1. Mechanisms of Evolution
Why does genetic change over time occur?

2. Introductory Concepts and Vocab
Populations, NOT individuals evolve
gene pool: all of a population’s genes
allele frequency: % of any allele in gene pool
genetic equilibrium: frequency of alleles remains the same over generations
Question: If a population is in Genetic
Equilibrium, is it evolving?
Answer: NO!

3. Mechanisms of Evolution
Evolution occurs whenever genetic equilibrium is disrupted!
In other words, anytime allele frequencies change…
Change can be caused by the following:
1. mutation:
2. genetic drift:
3. gene flow:
4. natural selection
5. non-random mating

4. CAUSES OF CHANGE
Mutation: Any change in the DNA sequence of an organism
caused by radiation, chemicals, chance etc. (any mutagen)
This is how new traits could arise…
See “A Mutation Story”

5. Genetic Drift
Alteration of allele frequencies by RANDOM, CHANCE events
In GENETIC DRIFT, survival has nothing to do with the fitness of an individual- it’s LUCK!
can greatly affect SMALL populations

6. Genetic Drift Examples
Bottleneck
Drastic reduction in population
Ex. Natural Disaster, …

7. Bottleneck Examples Northern Elephant Seal 1892- 50-100 seals
Today- 160,000 seal
Cheetah
Very low genetic diversity may be due to a bottleneck

8. Genetic Drift Examples
Founder effect
Part of the population founds a new colony
HD allele:
Dutch couple migrated to South Africa
Today- 30,000+ have gene that causes Huntington’s disease & can be traced back to the couple
Polydactyly
Amish population in Lancaster, PA

9. * migrating individuals entering and leaving a population
Gene Flow
* migrating individuals entering and leaving a population
Immigration
Emigration
Bu-Bye!
Hello!

10. Exit Questions Explain how genetic drift and gene flow are different.
Explain how genetic drift acts as a mechanism for evolution, giving at least one example.
Explain how gene flow acts as a mechanism for evolution, giving at least one example.

11. non-random mating-
choosing mates based on preferred characteristics. Ex. peacocks

12. Exit Question
Explain how non-random mating (aka sexual selection) may cause genetic change in a population (aka evolution).

13. Natural Selection
Selection of variations in organisms which enhance their survival
There are 3 types of Natural Selection that act on variation:
Stabilizing
Directional
Disruptive

14. Thought Questions on Natural Selection
Why does natural selection act on the phenotype rather than the genotype of an organism?
Why is genetic variation in a species valuable?
Ex. Potato famine
If natural selection is happening, why do some lethal alleles continue in populations? Shouldn’t diseases like sickle-cell anemia, cystic fibrosis and Tay-Sachs disappear?
See “HIV Immunity” and “Double Immunity”
Discuss the value of mutations and how natural selection affects which mutations persist.

15. Macroevolution and Speciation
Speciation Definition: the evolution of new species (REPRODUCTIVE ISOLATION = THE KEY!!)
Species: a group of organisms that can interbreed and produce fertile offspring in nature…
Reproductive Isolation:
Occurs when formerly interbreeding organisms can no longer mate & produce fertile offspring

16. Causes of Reproductive isolation:
a.geographic isolation: physical barrier divides a population (ex. Islands, mountains, rivers, canyons etc.)
Salamander Speciation
b. seasonal isolation:
breed at different times
c. mechanical isolation
physical incompatibility
d. behavioral isolation:
different courtship behaviors/signals
Albatross dance
e. changes in chromosome number (see next slide)
Example: mountain range or body of water dividing a population

17. Reproductive Isolation,cont’d:
Change in chromosome numbers
example: donkey (64) and horse (62) create mule (63) which is sterile
Polyploidy: having multiple sets of chromosomes (common in plants)
example: crop plants like wheat
mule

18. Assignment
Draw 3 comic strips showing how geographic isolation, seasonal isolation, and behavioral isolation could result in speciation (reproductive isolation) over time.
Explain how your comic shows how speciation might occur clearly below each comic.

19. Patterns of Evolution
Divergent Evolution: a pattern of evolution in which species that were once similar to an ancestral species, become increasingly distinct or “diverge”
example: Adaptive Radiation
evolution of species into an array of species to fit a number of diverse habitats.

20. * Convergent Evolution
pattern of evolution in which distantly related organisms evolve similar traits usually because of similar environments

21. END

22. Hardy-Weinberg Equilibrium
Equilibrium = no evolution, no genetic change!
Conditions for Hardy-Weinberg Equilibrium = exactly the opposite of conditions for evolution
1. NO mutation:
2. NO genetic drift (Very Large Population size) :
3. NO gene flow:
4. NO natural selection
5. random mating
Discussion: Why are the conditions for Hardy-Weinberg Equilibrium unlikely to occur in nature?

23. Hardy-Weinberg Equation
p=frequency of one allele (A); q=frequency of the other allele (a);
p+q= (p=1-q & q=1-p)
p2 =frequency of AA genotype; 2pq=frequency of Aa plus aA genotype; q2 =frequency of aa genotype;
Hardy Weinberg Equation:
p2 + 2pq + q2 = 1.0
Can be used to calculate allele and genotype frequencies if a population is in Hardy-Weinberg Equilibrium

24. Practice Problem
Calculate (predict) the frequency of each possible genotype in the following population of dogs.
25 % (0.25) of the dogs have blue eyes
75% (0.75) of the dogs have brown eyes.
Answer: homozygous recessive = 0.25 (25%), Homozygous dominant = 0.25 (25%), Heterozygous= 0.5 (50%).