1. HYBRID ZONES
UNIT 5 EVOLUTION

2. Hybrid Zones
Hybrid Zone – a region where two distinct populations or species found in close proximity to each other produce offspring of mixed ancestry.
HYBRID ZONE
POPULATION X
POPULATION Y

3. Hybrid Zone Patterns
Form when two species that lack reproductive barriers come in contact with each other
Can be any length or width
A species’ frequency decreases as they get closer to the hybrid zone

4. Hybrid Zones over Time
There are 3 possibilities for hybrid zones over time
Reinforcement of barriers
Fusion of Species
Stability

5. Reinforcement Strengthening of reproductive barriers
Occurs only when a hybrid is weaker than its parent species
Barriers are much stronger in sympatric populations than in allopatric populations

6. Fusion of Species Weakening of reproductive barriers
Occurs when the gene pools of two different species become increasingly alike
Can result in two species fusing into a single species
“Reverse Speciation”

7. Stability The constant and stable formation of hybrids
Occurs when the hybrids survive and reproduce better than their parent species

8. Darwin’s Questions How long does it take for a new species to form?
How many genes change when a single species splits into two?

9. Speciation over Time
Darwin gathered two pieces of evidence to measure speciation over time
The patterns of fossil records
Data showing the intervals between speciation events of a particular group of organisms

10. Fossil Record
Scientists have observed species inside of fossils that stayed for millions of years without change, and then soon disappeared
Punctuated Equilibria – when a species undergoes little to no morphological change in the layers of a fossil
Can result in extinction over time
Shows how speciation occurred at rapid rates

11. The Speciation Process
Speciation starts after gene flow between populations is interrupted
Speciation can be interrupted by environmental changes or natural events (storms, hurricanes, and floods)
Once gene flow is interrupted, populations start to become reproductively isolated

12. The Speciation Process
Can be affected by a single gene (birth defects)
A few amount of genes can be a reproductive barrier
If an organism has too much or less genes, it can influence the evolution of reproductive isolation
Reproductive Isolation can result in the emergence of a new species