1. Evolution as Genetic Change

2. Evolution as Genetic Change
Natural selection acts on phenotypes, survival and reproduction determine which alleles are inherited, changing relative frequencies of alleles in a population over time.
Thus evolution is any change in the relative frequencies of alleles in a population’s gene pool and acts on populations, not individuals.

3. Evolution of Single-Gene Traits
Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution.
One of the two phenotypes may make an organisms better fit, thus under pressure from natural selection and its relative frequency will increase

4. Single Allele Selection
Which phenotype has higher fitness?
Orange
Which allele’s frequency will decrease?
Green

5. Evolution of Polygenic Traits
Natural selection can affect the distributions of phenotypes in any of three ways:
Directional selection
Stabilizing selection
Disruptive selection

6. Graph of Directional Selection
Section 16-2
Directional Selection: When the entire bell moves left/right because there’s a higher fitness and increase in the number of individuals with the trait at one end of the curve.
Traits at one end or the other are selected for
Directional Selection
Key
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.

7. Directional Selection

8. Stabilizing Selection
Section 16-2
Stabilizing selection: When the bell becomes more narrow, because there’s a higher fitness and increase in the number of individuals with the trait in the center of the curve
The average trait is selected for
Key
Percentage of Population
Birth Weight
Selection against both extremes keep curve narrow and in same place.
Low mortality, high fitness
High mortality, low fitness
Stabilizing Selection

9. Evolution of Clutch Size

10. Disruptive Selection
Section 16-2
Disruptive selection: The bell can split into two, because there’s a higher fitness and increase in the number of individuals at both ends of the curve
Traits at both “extremes” are selected for
Disruptive Selection
Largest and smallest seeds become more common.
Number of Birds in Population
Beak Size
Population splits into two subgroups specializing in different seeds.
Key
Low mortality, high fitness
High mortality, low fitness

11. Disruptive Selection

12. Genetic Drift
Populations can also evolve without selection pressure through the process of genetic drift.
Genetic drift = random change in allele frequencies
In small populations, individuals that carry a particular allele may leave more descendants than other individual, just by chance. Over time, a series of chance occurrences can cause an allele to become common in a population.
Genetic drift can happen when a small group of individuals colonize a new habitat carrying different relative frequencies that the larger population.
2 special Cases:
Founder effect = allele frequencies change as a result of the migration of a small subgroup of a population
Bottle Neck= a population experiences a great reduction in the gene pool, leaving only a small subset of alleles behind. Results in inbreeding.

13. Genetic Drift- Chance

14. Genetic Drift- Founder Effect
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B

15. Genetic Drift- Founder Effect
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B

16. Genetic Drift- Founder Effect
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B

17. Genetic Drift Bottleneck

18. Genetic Equilibrium
Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more of a set of factors causes the population to change.
The following conditions that must be met to avoid evolution:
Random mating- no mate preferences, or choice (rare)
Large population- lots of diversity (less chance of genetic drift)
No movement into or out of the population- individuals don’t move between populations, carrying new alleles
No mutations- mutations change the DNA
No natural selection- all individuals have an equal chance of surviving and reproducing

19. Hardy-Weinberg
(p2) + (2pq) + (q2) = 1