1. The Evolution of Populations: Models of Change
Chapter 23

2. What you need to know!
How mutation and sexual reproduction each produce genetic variation.
How natural selection, genetic drift, and gene flow contribute to changing allele frequencies.

3. Changing Allele Freq.
Gene Flow: migration of fertile individuals into or out of a population. This tends to reduce variation between separate populations

4. Natural Selection
The only mechanism that consistently causes adaptive evolution
Fitness represents the reproductive success of an individual
Higher fitness means more babies
Natural selection acts directly on the phenotypes of individuals (indirectly on genotypes)

5. Genetic Variation
Genetic variation within a population are attributable to two sources:
Mutation: the ONLY source of new genes and alleles.
point mutations change one base in a gene
Chromosomal mutations are almost certainly harmful *but not always
Sexual Reproduction: crossing over, independent assortment (remember the math 2n), fertilization (2n x 2n)

6. Changing Allele Freq.
Natural selection: alleles passed in different proportions based on adaptations and fitness
Genetic drift: random, nonadaptive change in allelic frequencies in small populations. Two models include:
Founder effect: small piece of a larger population is isolated in a new region (different gene pool)
Bottleneck effect: most of the individuals in a population die, leaving a small sample of the original gene pool

7. Directional Selection
A single extreme adaptation is good, shifting the curve in that direction.
Example: peppered moths

8. Directional Selection

9. Stabilizing Selection
Extreme phenotypes are bad adaptations
Example: primate birth weight
Stabilizing Selection
Key
Low mortality, high fitness
High mortality, low fitness
Selection against both extremes keep curve narrow and in same place.
Percentage of Population
Birth Weight

10. Stabilizing Selection

11. Disruptive Selection Selection against the common phenotype
Selection for both extremes
Example: weeds are either very tall (difficult to uproot) or very short (survive lawn mowers)
Disruptive Selection
Largest and smallest seeds become more common.
Key
Low mortality, high fitness
Population splits into two subgroups specializing in different seeds.
Number of Birds in Population
Number of Birds in Population
High mortality, low fitness
Beak Size
Beak Size

12. Sexual Selection Selection for sexual phenotypes:
Male competition: fittest male mates most (antlers, horns, body size)
Female choice: attractiveness and mating behavior
Sexual dimorphism: diifferent appearance of males and females in certain species

13. Preservation of Variety
Diploidy: two alleles for every gene allows for dominant and recessive allele combinations
Heterozygote advantage: Certain genes provide advantages for being heterzygous (i.e. sickle-cell anemia heterzygotes are resistant to malaria, mutts)

14. Imperfection
Natural selection does not fashion the perfect organism because:
Selection only edits existing variation
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment interact