Last day talking about speciation ended discussing different

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 Sunfish Green 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 Squirrel Harris’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 - 25 -50% 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 Carl _______– Linnaeus – 1758 - first to consistently use binomials (e. g. Homo sapiens) “God creates, Linnaeus disposes” (or arranges, organizes)

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

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)

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

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