1. Speciation…
How new species form!

2. What exactly is a species?
A species is a population that can breed with each other to create viable offspring, but cannot create viable offspring with members of other species (Biological Species Concept)

3. How does Speciation Occur?
Two populations become reproductively isolated from each other
The populations can no longer make viable offspring together
Natural selection acts on the two populations independently

4. How does Speciation Occur?
Pre-zygotic (before zygote) Isolating Mechanisms:
1. Behavioral Isolation: mating rituals differ
2. Geographic Isolation: rivers, mountains, etc. separate two groups
3. Temporal Isolation: reproduce at different times of day or year

5. More Pre-Zygotic Barriers
4. Mechanical Isolation: Structural differences in genitalia prevent successful mating
5. Gametic Isolation: Mating occurs, but gametes fail to fertilize each other correctly

6. Post-Zygotic Barriers
If the two new species do try to mate, they will not make offspring because of:
1.Reduced hybrid viability: hybrid zygotes do not develop or reach sexual maturity
2. Reduced hybrid fertility: hybrids cannot reproduce
3. Hybrid breakdown: hybrids have unviable or infertile offspring

7. Results of Post-Zygotic Barriers
Male Ligers are infertile 
Many ligers die young from neurological disorders
Mules are infertile 