Genes in a Population

Population Genetics

Genes in a Population Population genetics is the study of allele behavior in a population. A population is a group of local interbreeding individuals of a single species Example: ABO blood type - as individuals - we carry only two alleles - have one blood type - as a population - we have three alleles - have four blood types

Features of a population - share a common gene pool - described by their allele frequency - genetically more diverse than that of an individual - individuals are subsets of the populations gene pool - offspring carry the gene pool to the next generation

Gene Pool All of the allele s in population s genes

Relative Frequency (Allele Frequency) Relative frequency of an allele is the number of times that alleles occurs in a gene pool, compared to the total alleles Evolution is any change in the relative frequency of alleles in a population

Genetic Equilibrium- a population in which the frequency of alleles remains the same over generations.

Changes in genetic equilibrium A population in genetic equilibrium is not evolving. Both alleles and phenotypes remain the same. Any factor that affects the genes in the gene pool can change allelic frequencies and shift the genetic equilibrium

Genetic Equilibrium It is believed that a population will stay in genetic equilibrium as long as it is not disrupted.

Bell Curve

Causes of Disruption Natural Selection Mutation Genetic Drift Migration Immigration Emigration

Natural Selection: What is it? The single most significant factor that disrupts genetic equilibrium. When environment changes and some individuals do not survive.

Natural Selection: How does it affect Genetic Equilibrium? Some individuals will die and that changes the make up of the gene pool. Therefore, the offspring created will look different than parental generations.

Peppered Moth

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Mutations: What are they? Spontaneous changes that occur constantly, at a very low rate and under normal conditions. The likelihood is increased with chemical and radiation exposure.

Mutations: Why do they affect genetic equilibrium? Mutations can affect genetic equilibrium by producing totally new alleles for a trait. Example

Examples: Color Mutations Back

Genetic Drift The phenomenon by which allele frequencies in a population change as a result of random events or chance. Example

Types of Genetic Drift Founder effects A founder effect occurs when a new colony is started by a few members of the original population.

An example of Founder effects The Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington s disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency.

Types of Genetic Drift Bottleneck- a population s size is reduced for at least one generation.

An example of a bottleneck: Northern elephant Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000.They have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

Elephant Seal: Bottleneck

Genetic Drift: How does this disrupt equilibrium? The frequency of alleles in the population will change. As a result, offspring will look different than the parental generation.

Migration Migration causes gene flow (the process of genes moving from one population to another).

Migration: Two types Immigration: the movement of individuals into a population. Emigration: The movement of individuals out of a population.