1. Mechanisms for Evolution
Natural selection
traits that improve survival or reproduction accumulate in the population
ADAPTIVE change
Genetic drift
frequency of traits changes in a population due to chance events
RANDOM change
Mutation, Gene Flow, Nonrandom Mating

2. Natural Selection
Selection acts on any trait that affects survival or reproduction
predation selection (speed, camouflage, defenses)
physiological selection (disease resistance, protection from injury)
sexual selection (attractiveness, fertility)

3. Modes of Selection
Directional: Darker mice are favored because they live in dark rocks.
Disruptive: Intermediate color at a disadvantage. Mice live in both light and dark rocks.
Stabilizing: Intermediate color is advantaged, extremes are not favored.
Examples: Directional – beaks of finches selected to accommodate food source; Stabilizing – birth weight -> either extreme can be harmful to baby or mother; Disruptive – black bellied seed crackers of South Africa -> seeds come in large and small variety so intermediate beak size is selected against.

4. Genetic Drift Chance events change frequency of traits in a population
not adaptation to environmental conditions
Founder effect
Bottleneck

5. Genetic drift (can greatly affect small populations)
CRCR
CRCW
CWCW
Only 5 of
10 plants
leave
offspring
Only 2 of
Generation 2
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3

6. Founder effect (Genetic drift)
A new population is started by a small group of individuals or are isolated from other population
just by chance some rare traits may be at high frequency; others may be missing
skews the gene pool of new population
less genetic diversity
Small founder group, less genetic diversity than Africans
All white people around the world are descended from a small group of ancestors
100,000 years ago
(Chinese are white people!)
albino deer Seneca Army Depot

7. Example: Distribution of blood types
Distribution of the O type blood allele in native populations of the world reflects original settlement
South & Central American Indians were nearly 100% type O for the ABO blood system. Since nothing in nature seems to strongly select for or against this trait, it is likely that most of these people are descendants of a small band of closely related "founders" who also shared this blood type

8. Bottleneck effect (genetic drift)
A large population drastically reduced by a disaster
famine, natural disaster, loss of habitat…
loss of variation by chance event
narrows the gene pool

9. Example: Cheetahs All cheetahs share a small number of alleles
less than 1% diversity
as if all cheetahs are identical twins
2 bottlenecks
10,000 years ago
Ice Age
last 100 years
poaching & loss of habitat

10. Gene Flow
Migration changes allelic frequencies in the gene pool

11. Nonrandom Mating Similar genotypes mate with one another Inbreeding
Assortive mating – increasing frequency of homozygous individuals (Ex: plant selfing)

12. Mutation Ultimate source of variaton
Random – only passed if in gametes
Mutagens

13. Question??? What do you think is the definition of a species?
Pair up and write down your definition of a species and how do we get new species?