Download Ch. 12 - Mechanisms of Speciation All life originated from a single

Ch. 12 - Mechanisms of Speciation
v All life originated from a single event 3.8 billion years ago; since then millions of
branching or speciation events have occurred between then and now
v In this discussion, we apply our knowledge of selection, mutation, migration and drift
to how populations diverged and speciation resulted
v Major topics to cover:
Ø What is a species? - Species concept
Ø Mechanisms of isolation
Ø Mechanisms of divergence
Ø Secondary contact and hybridization
Ø The genes of isolation and divergence
v Species Concept
Ø Ongoing debate as to the best definition and even if such a thing as a species
exists
Ø BSC - Biological Species
§ definition
§ Good when applied to many animal examples
§ Used as the definition of the Endangered Species Act
• Not good when…
• Good when…
Ø PSC - Phylogenetic (or evolutionary) Species Concept
§ definition
§ diagnostic (unique) traits that distinguish species
§ Proponents are mainly systematists
§ Advantage
§ Disadvantages
Ø Morphospecies concept
§ definition
§ Advantage
§ Disadvantages
• cryptic species.
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Application of the Morphospecies Concept
• Work by Jackson and Cheetham ('90, '94) on bryozoans. Tested whether
morphological differences they used to distinguish fossil species are
legitimate by analyzing similar differences in extant species
• experiment
• results
• conclusions
• Maybe underestimating species numbers .
Ø Application of the BSC and PSC to Red Wolf
§ Red wolf populations dwindling due to loss of habitat.
§ Appeared to be exhibiting characteristics of coyote that were also increasing
in frequency in the red wolf habitat
§ Looked like hybridization was occurring
§ US Fish &Wildlife captured breeding program
§ Morphological studies of skulls and teeth of specimens collected pre-1930
§ Post-1930 specimen studies
§ DNA studies of pelts (pre and post 1930)
§ conclusions.
v Mechanisms of Isolation
Ø Physical isolation as a barrier to gene flow - allopatric speciation model (Mayr,
1942) = speciation triggered by physical, geographic isolation (via dispersal or
vicariance) See figure 12.5
§ Geographic isolation by dispersal and colonization
• founder hypothesis
♦ Predictions based on founder hypothesis:
♦ Example: Hawaiian Drosophila speciation Fig. 12.7
§ Geographic isolation by vicariance
• Prediction
• Example of the snapping shrimp species pairs found on either side of the
Isthmus of Panama
Ø Changes in chromosomes as barrier to gene flow
§ Sometimes called saltation events - polyploidization or other changes in
chromosome karyotype.
§ In plants 40-70% of flowering plants are believed to arose by polyploidy
§ My dissertation on a diploid-tetraploid species pair of a grassland perennial
that occurred in southern Colorado.
v Mechanism of divergence
Ø Genetic Drift - definition; founder events; bottlenecks
Ø Natural Selection - definition; populations occupy different environments à
natural selection leads to divergence. Example of the apple maggot fly
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Rhagoletis pomonella - found to parisitize both hawthorne fruits and apple
fruits. Flies found on hawthorne trees and apple trees even when growing
together.
Are they from different populations or same population?
Is speciation possible without isolation (the first step)?
What selection factors have been overwelming enough to override the
exchange of occasional exchange of alleles?
How was the hypothesis tested?
What would you expect?
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Results.
Conclusions.
Ø Parapatric speciation - see box 12.2 - When can it occur?
v Reproductive Isolation - Consequences from secondary contact
The third step in speciation is reproductive isolation. If hybridization occurs, what is
the fate of the hybrids? What is the fate of the parental populations (species)?
Ø Reinforcement - the hypothetical mechanism proposed by Dobzhansky that since
hybrid offspring produced from matings between species populations in sympatry
that diverged in allopatry are usually going to have a lower fitness than the
parental populations and will therefore lower the mean fitness of the parental
populations, there should be selection for mechanisms that promote matings
between individuals of the same population (or species) = reinformement =
prezygotic isolation mechanism
Ø Reinforcement well studied in numerous Drosophila species pairs indicating:
populations in sympatry evolve prezygotic isolation mechanisms much faster than
populations in allopatry. This is what the reinforcement hypothesis predicts. Fig.
12.14. Evidence from other species?
Ø Gene flow through hybridization - Sorghum/johnsongrass example Fig. 12.15
Ø Hybrid speciation - hybrid becomes new species.
§ Hypothetical "superweed."
§ What factors may allow for hybrids speices to succeed?
§ Example in Helianthus - H. annuus, H. petiolaris, and putative hybrid H.
anomalous.
§ Evidence of reticulate evolution implies that species were derived through
hybrid speciation.
Ø Hybrid Zones
§ definition
Two circumstances produce hybrid zones:
§ 1. Secondary contact between allopatric species
2. Zone arising between parapatrically diverging populations
§ Characteristics of hybrid zones
• Hybrid zone broad and expanding
• Hybrid zone narrow and short-lived
• Hybrid zone stable and corresponds to ecotone or region of
"transitional" habitat
§ Sagebrush example to evaluate hybrid zone
• Two subspecies: mountain type and lowland type
• Hybrids are distinguishable from parent species and intermediate in form
(Fig. 12.16)
• hybrid zones are stable
• Fitness assessed relative to the three habitats. (Fig. 12.5)
Ø "Patterns of Evolution" video - be prepared to answer questions on exam (no
questions on quiz)