Last day… talking about speciation, ended discussingdifferent 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 _____________– 1758- first to consistently use binomials (e. g. Homo sapiens)

“God creates, Linnaeus disposes”(or arranges, organizes)

‘Modern’ system begins with Carl Linnaeus – 1758- 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 leucocephalus Steller’s Sea Eagle, Haliaeetus pelagicus

Species & genus only lowest levels in classification:Kingdom: AnimaliaPhylum: ChordataClass: AvesOrder: AccipitriformesFamily: AccipitridaeGenus: HaliaeetusSpecies: 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 planDarwin 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