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The Origin of Species
AP Biology
Biological species concept
• Species—group of organisms that is able to
interbreed and produce fertile offspring
– Reproductively isolated by various factors
preventing mixing with other species
Prezygotic Barriers
• Prevent fertilization of ova if members of
different species attempt to mate
– Habitat isolation
– Behavioral isolation
– Temporal isolation
– Mechanical isolation
– Gametic isolation
Habitat Isolation
• Different species that live in different
habitats within the same area may rarely
encounter one another
– 2 species of garter snakes in the genus
Thamnophis
• One lives mainly in water
• The other is primarily terrestrial
Behavioral Isolation
• Avoidance of mating
– Can recognize own
species
• Courtship behavior
– Mating calls, dances,
coloration
Temporal Isolation
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• Reproductively
active during
different times of
the day, season,
or years
• No mixing of
gametes possible
Mechanical Isolation
• Closely related species may attempt to
mate, but fail because they are anatomically
incompatible
Gametic Isolation
• Sperm-egg incompatibility
– Doesn’t recognize the molecules on the coat
surrounding the egg
• Sperm from one species may not survive in
the environment of the female reproductive
tract of another species
Postzygotic Barriers
• Reduced Hybrid Viability
• Reduced Hybrid Fertility
• Hybrid Breakdown
Reduced Hybrid Viability
• The hybrid offspring of two species do not mature
normally and usually die in the embryonic stage of
development. In crosses between different species
of irises, for example, the embryos die before
seeds form.
Reduced Hybrid Fertility
• Sometimes hybrid
offspring mature
normally but are not
able to reproduce
successfully.
• Often occurs because
the gametes are
abnormal in some
way. Mules are sterile
hybrids formed by
mating a female horse
with a male donkey.
Hybrid Breakdown
• If a mating between two F1 hybrids
produces a second hybrid generation, this F2
generation may be unable to reproduce
• The second-generation hybrids are defective
in some way that prevents successful
reproduction. Hybrid breakdown has been
demonstrated in sunflower hybrids.
Other Concepts of Species
• Morphological Species Concept—species based
on measurable physical features
• Recognition Species Concept—species recognize
signals of suitable mates
• Cohesion Species Concept—mechanisms that
maintain discrete phenotypes b/w species
• Ecological Species Concept—species’ roles in
their environment (niches)
• Evolutionary Species Concept—species
evolutionary lineages and ecological roles
Modes of Speciation
• Allopatric speciation—populations are
separated by a geographic barrier followed
by reproductive isolation
• Sympatric speciation—populations become
genetically isolated even though their
ranges overlap
Allopatric Speciation
• Speciation that occurs when two or more populations of a
species are geographically isolated from one another
sufficiently that they do not interbreed.
• In their separate niches, the two groups go their own
evolutionary ways, accumulating different gene mutations,
being subjected to different selective pressures,
experiencing different historical events, finally becoming
incapable of
interbreeding
should they ever
come together
again.
Sympatric Speciation
• New species evolve without geographic
isolation
– Most common way is genetic change as a result
of polyploidy (extra sets of chromosomes)
Occurs in
Plants
Some allopolyploids
are especially
vigorous because
they combine the
best qualities of both
parent species
Adaptive Radiation
• Evolution of many diversely adapted
species from a common ancestor
– Example: Darwin’s Finches
• Finches with different beak shapes fit
different ecological niches
Adaptive Radiation
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1. Seeds blown over from mainland
and form small colony
2. Gene pool isolated—evolves into
new species B
3. Storms/other agents blow seeds to
nearby island and evolve into
species C
4. Some of species C recolonize the
first island and cohabit with species
B and some populate a new island
5. Speciation continues between new
areas and previously colonized
areas
Punctuated Equilibrium
• Species diverge in spurts of relatively rapid
change, instead of slowly and gradually
• Long periods of stasis (equilibrium)
followed by episodes of speciation