Download Speciation - Winona State University

Jason Morgan and Dan SansCrainte
November 19, 2007
Speciation
Speciation – Evolution of reproductive isolation within an ancestral species,
resulting in two or more descendent species.
Modes of Speciation
Allopatric Speciation – The evolution of genetic reproductive barriers between
populations that are geographically separated by physical barriers such as
topography, water (or land), or unfavorable habitat.
Allopatric Speciation through Vicariance – Divergence of two large
populations caused by physical barriers.
- Example – The development of the Isthmus of Panama separated many
marine species into two separate populations. Some of these have
diverged into distinctly different species.
- Scientists agree that natural selection is the underlying cause of speciation
through vicariance.
Peripatric Speciation – Divergence of a small population from a widely
distributed ancestral form (also known as founder effect speciation).
Mayr’s Founder Effect
Founder Principal- The founders of a new colony of a species contain
inevitable only a small fraction of the total variation of the parental species.
- Populations of 20 founders diverged far more from each other than
populations founded by 4000 individuals each
- Founders carry only a small reservoir of genetic diversity
Success of Founder populations
- Use caution when generalizing success
o Selection in nature is primarily for over-all fitness
o Inbreeding in new colonies is not always harmful
o Brother-sister mating in certain organisms can be continued for
hundreds of generations, while in others it would lead to rapid
extinction
- Abundant evidence for occasional success of founder population
o New Zealand- numerous animals introduced with only a few
individuals as founders, yet were highly successful
Environment of founder populations
- Populations exposed to new biotic and genetic environmental factors
- Populations categorized into two categories:
o Central Populations
 Optimal physical and biotic environment
 Most mortality related to high population densities
Jason Morgan and Dan SansCrainte
November 19, 2007
o Isolated Populations (especially peripheral isolated populations)
 Usually establish in less than optimal conditions
 Populations can fluctuate
 Bad years Good years More capable of responding to new selection pressures and
new evolutionary opportunities
 Are in immediate position to utilize new gene combinations
Successful speciation
- Most species bud off peripheral isolates regularly
o Nearly all reestablish contact with parental species or die out
- “The odds are very much against a successful passing through the bottle
neck of reduced variability as well as the reaching of a new level of high
variability and of a new ecological niche” (Mayr 1966)
o It doesn’t matter if 98 or 99 out of 100 founder populations or
isolates fall by the wayside. As long as one discovers a new niche
once in awhile evolutionary progress continues.
o If the new niche is a whole new adaptive zone like the ones found
by the first amphibian, first bird etc. then there will be an
avalanche of successful speciations until that zone is completely
filled
Parapatic Speciation – Neighboring populations between which there is
modest gene flow, diverge and become reproductively isolated.
- Neighboring populations of the same species but live in different
habitats/use different resources.
- Example – Several species of grasses have evolved tolerances to heavy
metals found near mines. These grasses also flower at different times
and self pollinate more frequently then grasses further away from the
mines.
Sympatric Speciation – The evolution of reproductive barriers within a single,
initially random mating population.
- Requires a barrier to gene exchange without any spatial separation of the
species.
- Controversial (Mayr was very critical of sympatric speciation)
- Model of sympatric speciation - Certain homozygous genotypes have high
fitness for one or the other of two resources, and the intermediate
heterozygous form have lower fitness for both resources.
- Example – Apple Maggot Fly – The larvae of apple maggot flies develop in
ripe fruits. In the past the primary host plants were hawthorns, but about
150 years ago they started invading cultivated apple crops. Over time
Jason Morgan and Dan SansCrainte
November 19, 2007
allele frequencies at several loci have changed significantly between the
hawthorn flies and the apple flies.
Speciation through Polyploidy
- Polyploidy is an organism with more than two complements of
chromosomes (usually found in plants).
- Polyploidy arises through a failure of segregation in meiosis.
- Plants with odd-numbered ploidy are usually sterile (3n, 5n, etc.) Because
the hybrid between a tetraploid (4n) and its diploid (2n) is triploid (3n),
and generally sterile, the tetraploid (4n) is reproductively isolated from the
diploid (2n) and a distinct biological species.
Recombinational Speciation – Certain genotypes of hybrids between two
species may be both fertile and reproductively isolated from their parents. These
genotypes may increase in frequency and between a distinct species.
- Also known as hybrid speciation
- Rare in animals but more common in plants.
- Example – The hybridization between two species of sunflowers has given
rise to three other genetically isolated and fertile species of sunflowers.
How long does speciation take?
Speciation is a slow historical process, which can never be observed by an
individual observer (Mayr 1966). The rate of speciation varies greatly among
different species. For example, some sympatric species of Drosophila developed
in 80,000-200,000 years while a study on American song birds has shown
speciation to take as long as 5 million years. The speed of speciation differs
depending on the method of speciation. Speciation due to mutation and genetic
drift would be very slow while speciation due to ecological or sexual selection
would be much faster.
Selected References
Futuyma, Douglas. Evolution, Sunderland Massachusetts: Sinauer Associates,
2005.
Grant, Verne. Plant Speciation, New York: Columbia University Press, 1981
Mayr, Ernst. Animal Species and Evolution, Cambridge Massachusetts: The
Belknap Press of Harvard University Press, 1966.
Mayr, Ernst. This Is Biology, Cambridge Massachusetts: The Belknap Press of
Harvard University Press, 1998.
Jason Morgan and Dan SansCrainte
November 19, 2007