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Important Definitions
Allele: two or more forms of a gene (e.g. B is the allele for brown eyes and b is the allele for blue eyes)
Allele Frequency: the number of copies of an allele compared to the total number of alleles in a population
Gene Pool: the total number of genes of all of the individuals in a population
Factors That Change Allele Frequencies in Populations
1.
Mutations
- Mutations randomly introduce new alleles into a population
- Mutation changes allele frequencies
2.
Gene Flow (migration)
- Gene flow describes the net movement of alleles from one population to another as a result of the migration of individuals
- Gene flow can change allele frequencies in either or both populations through the movement of genes (alleles)
- E.g. a lone grey wolf can travel over 800km in search of new territory or a new breeding partner therefore bringing new alleles into the gene pool of the nearby population

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3.
Non-Random Mating
- During non-random mating, individuals in a population select their own mates, often on the basis of their phenotypes
- Non-random mating increases the proportion of homozygous individuals in a population, but does not affect the frequencies of alleles
- E.g. 1: Preferred Phenotypes - In herds of caribou, males compete for mates by using their antlers to spar against other males, chasing one another and fighting. This is a form of non-random mating because it prevents certain phenotypes from breeding. Only the individuals who successfully mate will contribute to the gene pool of the next generation.
- E.g. 2: Inbreeding - an extreme example of inbreeding is the self-fertilization of some flowers such as peas. Close relatives share similar genotypes, so inbreeding increases the frequency of homozygous genotypes. Inbreeding does not directly affect the distribution of alleles. However, as homozygous genotypes become more common, harmful recessive alleles are more likely to be expressed.
4.
Genetic Drift
- Genetic drift refers to random change in genetic variation from generation to generation due to chance.
- Genetic drift changes frequencies of alleles
- E.g. 1: The Founder Effect - a change in the gene pool that occurs when a few individuals start a new isolated

SBI3U population. This occurs frequently on islands. The Amish population of Philadelphia, Pennsylvania was founded in the
1700’s by only a few families. The current population of
Amish in the region has an unusually high rate of polydactylism (the presence of a 6 th finger or toe).
- E.g. 2: The Bottleneck Effect – changes in gene distribution that result from a rapid decrease in population size due to events like disease, starvation, human activities and natural disasters such as severe weather. In 1775 the small island called Pingelap in Micronesia was hit by a typhoon. There were fewer than 30 survivors from an original population of about 1600. One of the survivors had a genetic mutation that causes colour vision and now up to 10% of the current population currently has a colour vision deficiency. In the general population, this condition is relatively rare.
5.
Natural Selection
- Natural selection is the result of the environment selecting for individuals in a population with certain traits that make them better suited to survive and reproduce than others in the population.
- Over many generations, frequencies of alleles of many different genes may change, resulting in significant changes in the characteristics of a population

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Homework
1.
When discussing evolution, explain why it is necessary to keep in mind that populations evolve, not individuals?
It is important because the change occurs in a species over time. As the more fit individuals survive and the less fit individuals die, the gene pool changes to favour the more fit individuals’ genes. This leads to evolution over time.
2.
Explain why mutations may provide a selective advantage to some individuals of a population in a changing environment.
Some mutations can be beneficial because they lead to greater genetic diversity. Individuals that are different are more likely to survive in a changing environment. E.g. peppered moth, MRSA (methicillin resistant Staphylococcus aureus)
3.
Explain in your own words what is meant by the term “allele frequency”.
Allele frequency is the ratio of the number of copies of an allele compared to the total alleles in a population.
4.
Why does gene flow increase diversity in one population of a species while at the same time reducing genetic differences among many populations of the same species?
As an individual migrates, it brings its alleles with it, taking away from one gene pool and adding to another.
5.
Describe how non-random mating differs from random mating. Use the concept of preferred phenotypes to explain why random mating in nature is uncommon.
Non-random mating involves the selection of mates based on phenotype, song, dance, strength etc. Random mating involves mating with another random individual of the opposite sex. This rarely occurs.

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6.
Why are small populations more susceptible to genetic drift than large populations?
Small populations have a smaller gene pool. It is easier to eliminate alleles completely when a few individuals migrate or die. If alleles are eliminated then genetic drift occurs.
7.
Explain in your own words what is meant by the term founder effect?
The founder effect occurs when a few individuals begin a new isolated population. This causes a change in the gene pool.
8.
Explain in your own words what is meant by the term bottleneck effect?
The bottleneck effect occurs when there is a natural disaster that kills a large proportion of the population. The population reestablishes and there is a change in the gene pool.
9.
Describe the impact of the founder effect on the gene pool of a population.
When only a few individuals found a population there is little genetic diversity so the gene pool is small.
10.
Describe two events that might result in the bottleneck.
- Hurricane, mudslide, tornado, fire, disease outbreak