Transcript Speciation/Systematics

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
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