BIOL 4P08 Lecture Notes - Allopatric Speciation, Disruptive Selection, Evolutionary Radiation

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Sympatric speciation occurs without physical barriers
A partition of a gene pool without physical isolation.
What is required is some form of disruptive selection in which certain
genotypes have high fitness on one or the other of two resources.
Sympatric speciation via ecological isolation may be widespread among
insects, many of which feed on a single plant species.
BUT most common means of sympatric speciation is polyploidy—the
production within an individual of duplicate sets of chromosomes.
Polyploidy can arise from chromosomes duplication in a single species
(autopolyploidy) or from the combing of the chromosomes of two different
species (allopolyploidy).
Allopolyploids may also be produced when individuals of two different species
interbreed or hybridized.
What Happens when Newly Formed Species Come Together?
2Reproductive isolation can evolve as an incidental by-product of genetic
changes in allopatric populations.
Geographic isolation does not necessarily lead to reproductive isolation,
however, b/c genetic divergence does not cause reproductive isolation to
appear as a by-product.
Prezygotic barriers operate before fertilization
Mechanisms that operate before fertilization—prezygotic reproductive
barriers—may prevent individuals of different species or populations from
interbreeding:
oHabitat Isolation
oTemporal Isolation
oMechanical Isolation
oGametic Isolation
oBehavioural Isolation
Postzygotic barriers operate after fertilization
If individuals of two different populations lack complete prezygotic
reproductive barriers, postzygotic reproductive barriers may still prevent
gene exchange.
oLow hybrid zygote viability
oLow hybrid adult viability
oHybrid infertility
Individuals that mate with individuals of the related species should evolve
prezygotic reproductive barriers more rapidly that allopatric pairs of species.
Hybrid zones may form if reproductive isolation is incomplete
If contact is re-established between formerly isolated populations before
complete reproductive isolation has developed, members of two populations
may interbreed. Three outcomes of such interbreeding are possible:
oIn hybrid offspring are as fit as those resulting from mating within each
population; hybrids may spread through both populations and
reproduce with other individuals. The gene pools are then combined,
and no new species result from the period of isolation.
oIf hybrid offspring are less fit, complete reproductive isolation may
evolve as reinforcement strengthens prezygotic reproductive barriers.
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Document Summary

Prezygotic barriers operate before fertilization: mechanisms that operate before fertilization prezygotic reproductive barriers may prevent individuals of different species or populations from interbreeding, habitat isolation, temporal isolation, mechanical isolation, gametic isolation, behavioural isolation. If individuals of two different populations lack complete prezygotic reproductive barriers, postzygotic reproductive barriers may still prevent gene exchange: low hybrid zygote viability, low hybrid adult viability, hybrid infertility. Individuals that mate with individuals of the related species should evolve prezygotic reproductive barriers more rapidly that allopatric pairs of species. Hybrid zones may form if reproductive isolation is incomplete. If contact is re-established between formerly isolated populations before complete reproductive isolation has developed, members of two populations may interbreed. Three outcomes of such interbreeding are possible: in hybrid offspring are as fit as those resulting from mating within each population; hybrids may spread through both populations and reproduce with other individuals.

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