Last day… talking about speciation, ended discussing different forms of isolating mechanisms Included prezygotic mechanisms such as behavioral & mechanical isolation, and post-zygotic mechanisms…

Postzygotic barriers include: 1) Reduced hybrid viability (e.g. sunfish) Longear SunfishGreen Sunfish abnormal hybrid larva

2) Reduced hybrid fertility (mules) Horse, Donkey & Mule

3) Hybrid breakdown (rice & cotton)

How does speciation work? Most popular model of speciation: allopatric speciation - allopatry: inhabiting different ranges - suggests original pop. becomes divided by new geographic barrier, or colonization of new isolated area White-tailed Antelope SquirrelHarris’Antelope Squirrel

- isolated pops. gradually diverge; genetic diffs. develop due to selection in diff. environments, genetic drift, or founder effect - when & if pops. make contact isolating mechanisms may be reinforced (hybrids selected against)

An alternative model: sympatric speciation - sympatric: living in same area; two new species form while their geographic ranges still overlap - controversial – some systematists doubt it occurs in animals, most think probably rare (& hard to prove) allopatric sympatric

- might originate through host specialization (e.g. Apple Maggot Fly) or strong assortative mating

Sympatric speciation does occur commonly in plants via polyploidy Autopolyploidy: chromosomes fail to separate (nondisjunction), diploid gametes, may self-fertilize & form tetraploid offspring - new species, cannot interbreed with diploids

Allopolyploidy: more frequent - hybridization leads to unusual chromosome #, either non-disjunction or second hybridization event increases chromosome # - new polyploid species, can breed with other polyploids

Vegetative reproduction & self-fertilization keep hybrids alive until plant can reproduce sexually % of plant species may be formed this way (e.g. wheat & other crops)

Systematics Repeated speciation leads to tremendous diversity of life – needs to be organized Systematics – study of relationships between organisms & their classification & naming

‘Modern’ system begins with _____________– first to consistently use binomials (e. g. Homo sapiens) “God creates, Linnaeus disposes” (or arranges, organizes) ‘Modern’ system begins with Carl Linnaeus – first to consistently use binomials (e. g. Homo sapiens)

Specific epithet is label that (almost) always stays with sp. once named (Law of Priority) e.g. Bald Eagle: Haliaeetus leucocephalus Genus name indicates closest relatives (plural = genera) First part – genus name, second part – specific epithet Bald Eagle, Haliaeetus leucocephalusSteller’s Sea Eagle, Haliaeetus pelagicus

Species & genus only lowest levels in classification: Kingdom:Animalia Phylum:Chordata Class:Aves Order:Accipitriformes Family:Accipitridae Genus:Haliaeetus Species:leucocephalus Any ‘unit’ in the classification is a taxon (pl. taxa)

Systematists try to make classification reflect the phylogeny of the group Phylogeny – the pattern of evolutionary descent of a taxon Linneaus trying to decipher God’s plan Darwin recognized that classification reflected life’s family tree

What sort of groups should be recognized in a classification? Three types of groups can be distinguished: Monophyletic - all spp. share a common ancestor, & all descendants of that ancestor are included

Polyphyletic - does not include the most recent common ancestor of the species

Paraphyletic - includes the most recent common ancestor, but does not include all descendants