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Speciation
Definitions:
Species: three different definitions depending on what you
ascribe toMorphospecies: a group of morphologically similar
organisms
Biological species: group of organisms that interbreed
(offspring viable)Phylogenetic species: smallest monophyletic group
of organisms
Anagenesis: evolutionary change in a single lineage
Cladogenesis: a lineage splitting into two; the process of
speciation
Genetic drift: random changes in allele frequencies due to
stochastic events, affects smallpopulations more and can lead to
the loss of rare alleles
Introgression: movement of genes between differentiated
populations by backcrossing
Modes of speciation
Allopatric: population is divided by a physical barrier and then
populations diverge in
geographic isolationEx. Snapping shrimp on either side of
Panama
Peripatric: small founding population migrates to a new,
isolated, niche (Founder effect)Ex. Drosophila in Hawaii
Parapatric: a small founding population enters a new, adjacent,
niche (Ecological speciation)Ex. Sculpins
Sympatric: no physical isolation, speciation in the same place
occurs through polyploidy,hybridization, or host switching
Ex. Apple maggot flyEx. Desert sunflowers or other plants
Ex. Cichlids? Seems parapatric to me (adjacent niches in lake)
ask Matt!
Ring species:-Species spreads around some barrier and forms a
continuous gradient of morphs without sharp
distinction but by the time the ends of the ring rejoin there
have been too many accumulatedchanges and they are reproductively
isolated
Ex. Ensatina slamander around central valley
Character displacement:-To coexist there is pressure to occupy
differentiated niches, divergent selection leads toecological
speciation
Ex. Darwins finches
Reproductive isolation:-Can be prezygotic (different mating
songs or colors, timing of mating, etc) to keep mating from
occurring-Can be postzygotic (less fit or sterile hybrid) which
should provide evolutionary pressure to
evolve prezygotic barriers to avoid wasting resources on
hybrids-Keeps populations that have speciated allopatrically from
rejoining if barriers are strong enough
-Secondary contact (from strong -> weak reproductive
isolation):no hybridization, hybridization, limited introgression,
reunification
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Papers
Kocher, T.D. 2004. Adaptive evolution and explosive speciation:
the cichlid fish model.
-A classic example of parapatric selection because different
species have diverged to occupy
different niches in the same lake-The species in one lake are
more closely related to each other than to their analogous
morphotypes in other lakes (convergent selection to fill same
niches in different lakes)
-diversification of color patterns is a prezygotic barrier and
promotes reproductive isolation andreinforces speciation
Rieseberg, L.H. 2001. Chromosomal rearrangements and
speciation.
-Chromosomal changes such as duplication, deletion, inversion,
and translocation cause genetic
incompatability-Plants can survive chromosomal doubling if they
breed with other mutants that have the same
doubling (polyploidy) but animals cannot-Suppressed
recombination promotes speciation
