Download File - Mrs. LeCompte

MODES OF SPECIATION
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Reproductive barriers form boundaries around species, and the evolution of these barriers is the
key biological event in the origin of new species.
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Two Types:
1) Allopatric_Speciation = Speciation that occurs when the initial block to gene flow is a
geographical barrier that physically isolates the population.
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The two populations are then subjected to different environmental conditions…their
natural selection will be different
o Ex. The Grand Canyon is an impassable barrier to rodents, but is easily crossed by
birds. As a result, the same bird species populate both rims of the canyon, but each rim
has several unique species of rodents.
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Conditions favoring allopatric speciation
o A small isolated population is more likely to change
o Usually occurs at the edge of the parent population’s range
o Peripheral isolates are good candidates for speciation for three reasons:
o The gene pool of the peripheral isolates probably differs from that of the parent
population from the outset.
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Fringe individuals are often extremes of any genotype or phenotype cline
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Increases founder effect
o Genetic drift will continue to cause chance changes in the gene pool of the small
peripheral isolate until a large population is formed.
o Evolution caused by selection is likely to take a different direction in the peripheral
isolate than in the parental population
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Different areas/environments = different selection pressures
Special Case of Allopatric Speciation:
o Adaptive Radiation = the rapid evolution of several species from a single common ancestor.
o Common after the removal of a competitor, a predator, or a change in the environment
o Ecological Release = freedom for a species to expand its use of resources within
habitats where competition has been removed
o Ex. Galapagos Islands - Darwin’s finches
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Multiple invasions and allopatric speciations created their diversity
2) Sympatric Speciation = Formation of new species without the presence of a geographical
barrier
o Many plant species have originated from improper cell division that results in extra sets of
chromosomes = polyploidy
o Depending on the origin of the extra set of chromosomes, classified in two forms:
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Autopolyploid = An organism that has more than two chromosome sets, all
derived from a single species.
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Nondisjunction in the germ cell line results in diploid gametes
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Self fertilization to the tetraploid (4n)state.
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Tetraploids cannot interbreed with diploids of the parent population,
because hybrids would be triploid (3n) and sterile
o Ex. Many large strawberries (8n)
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Allopolypolid = polyploid hybrid resulting from two different species.
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More common than autopolyploidy
(will draw in class tomorrow!)
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25-50% of all plant species are polyploids

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Ex. bread wheat oats, cotton, potatoes, and tobacco
Plant geneticists are presently inducing these genetic accidents to produce new
polyploids with high yields and disease resistance.
Sympatric Speciation may also occur in animals if a group of animals become fixed on resources not
used by the parent population as a whole.

Ex. The great diversity of Cichlid fishes in Lake Victoria is a result of the use of different food
sources
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More common than allopatric speciation
Convergent Evolution
Convergent Evolution = when a similar biological trait evolves in two seperate species as a result of
being in similar environments

Ex. Cichlid fishes in different lakes on Africa
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Develop analogous traits = similar function, but different origin
o Ex. Bird and bat wings for flight