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Regents Living Environment
16-3: The Processes of Speciation
Oswego High School
Natural selection and chance events can change the relative frequencies of
alleles.
Speciation, the formation of a ___________ _________________ occurs as a
result of these changes in allele frequency.
Species: ________________________________________________________
________________________________________________________________
Since a population of a species has a _________________ gene pool, if a
change occurs, it can ____________ to other individuals in the population and
those individuals will ___________________.
Speciation occurs through various ________________ _____
_______________.
Isolating Mechanisms
In order for speciation to occur, the gene pools of two populations must become
_____________________.
Reproductive Isolation
Reproductive isolation happens when members of two ____________________
populations cannot __________________with each other and produce fertile
offspring. At this point these populations have ______________ gene pools.
Behavioral Isolation
Behavioral Isolation occurs when two populations are
______________________ of interbreeding, but may have different
___________________ rituals or other reproductive strategies that involve
_________________.
Example: Habitats of Eastern and Western meadowlarks ________________
with each other. However, these birds have very different ________________
____________. Eastern and Western meadowlarks will not respond to each
other’s mating calls.
Geographic Isolation
Occurs when tow populations are separated by __________________________
_____________________ like rivers, mountains or bodies of water.
Example: About 10,000 years ago the Colorado River split the Albert Squirrel into
two _____________________ populations. This resulted in two separate gene
pools being formed. Natural selection worked ____________________ and led
to the formation of a sub-species called the ______________ ______________.
These two squirrels are closely related, but differ in many ways.
Albert Squirrel
Kaibab Squirrel
Geographic isolation does not always __________________ formation of a new
species. It is possible that lakes can become connected during ______________
or _______________ _______________ may form between islands. This may
allow separated populations to __________.
If two separated populations can still ___________________, they are still of the
_____________ species.
Also, barriers can separate certain types of organisms but not others.
Temporal Isolation
Temporal isolation occurs when two or more species reproduce at ___________
times.
Example: Three similar species of _________________live in the same area of
rain forest. Each of these orchids ___________________ on different days, so
they will never pollinate each other.
Testing Natural Selection in Nature
Natural selection can be observed in nature. When Darwin first saw the
Galapagos Finches, he thought they were all _________________ birds.
Because they looked so ___________________ from one another. Once he
realized they were finches, Darwin proposed that they had evolved from a
common _________________.
This was a very reasonable hypothesis, but nobody thought it was
_________________ until ________________ and __________________
____________, who spent 20 years studying these finches. They realized that
Darwin’s hypothesis relied on two testable assumptions.
1. In order for beak size and shape to evolve, there must be enough
________________ ________________ to provide raw material for
natural selection.
2. Differences in beak size and shape must produce differences in
________________, which will cause natural selection to occur.
The Grants studied medium ground finches on _____________ _____________,
one of the Galapagos Islands.
Variation
First, the Grants ____________________ and __________________ as many
individual finches as possible. They recorded those who were still ____________
and those how had _____________. They also studied many anatomical
characteristics.
These characteristics appeared in a __________ _________________
distribution, which led the Grants to conclude that there was a great deal of
__________________ of _____________________ traits.
Natural Selection
Many researchers didn’t see the different finches __________________ or
eating ______________ foods. This is because when they visited, it was the
rainy season and there was ______________ of food.
However, when the dry season came, some food became ____________, while
other food sources ________________ altogether. During this time, beak size
can be the difference between ____________ and _____________. In order to
survive, birds become feeding ________________, meaning that each bird will
select the type of food best ______________ for its beak size.
The Grants discovered that individual birds with different sized beaks had
_________________ chances of survival during a drought. They found that
individuals with the _______________ beaks had the best chances of survival
when food was scarce.
Also, large beaks had a role in ________________. Large beaked finches mate
with other large beaked finches. Because of this, the average beak size
______________ in the population in a dramatic way over the course of time.
This increase in average beak size is an example of ____________________
__________________.
By documenting this change, the Grants were able to provide _______________
for the process of evolution and that it can happen _____________. This was
very different from what Darwin envisioned, which was that evolution happened
__________________.
Speciation in Darwin’s Finches
If we can combine what we have learned in this chapter, we can show now
natural selection can lead to ___________________.
1. Founders arrive

Many years ago, a small group of finches from the South American
mainland _____________ on the Galapagos Islands. We will call
this group Species A. Finches are small and cannot fly far
distances over open water. This means they must have been
__________ there by a storm.

Once they got to one of the islands, they were somehow able to
________________ and _________________
2. Geographic isolation

Some finches from Species A moved to another island in the
Galapagos. Since they don’t usually fly over open water, they
_____________ move from island to island.

These two finch populations were now __________________ from
each other and no longer shared a gene pool.
3. Changes in the Gene Pool

Over time, the populations on the two islands _________________
to their new environments.

The first island may have had small, ___________ shelled seeds,
while the second island may have had large, _________________
shelled seeds. Directional selection would have favored those with
_______________ beaks.

Over time, this population would have evolved larger beaks, leading
to the formation of a _______________ _____________________,
population B.
4. Reproductive Isolation
 Imagine that a few birds from population B were to return to the first
island. It would be very ______________ that birds form Species A
would breed with birds from Species B.

Finches choose their mates very _________________. Finches
choose to mate with finches with the ______________ beak size as
they have.

Differences in beak size combined with ____________________
rituals leads to reproductive isolation. The ______________
_____________ of the two populations remain _______________,
even though they live in the same place.

Because of this reproductive isolation, the two populations have
now become _______________ species, Species B.
5. Ecological Competition

As Species A and B live in the ____________ environment (the first
island), they will ________________ with each other for available
food. During the dry season, those who are most ______________
from each other will have the highest __________________.

More specialized birds have ______________ competition for
certain types of seeds and other food sources. The competition
among _________________ finches is also reduced.

Over time, the birds form Species B on the first island may
_______________ into a new species, Species C.
6. Continued Evolution
 This process of isolation, genetic change, and reproductive
isolation __________________ itself across the entire island chain
of the Galapagos. Over many generations, this led to the evolution
of the __________ different finch species that live on the
Galapagos Islands.
Studying Evolution Since Darwin
Darwin made very _____________ assumptions about heritable variation, the
age of the Earth, and relationships among organisms. New _____________ from
physics, biochemistry and genetics could have proved Darwin
_______________, but they didn’t. Scientific evidence supports the theory that
living species descended with _____________________ from common
ancestors.
Limitations of Research
The Grants’ research shows the effects of directional selection, as well as how
competition and climate change affect natural selection. However, their work
does have some __________________.
Even though the Grants observed changes in _____________ ______________
of the finches, they did not observe the formation of a new _________________.
Unanswered Questions
The Grants’ studies fit into an _____________________ body of scientific
research that _______________ the theory of evolution. Remember that a theory
is a well supported, __________________ explanation of natural phenomena
that accounts for a __________________ range of observations. The theory of
evolution fits this definition.
New discoveries have led to new ____________________ that ______________
or _________________ Darwin’s ideas. However, there are many questions that
remain unanswered.
Understanding evolution is important because it is still _____________________
to this day. Evolution drives changes like ______________ ________________
in bacteria and viruses and ___________________ resistance in insects.
Understanding evolution helps us _______________ to these changes in ways
that ______________ human life.