1. Evolution II Notes Evolution and Populations
Biology
Evolution II Notes
Evolution and Populations

2. Darwin’s Ideas Revisited
Developed his theory of natural selection which explains how species evolve
Published theory in 1859
Gregor Mendel’s findings were published in 1860 and the significance of Mendel’s work was not realized until after 1900
Without knowledge of genetics Darwin was unable to explain two factors that were key to understanding evolution
What is the source of variation?
How are inheritable traits passed on from one generation to the next?
Scientists were able to combine Darwin’s theory with basic principles of genetics in 1910.

3. Gene Pools
Gene Pool = combined genetic information of all members of a population
Typically contains two or more alleles for a trait
Relative Frequency = how often an allele occurs within the gene pool compare to other alleles
Usually expressed as a decimal or a percentage %
All alleles for a trait must add up to 1 or 100%
Practice Problem: In a population of people there are a total of 35 blue alleles, 40 brown alleles, and 25 green alleles. What is the relative frequency of each allele within the gene pool?

4. Lethal Recessive Alleles in Gene Pools
How are lethal recessive alleles maintained within a gene pool?

5. Sources of Genetic Variation
Mutations
Change made in DNA Sequence
Change does not always affect organisms phenotype
Changes that do affect an organisms phenotype may also affect an organisms fitness
Can make an organism more or less likely to survive and reproduce successfully
Give examples of mutations affecting an organisms phenotype that would make them more and less successful!
Gene Shuffling
Most inheritable differences are due to gene shuffling
Chromosomes assort randomly during meiosis resulting in new combinations of genes
Sexual Production therefore can produce many genetic differences between you, your parents, and your siblings

6. Is Genetic Variation In a Species Good or Bad?
Good! Why?
When natural disasters or changes in the environment occur, it is more likely for some members of the species to survive because they may have a variation that allows them to survive the change (adaptation).

7. Frequency of Phenotype
Single Gene vs Polygenic Traits The # of phenotypes produced for a given trait depends on the # of gene sites controlling the trait
Single Gene
1 gene site controlling the trait
Gene site has two or more alleles
Few Possible Phenotypes
Example = Hairline
Polygenic
More than one gene site controlling the trait
Each gene site has two or more alleles
Many possible phenotypes
Example = height 00 40 20
100 50
Frequency of Phenotype
(%)
Frequency of Phenotype
Phenotype (height)
Widow’s peak
No widow’s peak

8. Natural Selection On Single-Gene Traits
Leads to changes in allele frequencies and thus evolution (changing) of the species
Less complex than polygenic traits
Give Example!
Natural Selection On Polygenic Traits
Wide range of phenotypes for a trait
Natural selection can act in 3 ways
Directional Selection
Stabilizing Selection
Disruptive Selection

9. Directional Selection
One extreme for a trait is favorable, over time individuals in the population will look more like that extreme
Can work in both directions depending on what is being favored by nature!
Example = speed with gazelles
More fit to be fast than slow
Population will get faster from generation to generation
Key
Low mortality, high fitness
Directional Selection
High mortality, low fitness
Food becomes scarce.

10. Stabilizing Selection
Average individuals are more fit, over time population will look more like the average for a trait
Disadvantage to be at either extreme
Example = human birth size
to small = health risks
to large = difficulty in birthing process
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

11. Disruptive Selection
Either is extreme is more favorable than being average, over time population will look like both extremes with less individuals exhibiting the average trait
Individuals being average for a particular trait are at a disadvantage
Disruptive Selection
Largest and smallest seeds become more common.
Key
Population splits into two subgroups specializing in different seeds.
Low mortality, high fitness
Number of Birds in Population
Number of Birds in Population
High mortality, low fitness
Beak Size
Beak Size

12. Genetic Drift Random change due to chance (not natural selection)
Most likely to occur in a small populations
Founder Effect- small group separate from main group
-gene frequencies differ from main group
Bottleneck Effect- large number of indivuals die off.
-gene frequencies by chance (not selection) are different from original group