1. Shaping Evolutionary Theory
Chapter 15 Section 15.3
Shaping Evolutionary Theory

2. Shaping Evolutionary Theory
The theory of evolution continues to be refined as scientists learn new information
Darwin’s theory of natural selection remains a central theme in evolution
Scientists now know that natural selection is not the only mechanism of evolution
Population genetics and molecular biology have led to the development of evolutionary theory,

3. Mechanisms of Evolution
______________as the raw material
Population genetics
_______________________________: In the absence of forces that cause change, the ____________ frequency remains the same and ______________ doesn’t occur
When allelic frequencies remain constant, a population is in genetic equilibrium.
Figure pg. 431(owls)

4. Hardy-Weinburg Principle
Five conditions
The population is very large
There is no immigration or emmigration
Mating is random
Mutations do not occur
Natural selection does not occur
Table 15.3 pg.432

5. Genetic Drift
Any change in the allelic frequencies in a population that is due to chance
In large populations, enough alleles “drift” to ensure that the allelic frequency of the entire population remains relatively constant from one generation to the next
In smaller populations, the effects of genetic drift becomes more pronounced and the chance of losing an allele becomes greater

6. Founder Effect An extreme example of genetic drift
Can occur when a small sample of a population settles in a location separated from the rest of the population
Evident in the Amish and Mennonite communities in the United States
Old Order Amish have a high frequency of six-finger dwarfism

7. Bottleneck An extreme example of genetic drift
Occurs when a population declines to a very low number and then rebounds
Reduced diversity
Cheetahs in Africa: appear inbred which decreases fertility – may be a factor in the potential extinction of this endangered species

8. Gene Flow
The random movement of individuals between populations, or migration, increases genetic variation within a population and reduces differences between populations

9. Nonrandom Mating Rarely is mating completely random in a population
Usually organisms mate with individuals in close proximity – promotes inbreeding and could cause a change in allele proportions favoring individuals that are homozygous for particular traits

10. Mutation A random change in genetic material
Many mutations cause harm or are lethal
Occasionally a mutation provides an advantage to an organism
Mutations provide the raw material upon which natural selection works
How does mutation violate the Hardy-Weinburg Principle?

11. Natural Selection
Hardy-Weinburg Principle requires that all individuals in a population be equally adapted to their environment and contribute equally to the next generation
Natural selection acts to select the individuals that are best adapted for survival and reproduction
acts on an organism’s phenotype and changes allelic frequencies

12. Natural Selection
Figure page 434 Three main ways natural selection alters phenotypes
Stabilizing selection: selection against both extremes
Most common – little variation
Directional selection: selection against one extreme
Extreme variation – peppered moths (industrial melanism)
Disruptive selection: selection against the mean
Splits population into two groups
A fourth type: sexual selection – ability to attract a mate
- occurs in populations where males and females differ drastically in appearance (peacocks)