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16.3 The process of speciation
Natural selection and chance events can change the
relative frequencies of alleles in a population.
But how does this change lead to the formation of
new species?
What is a species?
A species is a group of animals that breed with one
another and produce fertile offspring.
Since members of a species share the same
gene pool, a change in one individual can
spread through the population as that
individual spreads its genes to offspring.
If that change increases fitness, the allele will
eventually become common in a
population.
Isolating mechanisms
In order for a new species to arise, one
population must be separated into 2 or
more gene pools.
In other words, one population is divided into
2 populations.
These populations must be reproductively
isolated from each other.
Example of reproductive isolation
Plate tectonics is the movement of earths
continents.
As continents divide, oceans fill the spaces
between them.
This creates 2 populations from one. The
individuals of one continent can not mate
with those on the other continent.
This is reproductive isolation
Now there are 2 populations living in separate
environments.
These 2 populations will experience natural
selection slightly differently based on
location on Earth.
Reproductive isolation can occur
in 3 ways.
1. Behavioral isolation
2. Geographic isolation
3. Temporal isolation
Behavioral isolation
Behavioral isolation occurs when two
populations are capable of interbreeding but
have differences in courtship rituals or
other reproductive strategies that involve
behavior
For example, there are 2 birds in the united
states – the western meadowlark and the
eastern meadowlark.
Their habitats overlap in the center of the
united states.
Even though they are the same species, and
therefore can interbreed, they never do.
Why?
They will not interbreed because of their
reproductive strategies.
In order to attract a mate, the males will sing a
song.
The western meadowlark song is different
from the eastern meadowlark song.
The females will not mate with a male that
does not sing the correct song.
These birds are separated by BEHAVIOR.
This is behavioral isolation
Geographic isolation
Geographic isolation is when a population is
separated by a geographical barrier such
as a river, mountain, or body of water.
The Galapagos islands are a perfect example
of geographical isolation.
Each island is separated by a body of water –
none of the animals on each island can go
to another island.
Another example is the Abert squirrel which
lives in the southwest of the United States.
About 10,000 years ago, the Colorado River
divided the Abert squirrel into 2 separate
populations.
In other words, the Grand Canyon separated
them.
This formed 2 different gene pools.
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Scan pg 405
Natural Selection worked independently on each side
of the Grand Canyon.
After many generations, natural selection led to the
formation of 2 different subspecies.
Now we have the Abert squirrel and the Kaibab
squirrel.
Both are very similar, yet they have striking
differences.
The Abert squirrel and the Kaibab squirrel have a
common ancestor.
This is geographical isolation.
Geographical barriers do not separate all
populations.
For example, a river might separate a
population of mice, but not water snakes.
Mice cannot cross the river, but water snakes
can.
Temporal isolation
Temporal isolation is an isolation of time.
Some populations that can interbreed do not
because they breed at different times of
the year.
For example, Three similar species of orchids
live in the same rainforest.
Each organism releases pollen (sperm) on a
single day.
Because they release pollen on different days,
the different fertilization patterns develop.
They cannot all pollinate (fertilize) one
another because of timing
Testing Natural selection in Nature
Can natural selection be observed in nature?
In order for evolution to be a theory, you must
be able to test it.
Natural selection can be tested.
The most important studies showing natural
selection involve the finches on the
Galapagos.
The finches that Darwin observed were so
different, he did not realize they were all
finches – genetic variation
Their beak sizes and feeding habits were very
different.
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Once Darwin realized that all of these birds
were finches, he hypothesized that they all
descended from a common ancestor.
He proposed that natural selection shaped the
birds’ beaks as they adapted to the
different foods available to them.
Nice hypothesis – how can you back it up
with a test?
It was thought during Darwin’s time that it
could never be tested.
Two scientists from Princeton University took
the challenge to test natural selection.
Their names are Peter and Rosemary Grant.
Peter and Rosemary Grant worked on the
puzzle for over 20 years on the Galapagos
Islands.
They have been measuring variations among
the finch population for over 20 years.
Scan a picture from text
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They realized that Darwin’s hypothesis relied
on two testable assumptions.
1. If beak size of finches was to change,
there must be enough heritable variation to
allow for the changes.
2. Differences in beak size and shape must
produce differences in fitness that cause
natural selection.
The Grants did their studies on the Galapagos
island “Daphne Major.”
This island was large enough to support a
wide range of finches, yet small enough for
them to capture all known species of finches
on the island.
