BIO1M03 Week 3

What is evolution?

Change in allele frequencies in a population over time.

What are the four processes that drive evolution?

Natural selection, genetic drift, gene flow, and mutation.

What does natural selection do?

Increases the frequency of alleles that contribute to reproductive success in a particular environment.

What is genetic drift?

Random change in allele frequencies.

What is gene flow?

Movement of individuals between populations followed by breeding, causing alleles to move between populations.

What does mutation do?

Introduces new alleles into a population.

Is natural selection the only process responsible for evolution?

No. Genetic drift, gene flow, and mutation can also cause evolution.

What is the Hardy-Weinberg principle?

A mathematical model that predicts allele and genotype frequencies in a population when none of the four evolutionary processes is occurring.

What is the gene pool?

The group containing all alleles from all gametes in a population that can combine to produce offspring.

What do p and q represent in Hardy-Weinberg?

p = frequency of allele A1; q = frequency of allele A2.

What is the relationship between p and q?

p + q = 1.

What are the three possible genotypes for two alleles?

A1A1, A1A2, and A2A2.

What is the Hardy-Weinberg equation for genotype frequencies?

p² + 2pq + q² = 1.

What does p² represent?

The frequency of the A1A1 genotype.

What does 2pq represent?

The frequency of the A1A2 heterozygous genotype.

What does q² represent?

The frequency of the A2A2 genotype.

What happens to allele frequencies under Hardy-Weinberg conditions?

They remain constant from generation to generation.

What are the five assumptions of Hardy-Weinberg equilibrium?

No natural selection, no genetic drift, no gene flow, no mutation, and random mating.

What does the no natural selection assumption mean?

All individuals contribute equal numbers of gametes to the gene pool.

What does the no genetic drift assumption mean?

The population is large enough that random sampling does not significantly change allele frequencies.

What does the no gene flow assumption mean?

No alleles enter or leave the population through immigration or emigration.

What does the no mutation assumption mean?

No new alleles are introduced into the gene pool by mutation.

What does random mating mean in Hardy-Weinberg?

Individuals mate randomly with respect to the gene being studied; there is no mate choice based on that gene.

What are the two conclusions of Hardy-Weinberg?

Allele frequencies do not change generation after generation, and genotype frequencies are p², 2pq, and q².

Why is Hardy-Weinberg useful?

It serves as a null hypothesis for determining whether evolutionary processes are occurring.

What is a null hypothesis?

A prediction of what should be observed if the hypothesis being tested is not correct.

What does it mean if observed genotype frequencies differ significantly from Hardy-Weinberg expectations?

At least one Hardy-Weinberg assumption may be violated and an evolutionary process may be occurring.

What are the steps for testing Hardy-Weinberg equilibrium?

1. Calculate observed genotype frequencies. 2. Calculate observed allele frequencies. 3. Calculate expected genotype frequencies. 4. Compare observed and expected counts using a chi-square test. 5. Make a conclusion.

How do you calculate observed genotype frequency?

Number of individuals with that genotype divided by the total number of individuals.

How do you calculate allele frequency from genotype frequencies?

For a homozygote, use its full frequency; for a heterozygote, use half its frequency.

How do you calculate the frequency of allele M?

Frequency of MM + ½(frequency of MN).

How do you calculate the frequency of allele N?

Frequency of NN + ½(frequency of MN).

How do you calculate expected genotype frequencies?

MM = p², MN = 2pq, NN = q².

What is a chi-square test used for in Hardy-Weinberg?

To compare observed genotype counts with expected genotype counts.

What does O represent in the chi-square equation?

Observed count.

What does E represent in the chi-square equation?

Expected count.

What does χ² represent?

The chi-square statistic measuring the difference between observed and expected counts.

What is the null hypothesis when testing Hardy-Weinberg equilibrium?

The population is in Hardy-Weinberg equilibrium.

What happens if χ² is greater than or equal to the critical value?

Reject the null hypothesis.

What happens if χ² is less than the critical value?

Fail to reject the null hypothesis.

What was the critical value in the example at α = 0.05 and df = 1?

3.84.

What did χ² = 0.715 < 3.84 mean in the MN blood-type example?

Fail to reject the null hypothesis; the observed genotype counts were not significantly different from Hardy-Weinberg expectations.

What is natural selection?

A process in which individuals with certain phenotypes produce more surviving offspring than individuals with other phenotypes.

How does natural selection cause evolution?

If favoured phenotypes are associated with certain alleles, those alleles increase in frequency while other alleles decrease.

What is genetic variation?

The number and relative frequency of alleles present in a population.

Why is genetic variation important?

Natural selection requires heritable variation, and variation allows populations to respond to environmental changes.

What can happen when a population has little genetic variation?

It may be less able to respond to environmental change, causing average fitness to decline and potentially leading to extinction.

What are the four modes of natural selection?

