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Process of Speciation
Process of Speciation
• How do natural selection and genetic drift
create new species?
– Speciation – formation of new species
– Species – group of organisms that breed with
one another and produce fertile offspring –
share a common gene pool
Isolating Mechanisms
• As new species evolve, populations become
reproductively isolated from each other
• When members of two populations cannot
interbreed and produce fertile offspring,
reproductive isolation has occurred
• Three types of isolating mechanisms:
– Behavioral isolation
– Geographic isolation
– Temporal isolation
Behavioral Isolation
• Two populations are capable of
interbreeding but have differences in
courtship rituals or other types of behavior
– this prevents interbreeding
– Ex. Birds using different “songs” to attract
members of their own species
Geographic Isolation
• Two populations are separated by
geographic barriers such as rivers,
mountains, bodies of water, or man-made
barriers (roads, walls, etc)
• Does not guarantee formation of new
species because lakes may be linked or a
land bridge may be present between
islands, allowing “mixing”
Temporal Isolation
• Two or more species
reproducing at different
times
– Ex. Orchid species
release pollen on
separate days
Testing Natural Selection in Nature
• Recall the finches Darwin observed while visiting
the Galapagos Islands. The finches varied
greatly in the sizes and shapes of their beaks
and feeding habits
• Darwin first thought they were all different
species of birds, but discovered that they were
all finches and hypothesized that they had
descended from a common ancestor. He
proposed natural selection shaped the beaks as
they adapted to different foods and feeding
behaviors
Testing Natural Selection in Nature
(continued)
• Darwin’s hypothesis was tested by Peter and Rosemary
Grant. They measured finches on the islands
– Variation – Grants’ had to identify and measure as many birds as
possible (they measured beak characteristics) and from their
data, they concluded there was a great variation of traits (these
traits are polygenic!)
– Natural selection – different sized beaks meant different chances
of survival during droughts – largest beaks survived when food
was scarce
– Rapid evolution – next generation of finches had larger beaks
and they found that natural selection took place frequently and
sometimes rapidly
Galapagos Island Finches
Speciation in Darwin’s Finches
• Hypothetical scenario for the evolution of
all Galapagos finches from a single group
of “founding” birds
Speciation in Darwin’s Finches
(continued)
• 1) Founders Arrive
– Many years ago, a few
finches from South America
(Species A) flew or were
blown by a storm to one of
the islands where they
managed to survive and
reproduce (finches don’t
normally fly over large
bodies of open water).
Speciation in Darwin’s Finches
(continued)
• 2) Separation of
Population
– Later on, some birds from
Species A crossed to
another island in the
Galapagos group. Because
finches rarely move over
open water, the finches
were essentially isolated
from each other, no longer
sharing a gene pool
Speciation in Darwin’s Finches
(continued)
• 3) Changes in the Gene
Pool
– Over time, populations on
each island became
adapted to their local
environments. Different
plants on the 2 different
islands caused different
beak sizes to become
apparent. Birds with bigger
beak sizes on the second
island survived better,
creating a new species,
Species B
Speciation in Darwin’s Finches
(continued)
• 4) Reproductive
Isolation
– A few birds from
Species B cross back to
the first island and do
not breed with Species
A because they choose
their mates carefully
and only breed with bigbeak birds
Speciation in Darwin’s Finches
(continued)
• 5) Ecological Competition
– Species A and B on the first
island compete with each
other for food. During the
dry season, individuals that
are most different from
each other have the highest
fitness and less
competition. Over time,
species evolve in a way that
increases the differences.
Species B birds on the first
island may then evolve in
Species C
Speciation in Darwin’s Finches
(continued)
• 6) Continued Evolution
– This whole process
repeated itself time and
time again across all
the Galapagos Islands.
Now, there are 13
different finch species
Speciation in Darwin’s Finches
(continued)