Download L567 lecture 22 speciation new

Speciation: the mystery of mysteries.
Questions
1. Can we explain speciation with microevolutionary forces (mutation,
selection, drift), or must we appeal to “macroevolutionary forces” like
species selection?
2. Can we explain saltation in the fossil record (phenotypic jumps)?
(Remember that Huxley said that gradual evolution was an “unnecessary
burden”)
3. Is there natural selection for speciation?
Species defined:
The biological species concept (BSC, Mayr 1942): Species are groups of
actually or potentially interbreeding individuals that are reproductively
isolated from other such groups.
The tautological species concept (Lively, unpublished). Species are the products of specation events
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Process of speciation
Step 1: Population of interbreeding organisms becomes broken into 2, or
more, smaller populations (in space or time).
The break may occur by
i) the establishment of geographical barriers within the population’s range, or
ii) by the migration by some individuals across existing barriers.
Step 2: Genetic divergence of isolated populations due to either
A. Genetic drift
B. Natural selection. i.e. different selection pressures on populations that are
isolated in space (or time).
C. Drift and selection (shifting balance) causing peak shifts on complex
adaptive topographies (more than one attractor).
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Step 3: Reproductive isolation
1. Prezygotic isolation: isolation prior to zygote formation, perhaps due to
mate choice, or timing of mating. (remember the Kirkpatric model of
runaway Sexual Selection?)
2. Postzygotic isolation: perhaps due to hybrid sterility or mortality.
(Remember the Zeh and Zeh paper? Incompatibilities between species for
the resolution of IGF2 and IGFr?)
Let’s consider the second step. Genetic divergence.
a. Local adaptation to different environments.
(see local adaptation model from scanned overheads)
There is genetic divergence due to selection to adapt to different environments
(for example on the wet side verses the dry side of a mountain range.
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b. Drift or selection in the same kind of environment (DobzhanskyMuller model).
(see Dobzhansky-Muller model from scanned overheads)
Different alleles go to fixation in different populations occupying the same
niche, where there is complex epistasis between loci. These alleles, which
evolved in allopatry, do not work well together, causing hybrid breakdown.
c. Drift and selection in the same kind of environment (shifting
balance).
(see “shifting balance” from scanned overheads)
One or more, but not all, populations drift across an adaptive valley, and then
are selected up a new adaptive peak
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Step 3. Reproductive isolation.
A. There is post-zygotic isolation due to any of the genetical explanations in
step 2.
Or
B. There is prezygotic isolation due to, for example, mate choice evolution in
allopatry.
Or
C. Prezygotic isolation is a evolutionary consequence due of selection not to
mate with member of the other subgroup in zones of sympatry. Speciation
by re-enforcement.
Species happen.
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