Directional, stabilizing, disruptive, and balancing selection.

What is directional selection?

Selection that favours phenotypes at one end of a trait distribution.

What happens to the average phenotype during directional selection?

It changes in one direction toward the favoured extreme.

How does directional selection affect genetic diversity?

It tends to reduce genetic diversity.

What does fixation mean?

An allele reaches a frequency of 1.0 in a population.

What does loss mean for an allele?

An allele reaches a frequency of 0.0 in a population.

What is purifying selection?

Selection that removes disadvantageous alleles from a population.

What happened to cliff swallows during the cold spell example?

Larger birds were more likely to survive because they had larger fat stores and were less affected by cold and starvation.

What is stabilizing selection?

Selection that favours intermediate phenotypes and selects against both extremes.

How does stabilizing selection affect the average phenotype?

The average phenotype does not change significantly over time.

How does stabilizing selection affect variation?

It reduces genetic variation in the trait.

What is an example of stabilizing selection?

Human newborn birth weight, where babies of average weight had the highest survival.

What happened to very small and very large human babies in the birth-weight example?

They had higher mortality than babies with average birth weight.

What is disruptive selection?

Selection that favours extreme phenotypes and selects against intermediate phenotypes.

How does disruptive selection affect variation?

It increases or maintains variation in a population.

What is an example of disruptive selection?

Black-bellied seedcrackers, where birds with very short or very long beaks survived better than birds with intermediate beaks.

Why were intermediate-beaked seedcrackers disadvantaged?

They had difficulty efficiently eating both small and large seeds.

What is balancing selection?

Selection that maintains multiple alleles in a population because no single allele has a consistent advantage.

What does balancing selection do to genetic variation?

It maintains genetic variation.

What is heterozygote advantage?

A form of balancing selection in which heterozygous individuals have higher fitness than either type of homozygote.

How does heterozygote advantage maintain genetic variation?

Selection favours heterozygotes, preventing either allele from being completely eliminated.

How can different alleles be favoured at different times or places?

Environmental conditions can vary across locations or over time, causing different alleles to be favoured in different situations.

What is frequency-dependent selection?

A form of balancing selection where an allele's fitness depends on how common or rare it is.

What happens in negative frequency-dependent selection?

Rare alleles are favoured while common alleles are disadvantaged.

What is an example of frequency-dependent selection?

Rare colour alleles in guppies may be favoured because predators learn to recognize common colours.

Which type of natural selection is the only evolutionary process that produces adaptation?

Natural selection.

Which selection favours one extreme phenotype?

Directional selection.

Which selection favours intermediate phenotypes?

Stabilizing selection.

Which selection favours both extreme phenotypes?

Disruptive selection.

Which selection maintains multiple alleles because no single allele has a consistent advantage?

Balancing selection.

Which types of selection generally decrease genetic diversity?

Directional and stabilizing selection.

Which type of selection increases or maintains genetic variation by favouring extremes?

Disruptive selection.

Which type of selection preserves genetic variation through balancing forces?

Balancing selection.

What is genetic drift?

Any change in allele frequencies caused by chance or sampling error.

Is genetic drift adaptive?

No. Genetic drift is random with respect to fitness.

When is genetic drift strongest?

In small populations.

Does genetic drift occur only in small populations?

No. It occurs in every population every generation, but its effects are much stronger in small populations.

What can genetic drift eventually cause?

Random fixation or loss of alleles.

How does genetic drift affect genetic variation?

It tends to decrease genetic variation within populations.

How does genetic drift affect differences between populations?

It can increase genetic differences between populations.

Why is genetic drift important for conservation?

Small populations in nature reserves or zoos are especially vulnerable to losing genetic variation through drift.

What is sampling error in genetic drift?

Random differences between the alleles sampled for reproduction and the allele frequencies in the original population.

What is a founder effect?

A change in allele frequencies that occurs when a small group establishes a new population.

Why do founder effects occur?

The small group may have allele frequencies that differ from those of the original source population.

Where are founder effects especially common?

When isolated habitats, such as islands, are colonized.

What is a population bottleneck?

A sudden decrease in population size caused by events such as disease outbreaks or natural catastrophes.

What is a genetic bottleneck?

A sudden reduction in the number of alleles in a population caused by a population bottleneck.

How does a bottleneck cause genetic drift?

Random survival of individuals changes allele frequencies in the surviving population.

What happened in the Pingelap Atoll bottleneck example?

Only about 20 people survived two natural disasters from an original population of several thousand.

What happened to the achromatopsia allele after the Pingelap bottleneck?

Its frequency increased to over 20%, likely because of the extreme population bottleneck.

What is gene flow?

The movement of alleles between populations.

How does gene flow occur?

Individuals leave one population, enter another, and reproduce.

What does gene flow generally do to populations?

It tends to equalize allele frequencies between populations.