What is a “population”?
A group of potentially interbreeding individuals of the same species living in a prescribed geographical location
Populations may have “structure” i.e. sub-groups whose members are more likely to breed with each other
What is the “gene pool”?
All the alleles in the population
How does evolutionary change occur?
Evolutionary change occurs via changes in allele frequencies (including creation and spread of new alleles) over time
What is the Hardy-Weinberg principle?
Allele and genotype frequencies will remain constant between generations n the absence of other evolutionary influences
What is the Hardy-Weinberg equation?
p^2 + 2pq + q^2 = 1
What equation do we often pair with the Hardy-Weinberg equation?
p + q = 1
What are the assumptions of the Hardy-Weinberg principle?
- Organisms are diploid
- Only sexual reproduction occurs
- Generations are non-overlapping
- Mating is random
- Population size is infinitely large
- Allele frequencies are equal in the sexes
- There is no migration, mutation or selection
What is the Hardy-Weinberg equilibrium?
When allele and genotype frequencies do not change between generations
What are the 5 requirements of the Hardy-Weinberg equilibrium?
- Random mating
- No mutation
- No migration
- A very large population
- No selection
What does it mean for a mate choice to be “positive assortive”?
It increases homozygosity
What does it mean for a mate choice to be “negative assortive”?
(“dissassortative”)
Increases heterozygosity
What are the effects of inbreeding?
- Positive assortative for many traits
- Alleles have common ancestor
- “Fix” traits
- Increases homozygosity
- Increases concentration of deleterious recessives
Briefly describe migration and its affect on the Hardy-Weinberg equilibrium.
Individuals may migrate from one population into another that has a different allele frequency; this can cause a deviation from the Hardy-Weinberg equilibrium (until the migrants have randomly interbred with the natives)
Briefly describe genetic drift
- Randomly, some individuals will not pass on alleles; others will pass on disproportionate numbers
- Effect particularly pronounced in small populations
- Given enough time, any allele frequency can drift to 1 (fixation) or 0 (extinction)
- Major cause of genetic differences between sub-populations
What is a “bottleneck”?
A population that has reduced dramatically
Briefly describe the effects of a bottleneck
- Survivors may have non-random distribution of alleles
- Different allele frequencies from the original population
What is the “founder effect”?
- A type of bottlneck
Briefly describe “selection”
- Selection results when fitness (the probability that an organism will pass on its genes) depends on genotype
- Certain alleles/combinations of alleles can increase or decrease fitness
- Whether an allele is influential will also depend on the genetic background (other genes) and environment
What is “directional selection”?
A mode of natural selection in which a single phenotype is favoured, causing the allele frequency to continuously shift in one direction
- E.g. Peppered Moth
What is “stabilising selection”?
The opposite of disruptive selection. Instead of favouring individuals with extreme phenotypes, it favours the intermediate variants. It reduces phenotypic variation and maintains the status quo.
- E.g. Human birth weight
What is “balancing selection”?
Refers to a number of selective processes by which multiple alleles (different versions of a gene) are actively maintained in the gene pool of a population at frequencies larger than expected from genetic drift alone.
- E.g. Sickle Cell Anaemia
What is “frequency dependent selection”?
An evolutionary process by which the fitness of a phenotype depends on its frequency relative to other phenotypes in a given population.
- In positive frequency-dependent selection, the fitness of a phenotype increases as it becomes more common.
- In negative frequency-dependent selection, the fitness of a phenotype decreases as it becomes more common. This is an example of balancing selection.
- E.g. Batesian mimicry & Mullarian mimicry
What is “disruptive selection”?
Also called diversifying selection
- Describes changes in population genetics in which extreme values for a trait are favoured over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups.
- E.g. Endler’s guppies
What is Fishers runaway hypothesis?
A sexual selection mechanism proposed to account for the evolution of exaggerated male ornamentation by persistent, directional female choice.
