1. The Evolution of Populations
Chapter 16

2. Genetic Equilibrium
Population Genetics: the study of evolution from a genetic viewpoint
Most popltns have variation amongst their traits
i.e. different heights, skin color, etc.
If graphed, looks like a bell

3. Effects of Natural Selection
Natural Selection can cause a shift in traits within a population
Depends on which traits are favorable in that environment
Those traits will increase over time
4 Different Effects:
Stabilizing Selection
Directional Selection
Disruptive Selection
Sexual Selection

4. Stabilizing Selection
Occurs when the average form of a trait is considered the highest fitness level (best for that environment)
i.e. height
Causes decrease in variation

5. Directional Selection
Occurs when one extreme is considered highest fitness level
i.e.: an anteater’s tongue length
Causes a shift in variation

6. Disruptive Selection
Occurs when both extremes are considered highest fitness level
i.e.: black/white limpets on barnacles
Causes splitting of phenotypes; may lead to formation of new species

7. Sexual Selection
Occurs when female organisms select a mate based on phenotype
May require extreme male traits
i.e. bright colors, unique mating calls, etc.

8. Speciation
Chapter 16

9. Formation of Species Speciation: process of forming new species
Results in many related popltns
May remain similar or become quite different
Manchester
Terrier
Dandie
Terrier

10. Concepts of “Species” 1) Morphological: species defined by appearance
Does not account for individual variations
2) Biological: species defined by reproduction patterns
Does not apply to ancient organisms
3) Modern: species are similar in appearance and can interbreed to produce fertile offspring
Concept currently used in Biology

11. Isolating Mechanisms
Speciation occurs when members of a species stop interbreeding and split
Become isolated from each other
2 Types Isolating Mechanisms:
1) Geographic
2) Reproductive

12. Geographic Isolation
Geographic Isolation: the physical separation of members of a popltn
i.e.: by canyons, mountains, rivers, etc.
Once physically separated, will continue to diverge until they can no longer mate

13. Reproductive Isolation
Reproductive Isolation: members of a popltn no longer able to mate successfully
i.e. due to disruptive selection, changes in mating habits, etc.
Once separated, will continue to diverge

14. Rates of Speciation
Gradualism: some species evolve gradually & at stable rate
Although speciation may take millions of years, it is not always gradual

15. Punctuated Equilibrium
Punctuated Equilibrium: when species evolve in rapid spurts of great change
Involves long periods of no change between “growth spurts”
Usually due to natural disasters

16. The Gene Pool
Gene Pool: the total genetic information available in a popltn
All the possible alleles available
Allele Frequency: # certain allele
total # alleles in gene pool
Used to predict phenotypes of future offspring

17. Hardy-Weinberg Principle
“Allele frequencies in a popltn will remain constant unless acted upon by an outside force”
AKA: Evolution will not occur unless…
5 conditions must be met for this to hold true….

18. Hardy-Weinberg Conditions
1) No Mutations can occur
BUT, mutations occur constantly (can’t control)
2) Individuals may not enter or leave
BUT, individuals immigrate & emigrate constantly
3) Population size must be VERY large
BUT, popltn size is limited by many factors
i.e.: space, food source, competition, etc.

19. Hardy-Weinberg Conditions (con’t)
4) Mating must be completely random
BUT, mate selection is influenced by proximity, physical charac., relation, etc.

20. Hardy-Weinberg Conditions (con’t)
5) Natural Selection does not occur (all traits considered “favorable”)
BUT, “favorable” traits are determined by environment
4 Types Natural Selection:
Stabilizing
Directional
Disruptive
Sexual