1. Do Now
Explain the difference between directional and disruptive selection.
Directional – one extreme of the variation of the trait is most fit
Disruptive – BOTH extremes of the variation is most fit
What type of selection is most common?
Stabilizing selection
Homework: finish book work from class

2. Speciation

3. What is Speciation?
A species is defined as a group of organisms that can successfully interbreed, producing fertile offspring.
Speciation – the formation of new species
How does a species evolve into two new species?
The gene pools of two populations must become separated.
Over time genetic changes cause reproductive isolation between the populations.

4. Reproductive Isolation
Reproductive isolation means that the organisms can no longer interbreed successfully.
There are 2 main types of Reproductive Isolation:
Pre-zygotic – prevents fertilization from taking place
Examples: Geographical, Behavioral, Temporal
Post-zygotic – prevents fertile offspring from being produced after fertilization occurs

5. Geographic Isolation
Geographic isolation occurs when populations are physically separated from each other.
Natural selection works separately on each population.
Differences in environment can cause genetic differences.
Over time, genetic changes can lead to formation of 2 separate species.
Geographic isolation does not always lead to the formation of new species.
Geographical barriers may be temporary.
Geographical barriers may not affect all organisms (Ex: River would separate land animals but not necessarily birds.)

6. Geographic Isolation Examples
Pupfish lived in a large lake in Death Valley
After Ice age, the species were isolated into different ponds
The environments in the ponds differet and the poputions diverged, forming two different species
Abert squirrel on left, kaibab squirrel on right – used to be one species but colorado river split it over 10,000 years ago. Look similar but are now different species

7. Behavioral Isolation
Occurs when species do not interbreed because of behavioral differences.
Species do not recognize the mating calls or courtship behaviors of other species.
Bird Rituals

8. Temporal Isolation
Many species do not breed all year long and have specific mating seasons.
Temporal Isolation occurs when species sharing the same environment may reproduce at different times in the year.

9. Post-zygotic Isolation
Post-zygotic isolation prevents fertile offspring from being produced from the mating of two different species.
Several different ways that post-zygotic isolations occurs:
Offspring may die before birth
Offspring may die before reproductive age
Offspring may live, but may not be fertile

10. Example of Post-zygotic Isolation
Horses and donkeys can produce offspring, even though they are different species.
The offspring of a female horse and male donkey is called a mule.
Mules are sterile and cannot reproduce.