Download L567 19 October 2006

Lecture 22 L567
L567. The ecology of speciation
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 (TSC, Lively 1994):
species are the products of speciation events.
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Lecture 22 L567
Process of speciation (aside on teaching
undergraduates)
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 erection of geographical
barriers within the population’s range, or ii) by the
migration by some individuals across 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).
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?)
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Lecture 22 L567
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.
b. Drift or selection in the same kind of
environment (Dobzhansky-Muller 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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Lecture 22 L567
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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