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Speciation
The formation of new species
Speciation can occur via 3
main processes:
• 1.) Allopatric Speciation
• Occurs when a population is divided by a geographic barrier
so that breeding between two populations is prevented.
• Common barriers:
– Mountains, rivers, areas devoid of food or water, areas covered
with lava.
• Once reproductively isolated, gene frequencies in the 2 pops
can diverge due to natural selection, mutation or genetic drift.
• If gene pools diverge significantly, interbreeding between the
populations will not be possible, even if the barrier is
removed.
• As a result, new species have formed.
• 2.) Sympatric Speciation:
• Formation of new species without a geographic barrier.
• Can happen in 3 ways:
– A.) Balanced polymorphism
» Organisms of same species who share similar traits will
associate and mate. Since they keep away from others in
their species who do not share their traits, their gene pools
can diverge like in allopatric speciation.
» Ex: color patterns and insects.
– B.) Polyploidy
» Possession of more than the normal 2 sets of
chromosomes found in diploid cells.
» Occurs via non-disjunction, often in plants.
» Can result in triploid, tetraploid and higher ploidy numbers.
• C.) Hybridization
• Creation of a hybrid progeny from two distinctly
different forms of a species, or two related species
that are normally reproductively isolated.
• 3.) Adaptive Radiation
• Rapid evolution of many species from a single ancestor.
• Occurs when ancestral species is introduced into an area
where diverse geographic/ecological conditions are available
for colonization.
• Large varieties of niches encourage speciation because
groups will fill different niches.
• Ex:
– Marsupials of Australia began with colonization and subsequent
adaptive radiation of a single ancestral species.
– The 14 species of Darwin’s Finches on the Galapagos Islands
evolved form a single ancestral South American species.
Maintaining Reproductive Isolation
2 ways:
• 1.) Prezygotic isolating mechanisms
– A.) Habitat isolation
– B.) Temporal isolation
– C.) Behavioral isolation
– D.) Mechanical isolation
– E.) Gametic isolation
• 2.) Postzygotic isolating mechanisms
• A.) Hybrid inviability
• B.) Hybrid sterility
• C.) Hybrid breakdown
Patterns of Evolution
•
1.) Divergent evolution
• 2 or more species that originate from a common ancestor and have become
increasingly different over time. Due to allopatric or sympatric speciation or adaptive
radiation.
•
2.) Convergent evolution
• 2 unrelated species share similar traits due to adapting to similar environments.
• a dolphin’s and a shark’s body shape
•
3.) Parallel evolution
• 2 related species that have made similar evolutionary changes after divergence from
common ancestor.
• Flying squirrels and sugar gliders
•
4.) Coevolution
• Evolution of one species in response to new adaptations that appear in another
species.
• Often seen in predator-prey relationships; plants and plant-eating insects; polinators
and flowering plants; pathogens and immune systems