speciation darwin explained evolution, but didn’t do so well with speciation natural selection...
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Speciation
Darwin explained evolution, but didn’t do so well with speciation
Natural selection causes change in a pop., but what causes pop. to split?
- had idea of ‘diversification’, but not of gene pool
What is a species?
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Most popular idea (still?) is Biological Species Concept (Ernst Mayr):
- pop. or group of pops. whose members have potential to interbreed & produce fertile offspring
- largest interbreeding unit, genetically isolated from other pops.
1904-2005
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Species usually look different, but may be very hard to distinguish (or a species may be very variable)
Eastern Meadowlark
WesternMeadowlark
Polymita
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In practice, many species hybridize – if only rare, generally considered distinct spp.
Blue-winged Warbler
Golden-winged Warbler
‘Lawrence’s Warbler’
‘Liger’
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Hard to classify if: - disjunct range
- asexual reproduction
- fossils
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Can have ‘ring species’ – don’t interbreed in 1 zone of contact, but connected by interbreeding pops
EnsatinaDeer Mouse
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Difficulties expected if species arise by gradual divergence of pops.
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- other species concepts proposed, but they have difficulties too…
Morphological Species Concept – based on similar structure & appearance
- scientists may not agree on what differences define a sp.
Phylogenetic Species Concept – species is smallest group of related organisms that share a defining feature
- could greatly increase # of species
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Another possibility: Genetic Species Concept - a group of genetically compatible interbreeding natural populations that is genetically isolated from other such groups.
- allows use of extensive genetic data, but not clear how much difference is required to qualify as a species
Little Penguin – Australia?
Little Penguin – New Zealand
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For sexual species, most popular concept is still Biological Species Concept
Northern (‘Yellow-shafted’) Flicker Northern (‘Red-shafted’) Flicker
- relies on idea that reproductive isolation (lack of interbreeding between populations) maintainsseparate gene pools (total set of genes in a population)
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Reproductive isolation important to speciation – can be achieved in various ways:
A – Prezygotic barriers 1) Habitat isolation (flycatchers)
DuskyFlycatcher
Hammond’sFlycatcher
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2) Behavioral isolation (fireflies)
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3) Temporal isolation (spotted skunks)
Eastern Spotted Skunk Western Spotted Skunk
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4) Mechanical isolation (flowers, insects)
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5) Gametic isolation (marine animals, flowers)
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B May have 2 individuals of different spp. mate, & yet maintain distinctiveness through postzygotic isolating mechanisms or…– Postzygotic barriers
1) Reduced hybrid viability (sunfish)
Longear Sunfish Green Sunfish
abnormal hybrid larva
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2) Reduced hybrid fertility (mules)
Horse, Donkey & Mule
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3) Hybrid breakdown (rice & cotton)
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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
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- isolated pops. gradually diverge; genetic diffs. develop due to selection in diff. environments, drift, or founder effect
- when & if pops. make contact isolating mechanisms may be reinforced (hybrids selected against)
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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
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- might originate through host specialization (e.g. Apple Maggot Fly) or strong assortative mating
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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
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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
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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)
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Systematics
Repeated speciation leads to tremendous diversity of life – needs to be organized
Systematics – study of relationships between organisms & their classification & naming
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Modern system begins with Carl Linnaeus – 1758- first to consistently use binomials (e. g. Homo sapiens)
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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
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Species & genus only lowest levels in classification:Kingdom: AnimaliaPhylum: ChordataClass: AvesOrder: FalconiformesFamily: AccipitridaeGenus: HaliaeetusSpecies: leucocephalus
Any ‘unit’ in the classification is a taxon (pl. taxa)