Chapter 17 Notes

The Process of Speciation


The ultimate outcome of natural selection is the evolution of a new species.

·    Species: A population of interbreeding organisms capable of producing fertile offspring.

·    Speciation: Changes that lead to the formation of a new species.


Speciation occurs when:


§    Species cannot interbreed and cannot produce fertile offspring.

§    The gene pools of a population becomes separated or geographically isolated from one another.

§    As species evolve they become reproductively isolated from each other.

Example:  The donkey and the horse are two different species.  If they breed they will produce a mule. The mule is infertile (incapable of producing offspring of its own.)






Reproductive isolation: the members of two populations cannot interbreed and produce fertile offspring.


Reproductive isolation can develop in a variety of ways:


1. Behavioral isolation: two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. 


Example: The eastern and western meadowlarks.  The members of the two species will not mate with each other because they use different songs to attract mates.  They will not respond to one another’s song.

 Meadow lark


2. Geographic Isolation: two populations are separated by geographic barriers such as rivers, mountains, volcanic eruptions, deforestation, or bodies of water.



Example: The Albert squirrel lives in the Southwest.  They were separated by the Colorado River causing the species to split into two separate populations.  Two separate gene pools are formed.


3. Temporal Isolation:  two or more species reproduce at different times.


Example: Three similar species of orchid all live in the same rain forest.  Each species releases pollen only on a single day.  Because the three species release pollen on different days, they cannot pollinate one another.



Speciation in Darwin’s Finches:


Speciation in the Galapagos finches occurred by founding of a new population, geographic isolation, changes in the new population’s gene pool, reproductive isolation, and ecological competition.




1. Founders arrive: Species A finch flew over or was blown over by a storm from South America to the Galapagos.

2. Separation of Populations: Some birds from Species A crossed to another island in the Galapagos.

3.  Changes in the Gene Pool: Populations on each island became adapted to their local environments and the food that was available. 


§    The Galapagos Islands are all very different.  Some are desert and some tropical. 

§    Those who could eat the food lived and passed on their genes for their beak type to their offspring. 

§    Those who could not eat the food died.


4. Reproductive Isolation: A few birds from the second island cross back to the first island.  Birds prefer to mate with birds with the same sized beaks. 

§    Because the birds on the two islands have different-sized beaks, it is likely that they would not mate. 

§    The gene pools remain isolated.




5. Ecological Competition: As these two new species live together in the same environment, they compete with each other for available seeds. 


§    Dry season: birds most different from each other have the highest fitness. 

§    The more specialized birds have less competition.

§    This increases these differences over time.


6. Continued Evolution:  This process repeated itself many times until there were 13 different species on the islands.

Peter and Rosemary Grant study Darwin’s finches today.  They have found that directional selection of the bird beaks is the type of natural selection that is taking place today.