1. Modes and Pace of Evolution
Biology

2. Two Major Modes of Evolution
Allopatric speciation
Populations speciate after they become geographically separated
Separated in space
Sympatric speciation
Populations speciate despite being present in the same geographic area
Occur in the same area

3. Allopatric Speciation
When two or more populations are separated by geographic barriers, it allows distinctly different populations to evolve.
Alleles are not being mixed between populations
No interbreeding
Newly formed alleles in one population do not become introduced in the other populations
Typically, if the divergent species are re-united, reproductive capacity is limited or non-existent
Due to genetic differences that developed through time as a result of separation.
Examples:
Galapagos tortoises
Anolis experiments

4. Allopatric Speciation
When two or more populations are separated by geographic barriers, it allows distinctly different populations to evolve.
Alleles are not being mixed between populations
No interbreeding
Newly formed alleles in one population do not become introduced in the other populations
Typically, if the divergent species are re-united, reproduction capacity is limited or non-existent
Due to genetic differences that developed through time as a result of separation.
Examples:
Galapagos tortoises
Panamanian snapping shrimp
Anolis experiments

5. Allopatric Speciation

6. Sympatric speciation
Populations can diverge without the influence of geographic separation.
Alleles are not being mixed between segments of the population
Isolated breeding
Newly formed alleles in one population do not become introduced in the other populations
Examples
Stalk-eyed flies
Apple maggot flies

7. Sympatric speciation
Populations can diverge without the influence of geographic separation.
Alleles are not being mixed between segments of the population
Isolated breeding
Newly formed alleles in one population do not become introduced in the other populations
Examples
Stalk-eyed flies
Apple maggot flies

8. Sympatric Speciation

9. Sympatric Speciation

10. Allopatric vs. sympatric

11. Pace of Evolution Punctuated Equilibrium
Suggests that species remain relatively unchanged for long periods of time, and only over short periods of time is there speciation
Species will separate quickly into two distinct forms
Cladogenesis
There is no gradual change of one species to another
Eldredge and Gould argued that the fossil record doesn’t support Charles Darwin’s idea of gradualism
Fossil record is incomplete (“gappy”)

12. Pace of Evolution Gradualism
Suggests that species are constantly changing through time
Accumulation of minor changes leads to major change given long periods of time
Speciation occurs slowly from one form into another*
Anagenesis*
Darwin adapted Hutton and Lyell’s ideas of gradualism in geological processes
He argued that quickly occurring radical changes would be more likely to be selected against
Current observation supports his idea of gradualism
Not the same as “phyletic gradualism*”…next slide

13. Pace of Evolution

14. Anagenesis vs. cladogenesis

15. Thinking Deeper
How did the break up of the supercontinent Pangea influence the evolution of species?
What type of speciation most likely followed as a result?
Identify two ways in which sympatric speciation might occur?
Any others?
Which model on the pace of evolution do you think is supported by Mendelian genetics?
Do small changes accumulate over many generations or do major changes appear suddenly over a few?