BIOL 201 Evolution and Ecology Midterm 1

What is Hardy-Weinberg equilibrium?

• A null model

• States that allele frequencies in a given population will remain stable from generation to generation if the 5 assumptions are met

What is evolution?

A change in allele frequencies over generations

Mechanisms of evolution

• Mutation

• Genetic drift

• Gene flow (migration)

• Selection

How are new alleles formed?

Exclusively through mutation

Conditions/assumptions of Hardy-Weinberg equilibrium

• Random mating

• Infinitely large population

• No genetic drift

• No gene flow/migration

• No selection

When is genetic drift most likely?

• When the population is small

• E.g., when the population size has been reduced due to the bottleneck or founder effect

The bottleneck effect

The size of a population is rapidly reduced (such as by disease or a natural disaster) → small population that isn’t necessarily a representative sample of the alleles present in the previously large population → allele frequencies shift and some are lost (genetic drift)

The founder effect

A small group is separated from the larger population (such as by migration or environmental changes) → small population that isn’t necessarily a representative sample of the alleles present in the previously large population → allele frequencies shift and some are lost (genetic drift)

Selection is, by definition, (random / non-random)

non-random

Null hypothesis

• Hypothesis stating that something will not occur

• Default assumption that there will be no statistically significant difference between two groups or variables

Does inbreeding cause evolution? Why or why not?

• No, it does not change the frequencies of alleles in a population

• It only changes genotype frequencies, referring to how existing alleles are paired

Effects of inbreeding

Increased rate of homozygosity due to pairing a higher number of identical alleles

When is the null hypothesis rejected?

When χ² > χ²_crit

Binomial distribution

• Gives the probability of X successes in n trials

• Applies when each trial has 2 possible outcomes (one of which is arbitrarily called a success), the trials are independent, and the probability of success, p, is the same in every trial

• Randomly sampling n individuals from a large population meets these conditions

What does it mean (in terms of evolution) if a population is NOT in Hardy-Weinberg equilibrium?

• Simply means that one of the five assumptions of HWE were not met

• Often means the population is evolving, but not necessarily if the only unmet condition is random mating (as some types of random mating do not shift allele frequencies)

Which mechanisms of evolution are random (1) and which are not (2)

1. Mutation, genetic drift, gene flow

2. Selection

Wahlund effect

• Reduction in heterozygosity (more than would be expected in a population)

• Occurs when two small populations of the same species are geographically separated, and the subpopulations are analyzed as if they are a single population

• E.g., population 1 has a high frequency of A and a low frequency of a, while population 2 has the opposite frequencies. Combined, it will appear that there are fewer Aa individuals than would be expected for a single population given the amount of both types of homozygotes

Examples of non-random mating

• Inbreeding (doesn’t change allele frequencies)

• Assortative mating (mates are similar; doesn’t change allele frequencies)

• Disassortative mating (mates are opposite; doesn’t change allele frequencies)

• Sexual selection (a type of natural selection; is likely to change allele frequencies)

Does non-random mating always cause evolution?

• NO

• Only when it results in changes in allele frequency in the population over generations

When do mutations in DNA occur (2)?

• DNA replication

• DNA repair

When do beneficial alleles become fixed in the population?

• The allele is recessive

• The allele is additive

When are deleterious alleles erased from a population?

• The allele is dominant

• The allele is additive

Can additive alleles be masked or mask other alleles?

NO

The frequency of a lethal allele in a population to its ______ ___ because…

• mutation rate

• Individuals carrying the allele will be dead and therfore unable to reproduce, so the allele dies with them. They are only as frequent as their formation

Mutation rate (1) vs. substitution rate (2)

1. The rate at which new alleles are formed by errors in DNA replication and repair. Before evolution can act on the alleles

2. Closer to the observed *1*: deleterious alleles become less prevalent and are substituted for more beneficial alleles as they increase in the population. After evolution has acted on the alleles

• Equilibrium frequency formula for deleterious recessive case

• μ = mutation rate

• s = selection coefficient (the strength of selection against the allele, from 0 to 1)

• These tend to reach higher frequencies because they can hide in the population within heterozygotes

• Equilibrium frequency formula for deleterious dominant, but non-lethal case

• μ = mutation rate

• s = selection coefficient (the strength of selection against the allele, from 0 to 1)

Which phase of meiosis is this? What occurs here?

