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Copyright Pearson Prentice Hall

16-3 The Process of Speciation17-3 The Process of Speciation

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16-3 The Process of Speciation

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Natural selection and genetic drift 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 whose members can interbreed with one another and produce fertile offspring.

This means that the members of a species share a common gene pool. A genetic change in one individual can spread through the population if it increases fitness.

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Isolating Mechanisms

What types of isolation lead to the formation of new species?

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

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When populations become reproductively isolated, they can evolve into 2 separate species.

When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred. The populations then have 2 separate gene pools.

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

• behavioral isolation

• geographic isolation

• temporal isolation

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Behavioral Isolation

Behavioral isolation occurswhen two populations arecapable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior.

For example, 2 populations of meadowlarks, known as eastern and western meadowlarks, can but do not mate with one another primarily because they use different songs to attract mates.

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Geographic Isolation

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

An example of this occurred about 10,000 years ago when the formation of the Grand Canyon split up a population of squirrels.

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The Kaibab squirrel (black) on the north rim evolved separately from the Abert squirrel (grey) on the south rim.

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Temporal Isolation

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

There are 3 different species of orchid, for example, that live in the same rain forest. Each species releases pollen in one day but, since the pollen is released on 3 different one day periods, they cannot pollinate one another.

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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. He proposed that natural selection shaped the beaks of different bird populations as they became adapted to eat different foods.

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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.

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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.

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

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This was evidence of the process of evolution.

The Grants could then conclude that beak size can be changed by natural selection.

•They documented the directional nature of the selection.

•Their data showed competition & climate drive natural selection.

•Their work also suggested that natural selection can lead to speciation.

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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

• behavioral isolation

• ecological competition

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Founders Arrive

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

Because of the founder effect, the allele frequencies of this founding finch population could have already differed from those in the South American population.

Species A survive and reproduce.

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Geographic Isolation

Some birds from species A cross to a second island.

The two populations no longer share a gene pool.

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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.

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Behavioral Isolation

If population B birds cross back to the first island, they will not mate with birds from population A. Sexual selection would occur – species B birds would look for mates that had a large beak like their own. Differences in other mating behavior might also work to keep their gene pools separate.

Populations A and B would now be 2 separate species.

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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. This would insure less competition.

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Continued Evolution

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

In the end, it produced the 13 species of finches we see today.

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