Module 5: Drift, Gene Flow, Non-Random Mating

Genetic Drift

Any change in allele frequencies due to chance

• Can eventually lead to the fixation or loss of alleles — decreases genetic variation

• Random with respect to fitness

• Most pronounced in small populations

• Because drift is caused by sampling error, it can occur by any process or event that involves sampling

Genetic Marker

A genetic locus that can be identified and traced in populations by laboratory techniques or by distinctive visible phenotype

Founder Effect

A change in allele frequencies that often occurs when a population is established from a small group of individuals due to sampling error

• Occurs when a small group from the main population leaves and establishes a new population

Genetic Bottleneck

A reduction in the diversity of allele in a population resulting from a sudden reduction in the size of that population due to a random event

Gene Flow

The movement of alleles between populations

• Occurs when individual’s leave one population, join another and breed

• Random with respect to fitness

• Increases genetic variation in a recipient population if new alleles arrive with immigrating individuals, but decrease variation in the source population if alleles leave with emigrating individuals

Mutation

Increases genetic diversity over time and introduce entirely new alleles

• Can have a very large effect on evolution when combined with genetic drift, gene flow, and selection

  • Even if selection and drift are eliminating genetic diversity, mutation renews it

• If mutation did not occur, evolution would eventually stop

1. Point Mutations: if duplicated genes diversify via point mutations, they can lose their function or create new alleles

2. Chromosome-Level Mutations: gene duplication, increases the number of copies of a gene

3. Lateral Gene Transfer: the transfer of genes from one species to another, rather than from parent to offspring

Deleterious Alleles

Alleles that lower fitness

• Most mutations in sequences that code for a functional protein or RNA result in deleterious alleles

• Tend to be eliminated by purifying selection

Neutral Alleles

Alleles with no effect on fitness

1. Inbreeding

Mating between closely related individuals/relatives

• Increases homozygosity

• Leads to a decline in the average fitness via selection

  • Inbreeding Depression: the decline in average fitness that takes place when homozygosity increases and heterozygosity decreases

    • Many recessive alleles represent loss-of-function mutations

    • Many genes, especially those involved in fighting disease, are under intense selection for heterozygote advantage, a selection that favours genetic diversity

• Does not cause evolution — allele frequencies do not change in the population as a whole, changes only genotype frequencies

2. Assortative Mating

Mating that is nonrandom with respect to specific traits

• Positive Assortment: individuals choose mates that share a particular phenotypic trait

• Negative Assortment: individuals choose mates that differ in specific phenotypic trait

3. Sexual Selection

A type of natural selection that favours individuals with traits that increase their ability to obtain mates

• Occurs when individuals within a population differ in their ability to attract mates

• Causes certain alleles to increase or decrease frequency and results in evolution

Intersexual Selection

An individual of one sex chooses a particular individual of the other sex for mating

• Females are usually the gender that is pickiest about mate choice because females spend a disproportionate amount of energy on their gametes and offspring compared to males

Intrasexual Selection

Driven by competition among members of one sex (usually male-male) for an opportunity to mate

• Increased fitness due to reproductive success

The Fundamental Asymmetry of Sex

It is characteristic of almost all sexual species and can have two important consequences

1. Eggs are large and energetically expensive, females produce relatively few young over the course of a lifetime

2. Sperm are energetically inexpensive to produce that a male can father an almost unlimited amount of offspring

The Bateman-Trivers Theory

A claim about a pattern in the natural world and a process that causes that pattern

PATTERN: traits that attract members of the opposite sex are much more highly elaborated in males

PROCESS: the energetic cost of creating a large egg is enormous, whereas sperm contain few energetic resources

• If females invest a great deal in each offspring, then they should protect that investment by being choosy about their males — If males invest little in each offspring, then they should be more willing to mate with almost every female

• If there is an equal number of males and females in the population, and if males are trying to mate with any female possible, then males will compete with each other for mates

• If male fitness is limited by access to mates, the any allele that increases a male’s attractiveness to females or success in male-male competition should increase rapidly in the population — thus, sexual selection should act more strongly on males than on females

Territory

An area that is actively defended by an animal and provides exclusive or semi-exclusive use of its resources by the owner

Sexual Dimorphism

Any trait that differs between males and females

• Sexually selected traits often differ sharply between the sexes

Ecological/Environmental Selection

A type of natural selection that favours individuals with heritable traits that enhance their ability to survive and reproduce in a certain physical and/or biological environment, excluding their ability to obtain a mate