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.

• 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.

•

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

•

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.

•

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.

•

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.

•

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.

•

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

•

•