Copyright Pearson Prentice Hall 16-3 The Process of Speciation.

transcript

Copyright Pearson Prentice Hall

16-3 The Process of Speciation

Natural selection and chance events can change the relative frequencies of alleles in a population and lead to speciation.

Speciation is the formation of new species.

A species is a group of organisms that breed with one another and produce fertile offspring.

What is a species?What is a species?

Its not as straightforward a question as most believe.Its not as straightforward a question as most believe.

Evolution creates (and destroys) new species, but … Evolution creates (and destroys) new species, but …

TheseThese are members of different species - eastern (left) are members of different species - eastern (left) and western (right) meadowlark.and western (right) meadowlark.

What is a Species?What is a Species?

There is only one existing human species. There is only one existing human species.

What is a Species?What is a Species?

A species is a group of individuals capable of A species is a group of individuals capable of interbreeding to produce fertile offspring.interbreeding to produce fertile offspring.

Determining What Is and What Isn’t a Distinct Species Can Determining What Is and What Isn’t a Distinct Species Can Have Economic ConsequencesHave Economic Consequences

Northern spotted owl (left) and barred owl (right).Northern spotted owl (left) and barred owl (right).

How Many Species Are There?How Many Species Are There?

We don’t know. We don’t know.

About 2 million species have been described.About 2 million species have been described.

Estimates of existing species number range Estimates of existing species number range from 4 million to 100 million (with 10-15 million from 4 million to 100 million (with 10-15 million being a more commonly considered upper being a more commonly considered upper estimate).estimate).

Isolating Mechanisms

What factors are involved in the formation of new species?

The gene pools of two populations must become separated for them to become new species.

As new species evolve, populations become reproductively isolated from each other.

When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.

Reproductive isolation can develop in a variety of ways, including:

• behavioral isolation

• geographic isolation

• 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.

Courtship rituals, Courtship rituals, like these, are like these, are critical for mating critical for mating within a species, within a species, but ineffective for but ineffective for attracting attracting members of other members of other species.species.

Jumping SpidersWoods and Fields of New England

Differ in Appearance and Mating DanceDiffer in Appearance and Mating Dance

Behavioral IsolationBehavioral Isolation

Horse Horse (2N=64)(2N=64) X Donkey X Donkey (2N=62)(2N=62) = Mule = Mule

Geographic isolation occurs when two populations are separated by geographic barriers such as rivers or mountains.

Two species of ground squirrel are postulated to have Two species of ground squirrel are postulated to have descended from a common ancestral population that descended from a common ancestral population that was separated by formation of the Grand Canyon.was separated by formation of the Grand Canyon.

Harris’ antelope squirrelHarris’ antelope squirrel White-tailed antelope squirrelWhite-tailed antelope squirrel

Geographic barriers do not guarantee the formation of new species.

If two formerly separated populations can still interbreed, they remain a single species.

Potential geographic barriers may separate certain types of organisms but not others.

Temporal isolationTemporal isolation occurs when two or occurs when two or more species more species reproduce at different reproduce at different times. times.

Mating at different times of year

Testing Natural Selection in Nature

Studies showing natural selection in action involve descendants of the finches that Darwin observed in the Galápagos Islands.

The finches Darwin saw were different, but he hypothesized that they had descended from a common ancestor.

Peter and Rosemary Grant tested Darwin’s hypothesis, which relied on two testable assumptions:

• For beak size and shape to evolve, there must be enough heritable variation in those traits to provide raw material for natural selection.

• Differences in beak size and shape must produce differences in fitness, causing natural selection to occur.

The Grants tested these hypotheses on the medium ground finch on Daphne Major, one of the Galápagos Islands.

During the rainy season, there is plenty of food.

During droughts, food becomes scarce.

Individual birds with different-sized beaks had different chances of survival during a drought.

When food was scarce, individuals with large beaks were more likely to survive.

The Grants provided evidence of the process of evolution.

Beak size can be changed by natural selection.

Conclusion: Natural selection can occur often and rapidly.

Speciation in Darwin’s Finches

Describe the process of speciation in the Galápagos finches.

Speciation in Darwin's Finches

Speciation in the Galápagos finches occurred by:

• founding of a new population

• geographic isolation

• changes in new population's gene pool

• reproductive isolation

• ecological competition

Founders Arrive

A few finches—species A—travel from South America to one of the Galápagos Islands.

There, they survive and reproduce.

Geographic Isolation

Some birds from species A cross to a second island.

The two populations no longer share a gene pool.

Changes in the Gene Pool

Seed sizes on the second island favor birds with large beaks.

The population on the second island evolves into population B, with larger beaks.

Reproductive Isolation

If population B birds cross back to the first island, they will not mate with birds from population A.

Populations A and B are separate species.

Ecological Competition

As species A and B compete for available seeds on the first island, they continue to evolve in a way that increases the differences between them.

A new species—C—may evolve.

Continued Evolution

This process of isolation, genetic change, and reproductive isolation probably repeated itself often across the entire Galápagos island chain.

Studying Evolution Since Darwin

Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the ancient past.

Scientists predict that as new fossils are found, they will continue to expand our understanding of how species evolved.

The most important factor involved in the evolution of the Harris’ antelope and white-tailed antelope squirrels of the American Southwest appears to be

a. temporal isolation.

b. geographic isolation.

c. behavioral isolation.

d. different food sources.

When two species do not reproduce because of differences in mating rituals, the situation is referred to as

d. reproductive isolation.

All of the following played a role in speciation of Galápagos finches EXCEPT

a. no changes in the gene pool.

b. separation of populations.

c. reproductive isolation.

d. natural selection.

Beak size in the various groups of Galápagos finches changed primarily in response to

a. climate.

b. mating preference.

c. food source.

d. availability of water.

One finding of the Grants' research on generations of Galápagos finches was that

a. natural selection did not occur in the finches

b. natural selection can take place often and very rapidly.

c. beak size had no effect on survival rate of the finches.

d. natural selection was slow and permanent.

The most important factor involved in the evolution of the Kaibab and Abert squirrels of the American Southwest appears to be