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Test - Variation
If beak size of finches was to change, there
must be enough heritable variation to allow for
the changes.
The Grants identified and measured as many
individual birds as possible on the island.
They also compared the data to the fossils found on
the island.
They compared beak size, wing length, leg length,
beak color, and total mass.
The characteristics of each species appeared as a bell
curve indicating typical polygenic traits.
A bell curve demonstrates that there is enough
heritable variation to allow for the
changes.
This is check point one – remember heritable
variation is necessary in order to test
natural selection.
Fact – there is enough heritable variation to
allow for changes.
Test-Natural selection
Differences in beak size and shape must produce
differences in fitness that cause natural selection.
During the rainy season on Daphne Major,
there is plenty of different foods.
Under these conditions, finches will eat the
most abundant source of food.
During the dry season or drought, many foods
becomes scarce or disappear.
During this season, beak size can mean the
difference between life or death.
Birds are forced to eat the food available that
is best suited for the size of their beaks.
Since birds with large beaks can eat small and
large seeds, they have a wider variety of
diet and therefore are more fit for the dry
season.
The Grants also observed sexual selection –
Females with large beaks were more likely
to mate with males with large beaks.
The Grants observed that beak size in that
population increased dramatically over
time.
This was proof of directional selection.
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Check point 2 - The bell curve demonstrates
that differences in beak size and shape
does produce differences in fitness that
cause natural selection.
Fact - Differences in beak size and shape will
produce differences in fitness that cause
natural selection.
Remember from earlier, that if
natural selection was to be tested,
these conditions must be met.
1. If beak size of finches was to change,
there must be enough heritable variation to
allow for the changes.
2. Differences in beak size and shape must
produce differences in fitness that cause
natural selection.
The Grants studies confirmed these 2
conditions.
This is proof of natural selection and
evolution.
This is proof that evolution can take place
rather quickly – decades (the length of time
the Grants spent on their observations)
So why is Evolution a theory and not a fact?
Just because natural selection does happen,
doesn’t mean that it did in the past –
humans were not around to witness (make
observations) during the history of life.
Evolution remains a theory.
In order for a hypothesis to become a theory, it must
be well tested and unify a broad range of
observations.
Evolution has been tested.
Example
Question – Is there enough heritable variation in
beak size to allow for changes?
Hypothesis – There is enough heritable variation in
beak size to allow for changes.
Design experiment – See Peter and Rosemary
Grant’s work
Data – Grants graph
Conclusion - there is enough heritable variation in
beak size to allow for changes.
So now we know that natural
selection does occur….How are new
species formed?
Speciation in the Galapagos finches occurred by
1. founding of a new population,
2. geographic isolation,
3. changes in the new population’s gene pool,
4. reproductive isolation,
5. and ecological competition.
Founders arrive
Many years ago, a few finches from the South
American mainland – Species A – flew to
or were blown over to one of the
Galapagos islands.
Since finches are small birds and do not
usually make long journeys over oceans,
they remained on the islands where they
learned to survive.
Geographic isolation
Eventually, the finches crossed over to other
islands where they made their homes.
New populations of finches developed on
each island – separated by sea.
Changes in the gene pool
Over time, populations on each island became
adapted to their local environments.
Each island contained different plants which
went through a similar circumstances.
Different plants will produce different seeds.
Finches on islands with large seeds had to
adapted larger beaks for cracking them
open.
Now we have finches on different islands with
different traits.
Reproductive isolation
Finches with large beaks did travel back and forth
between islands, but females would only mate
with other finches with large beaks.
This is behavioral reproductive isolation – they can
mate, but they don’t.
The gene pools of each population remained isolated
– even though they were living together.
These 2 populations are now considered to be
separate species.
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Ecological competition
The two species living on the same island must
compete with each other for food.
The birds that are the most different from one
another have the greatest fitness.
A more specialized bird will have less competition.
Over time, species will evolve in a way that
increases their differences.
A third species could eventually evolve.
This is an example where disruptive
selection will separate further.
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Continued evolution
This process repeated many times on the
different islands.
Today there are 13 species of finches on the
Galapagos.
Studying evolution since Darwin
It is useful to critique the strengths and
weaknesses of evolution.
However, scientific evidence supports the
theory that living species descended with
modification from common ancestors that
lived in the ancient past – Evolution
There are still many unanswered questions –
science
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