Download Mechanisms of Divergence •Natural selection •Genetic Drift •Sexual

Mechanisms of Divergence
•Natural selection
•Genetic Drift
•Sexual Selection
Genetic drift
•Random changes in populations due to incomplete
“sampling” of genetic variation (alleles).
•Affects small populations
•The main consequence is a rapid (very likely nonadaptive) divergence from the original population.
•Selection will filter variance down after a rapid
population expansion
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Bottlenecks are periods of very low population size or near
extinction. This is a special case of genetic drift
The result of a population bottleneck is that even if the
population regains its original numbers, genetic variation is
drastically reduced
Natural Selection
•Natural selection is expected to play a role in genetic
divergence if one of the populations has a novel habitat or
lifestyle (e.g., new food source)
•Natural Selection could be visualised as a filter which
stringency will depend greatly on the available resources
•Natural Selection is therefore highly dependent on the
effective population size, being directly proportional to it
Darwin’s Finches in the Galápagos Islands
From an initial founder
population from South
America, these finches
have radiated out into 15
species. Similar in body
size and colour, but highly
differentiated in size and
shape of beak. Each has
adapted to a different
food source
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•In the case of Darwin’s Finches, this was facilitated by the
absence of competitors for the new food sources on the
Galápagos Islands.
•The speciation of the Drosophilids in Hawai’i is also an
example of an adaptive radiation.
•Adaptive radiation also took place in mammals after new
niches were opened up by the mass extinction of
dinosaurs.
•Adaptive radiation is is a burst of speciation caused by
adaptation to new niches, or new food sources ……
•The speciation of Darwin’s Finches is a classic example of
adaptive radiation (= when a single [or few] ancestral
species diversifies into a large number of descendent
species
Sexual Selection as a Mechanism of Genetic Divergence
•Specific kind of selection only concerned with the ability to
find a mate
•Changes in the way that members of a population choose
mates can lead to rapid divergence and speciation. This
kind of selection directly affects gene flow.
Secondary Contact (Mechanisms of reproductive
isolation)
•They may form hybrids.
•The fate of these hybrids determines the fate of the
speciation process.
•If the hybrids are successful, the speciation process
may be erased or they may result in an entirely new
species.
•If they have reduced fitness, then the original speciation
may be reinforced.
Mechanisms of reproductive isolation:
•What happens when populations that have been isolated,
and have started to diverge, come back into contact with
each other?
-Reinforcement
-Prezygotic
-Postzygotic
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Reinforcement
1. Reinforcement Selection (reinforcement of
speciation)
•
After a second contact between the separated
populations, the hybrids may have reduced biological
fitness when competing with non-hybrid individuals
•
This will lead to a selection for assortative mating i.e.,
choosing a mate from the same population
•
Mechanisms to prevent hybridisation will evolve
Prezygotic isolation
•
reproductive isolating mechanisms (RIMs) occur to
prevent fertilisation. These may include mutations
that alter mate choice or the timing of breeding (life
history changes).
Postzygotic isolation
• RIMs do not prevent fertilisation but render the hybrid
organism inviable or infertile (e.g., different
chromosome number, other genetic incompatibility).
• Postzygotic RIMs are considered to be wasteful of
the resources/energy of the parent, and some
models predict that there will be selection to develop
prezygotic RIMs.
• Note, however, that reinforcement is not required for
speciation to occur.
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