1. Population Genetics

2. Variation of Traits Within a Population
Causes of Variation
Traits vary and can be mapped along a bell curve, which shows that most individuals have average traits, whereas a few individuals have extreme traits.
Variations in genotype arise by:
1.) Mutations
2.) Recombination (Crossing-Over)
3.) Random pairing of gametes

3. The Gene Pool
Members of a species can interbreed & produce fertile offspring
Species have a shared gene pool
Gene pool – all of the alleles of all individuals in a population

4. The Gene Pool Different species do NOT exchange genes by interbreeding
Different species that interbreed often produce sterile or less viable offspring (i.e., mule)

5. Populations A group of the same species living in an area
No two individuals are exactly alike (variations)
Individuals with great fitness can survive & reproduce to pass on their traits

6. Hardy-Weinberg Genetic Equilibrium
Says: Allele frequencies in the gene pool do not change unless acted upon by certain forces.
Hardy-Weinberg genetic equilibrium is a theoretical model of a population in which NO evolution occurs and the gene pool of the population is stable.

7. 5 Assumptions of H-W Equilibrium
1.) “All traits allow for equal fitness”
2.) “No new alleles spontaneously added”
3.) “No gene flow can occur”
5.) “No sexual selection (being picky about a mate) can occur”
4.) “No genetic drift can occur”

8. Hardy-Weinberg Genetic Equilibrium
Used to describe a non-evolving population.
 Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium.
Deviation from Hardy-Weinberg equilibrium usually results in evolution

9. Microevolution
Changes occur in gene pools due to mutations, natural selection, & genetic drift
Gene pool changes cause more VARIATION in individuals in the population
This process is called MICROEVOLUTION
Example: Bacteria becoming unaffected by antibiotics (resistant)

10. Causes of Microevolution
Genetic Drift
- The change in the gene pool of a small population due to chance
Natural Selection
Success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring
Gene Flow
-Genetic exchange due to the migration of fertile individuals or gametes between populations

11. Causes of Microevolution
Mutation
A change in an organism’s DNA
Mutations can be transmitted in gametes to offspring
Non-Random Mating (Sexual Selection)
- Mates are chosen on the basis of the best traits

12. Factors that Cause Genetic Drift
Bottleneck Effect
A drastic reduction in population (volcanoes, earthquakes, landslides …)
Reduced genetic variation
Smaller population may not be able to adapt to new selection pressures

13. Modes of Natural Selection

14. Modes of Natural Selection
Directional Selection
Favors individuals at one end of the phenotypic range (at one extreme)
Disruptive Selection
Favors individuals at both ends of the phenotypic range (at both extremes) over intermediate phenotypes

15. Modes of Natural Selection
Stabilizing Selection
Favors intermediate over extreme phenotypes
Reduces variation and maintains the current average
The typical bell curve shape

17. Forming New Species
“Speciation”

18. Speciation Formation of new species
One species may split into 2 or more species
A species may evolve into a new species
Requires very long periods of time

19. What is a “Species”? Mechanisms Contributing to Speciation:
According to the biological species concept, a species is a group of organisms that can successfully interbreed but cannot breed with other groups.
Mechanisms Contributing to Speciation:
1.) Allopatric Isolation (different geographical areas)
2.) Sympatric Isolation (same geographical area)
3.) Reproductive Isolation (difference in reproductive times, seasons, mating calls, or anatomy)

20. Allopatric Isolation
What are now different species may have at one time been one species, but became separated due to geographical isolation
Can be caused by the formation of a geographic barrier, such as a canyon or river
Allopatric Isolation: Speciation that occurs when biological populations of the same species become isolated from each other, such that they now inhabit different geographic regions
Prevents gene flow
As time passes, they become different species

21. Allopatric Isolation

22. Sympatric Isolation
Sympatric Isolation: Speciation that occurs with organisms living in the same geographic region
Often due to reproductive isolation
Reproductive isolation results from the separation of population subgroups by barriers to successful breeding. Can be due to different:
Mating seasons
Mating calls
Anatomy

23. Sympatric Isolation

24. Reproductive Isolation
“Prezygotic Barriers”

25. Reproductive Isolation
“Postzygotic Barriers”
“Reduced hybrid viability” = miscarriage during embryonic development
“Hybrid breakdown” = F1 generation is fertile; later generations are not

26. Rates of Speciation
In the gradual model of speciation (gradualism), species undergo small changes at a constant rate.
Under punctuated equilibrium, new species arise abruptly, differ greatly from their ancestors, and then change little over long periods.

27. Rates of Speciation