophyletic groups • - using phylogeny to determine • evolutionary independence lots of work! monophyly – a single branch  all living descendents of a common ancestor

5. Tracing evolutionary history phylogeny – pattern and timing of evolutionary branching events (“evolutionary tree”) A B C D common ancestor of C & D common ancestor of A & B common ancestor of A, B, C, D • branching happened in the past • common ancestors cannot be observed • must infer from data

6. change over time Tracing evolutionary history Evolution = “descent with modification” common ancestry - homology • descendants should be the same as ancestors except for modifications • evidence of evolution may also reveal evolutionary history

7. Evidence of evolution homology – trait shared because of inheritance from common ancestor example:similarity in limb structure among mammals

8. Structural homology Developmental homology Biochemical homology Homology – similarities inherited from common ancestor

9. Tracing evolutionary history • Inferring phylogenies: • homologous traits may reveal evolutionary history • closely related species should have traits in common • – inherited from their common ancestor (homology) • ancestral – form of the trait in the ancestor of the group • derived – modified form of the trait

10. Tracing evolutionary history • Inferring phylogenies: • closely related species should have traits in common • – inherited from their common ancestor (homology) • – informative traits are shared derived traits • (homologies that are not the ancestral state) • identifying shared derived traits requires determining: • – homologies • – ancestralvs. derived states (direction of change)

11. outgroup – provides information about ancestral state – roots the tree Which traits are ancestral? Which traits are derived?