Last day… talking about speciation, ended discussing different forms of isolating mechanisms...
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Transcript of Last day… talking about speciation, ended discussing different forms of isolating mechanisms...
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