6: Families to Populations

What is genetic variation?

Differences in DNA sequences or alleles among individuals in a population

What is qualitative variation?

Variation in a trait that falls into distinct catagories, such as blood type.

What is quantitative variation?

Variation in a trait that can take a continuous range of values, such as height or body mass

What is a phenotype?

The observable characteristics of an organism, influenced by its genotype and environment

What is a genotype?

The combinatipn of alleles an individual possesses at a particular locus

What is a polygenic trait?

A characteristic influenced by multiple genes, often producing quantitative variation

What is an allele?

An alternative version of a gene

What is a locus?

The specific location of a gene or DNA sequence on a chromosome

What is a gene pool?

All the alleles present in a population

What is allele frequency?

The proportion of copies of a particular allele among all copies of that gene in a population (how often we see each alelle)

What is genotype frequency?

The proportion of individuals in a population within a particular genotype (how often we see each genotype)

What is the difference between allele frequency and genotype frequency?

Allele frequency counts allele copies, while genotype frequency counts individuals

How do you calculate allele frequency from observed genotype frequencies?

By calculating the frequency with a homozygous dominant and homozygous recessive genotype.

p = f(AA) + ½ f(Aa)

q = f(aa) + ½ f(Aa)

p + q = 1

How does dominance status affect an allele’s frequency with no relation to fitness?

If an allele does not affect fitness, its dominance status does not determine its frequency.

How does dominance status affect an allele’s frequency WITH relation to fitness?

When an allele affects fitness, dominance influences how effectively natural selection acts on it.

How does selection against a dominant allele (recessive advantage) affect its frequency?

The dominant alleles are most apparent in the phenotype, making them eventually go extinct when there is a disadvantage.

How does selection against a recessive allele (dominant advantage) affect its frequency?

The recessive alleles can still be carried by heterozygotes, making them decrease, but not go extinct.

• Recessive alleles can “hide out”

What is selection?

Differences in genotypes in regards to their fitness

What is evolution?

A change in allele frequencies in a population

Can selection occur without evolution, and why?

Yes, because selection involves differences in reproductive success, while evolution requires change in allele frequencies.

How does selection against a homozygote (heterozygote advantage) affect allele frequency?

Heterozygote selection maintains genetic variation because neither alleles disappear.

• Allele frequency eventually reaches an equilibrium frequency

• No more changes to allele frequency

• Starting allele frequencies do not matter

What happens when allele frequencies reach equilibrium?

Evolution is not occuring as there is no change in allele frequencies.

How does selection against a heterozygote (homozygote advantage) affect allele frequency?

The allele with the higher frequency is favoured and increases until it hits fixation.

• The allele with a lower frequency decreases continuously

• Genetic variation decreases

• Starting allele frequencies are critical

What 5 conditions determine whether a population is in Hardy-Weinberg Equilibrium (HWE)?

To be in HWE, the population must be:

• Large

• Random-mating

• Little to no mutation rate (negligible)

• No migration/gene flow

• No selection

What are the expected frequencies under HWE?

p² + 2pq + q² = 1

p²: expected AA frequency

2pq: expected Aa frequency

q²: expected aa frequency

How do you check HWE in 3 steps?

1. Calculate the p and q using observed genotype frequencies

2. Calculate expected frequencies using p² + 2pq + q² = 1

3. Compare observed frequencies with expected frequencies

If they are the same, the population is in HWE

What does it mean if a population is at HWE?

It is not evolving.

What does it mean if a population is NOT at HWE?

It may or may not be evolving

How does dominance status of various alleles, and different types of selection, affect the maintenance of genetic variation?

• Heterozygote advantage: maintains genetic variation by favouring heterozygotes

• Heterozygote disadvantage: reduces genetic variation by favouring fixation of one allele

What is a mutation?

A change in DNA that can create new alleles —> increases variation

How can mutations take a population out of HWE?

It can change allele frequencies and introduce new variants not accounted for in the HWE expected frequencies

What is gene flow?

Movement of alleles between populations through migration and reproduction —> usually increases variation

How can gene flow take a population out of HWE?

It can change allele frequencies and genotype proportions

What is genetic drift?

Random changes in allele frequencies due to change —> usually decreases variation

What are the 2 main reasons for genetic drift?

1. Founder effect: some individuals colonize a population and create a new population, reducing genetic diversity

2. Population bottlenecks: Disease, starvation, or hunting kill a large portion of a population, reducing genetic diversity

How can genetic drift take a population out of HWE?

Can randomly increase or decrease allele frequencies

What is assortive mating?

Individuals preferentially mate with others having similar traits as them.

How can assortive mating take a population out of HWE?

Often increases homozygosity at relevant loci

What is disassortive mating?

Individuals preferentially mate with others having different traits as them

How can disassortive mating take a population out of HWE?

Often increases heterozygosity at relevant loci

What are the different kinds of phenotype selection?

1. Directional: favours one extreme phenotype

2. Stabilizing: favours other phenotypes over extremes

3. Disruptive: favours both extreme phenotypes over intermediates

What is balancing selection?

Natural selection that maintains multiple alleles in a population —> maintains genetic variation

What are the 2 reasons for balancing selection?

1. Heterozygote advantage: heterozygotes have better fitness

2. Negative frequency-dependent selection: an allele or phenotype becomes more advantagous when it is rare

What is absolute fitness? Provide examples.

A measurable quantity of a genotypes fitness, represented by W

• Average lifespan, number of eggs, number of offspring

What is relative fitness? Provide examples.

A genotype’s absolute fitness divided by the absolute fitness of the most successful genotype, represented by w.

• w = W / W_max

How does genetic drift affect variation in the absence of bottlenecks and founders events?

Genetic drift is more random, unlike selection

• Over many generations, drift tends to remove genetic variation

• Usually (not always) the rarer allele gets removed

What is inbreeding and how does it affect genotypes?

Mating between genetically related individuals

• Increases homozygosity and reduces heterozygosity

What is inbreeding avoidance and how does it affect genotypes?

Avoiding mating with genetically related individuals

• Can reduce inbreeding and preserve heterozygosity

What can we assume if there are fewer heterozygotes in a population than expected?

Inbreeding, assortative mating, selection against heterozygotes

What can we assume if there are more heterozygotes in a population than expected?

Disassortative mating or heterozygote advantage

What can we assume if allele frequencies change randomly between generations?

Genetic drift

What can we assume if new alleles appear in a population?

Mutation or gene flow