• Prophase I

• Homologous chromosomes pair up

• Crossing over

Which phase of meiosis is this? What occurs here?

• Metaphase I

• Homologous chromosomes (in sets of sister chromatids since they’ve been replicated) line up beside each other along the metaphase plate

• Independent assortment of chromosomes

Which phase of meiosis is this? What occurs here?

• Anaphase I

• Nondisjunction could occur here (would result in 2 gametes with n-1 chromosomes and 2 gametes with n+1 chromosomes)

Which phase of meiosis is this?

Telophase I

Which phase of meiosis is this?

Prophase II

Which phase of meiosis is this? What occurs here?

• Metaphase II

• The chromosomes (each with 2 sister chromatids) line up along the metaphase plate

Which phase of meiosis is this? What occurs here?

• Anaphase II

• Nondisjunction could occur here if sister chromatids failed to separate (would result in 2 normal gametes, one with n-1 chromosomes, and one with n+1 chromosomes, if it only occurred once in one of the two cells going through anaphase II)

Which phase of meiosis is this? What occurs here?

• Telophase II

• Four total unique gametes are formed

In which stage of meiosis does crossing over occur (including I or II)?

Prophase I

In which stage of meiosis does independent assortment occur (including I or II)?

Metaphase I (separation occurs during anaphase I)

Independent assortment

Chromosomes are sorted into gametes independently of one another

What is the advantage of sexual reproduction?

Increases genetic variation by making new combinations of chromosomes (through independent assortment) and alleles (through recombination)

How does recombination contribute to genetic variation?

Increases genetic variation by mixing and matching alleles (e.g., crossing over)

Nondisjunction

• The failure of homologous chromosomes or sister chromatids to separate during anaphase

• Can occur during anaphase I, anaphase II, or mitosis

• Results in daughter cells or gametes with unequal numbers of chromosomes

During what phase(s) of meiosis can nondisjunction occur?

• During anaphase I or II

• Can be due to problems during metaphase, such as spindle fiber attachment, but actually occurs in anaphase

Meiosis (increases / reduces / maintains) ploidy

reduces

Crossing over vs. recombination

• Recombination is a broader term referring to when DNA is broken and rearranged/inserted with other DNA to create new combinations of alleles

• Crossing over is a mechanism that leads to recombination

• Crossing over specifically refers to the swapping of DNA segments between non-sister chromatids on homologous chromosomes during prophase I of meiosis

Crossing over is more likely to occur between genes located (close together / far apart) on the chromosome

far apart

Phenotypic plasticity

A single genotype can produce different phenotypes (physical traits, behaviour, or physiological states) in response to environmental conditions

Broad sense heritability

• Proportion of phenotypic differences that can be attributed to genetics (encompassing all aspects of genetics) as opposed to environmental factors

• H² = V_G / (V_E + V_G)

Phenotypic variation (V_P)

The sum of genetic variation (V_G) and environmental variation (V_E)

Narrow sense heritability

• Proportion of phenotypic differences that can be attributed to specifically additive alleles as opposed to other genetic factors or environmental factors

• h² = V_A / (V_E + V_A + V_D + V_I)

Epistatic gene

A gene that masks or modifies the expression of another separate gene

Hypostatic gene

A gene that is masked or modified by another separate gene, altering the phenotype asociated with it

Which type of selection is this?

Directional selection

Which type of selection is this?

Stabilizing selection

Which type of selection is this?

Disruptive selection

𝑅 = ℎ² × 𝑆

• Multiplying the strength of selection (the difference between the average of the trait in the population and the reproducing individuals) by the narrow-sense heritability gives a predicted value for how much the trait that is being selected for or against will change from one generation to the next

• E.g., if the mean mass of toads in a population is 623 grams, but the mean mass of reproducing individuals is 702 grams, then the selection strength is 79. If narrow-sense heritability is 0.4, then 0.40×79=32 grams, which is approximately how much more the new generation of toads will weigh on average compared to the general population

Linkage equilibrium

• Occurs when/a state in which alleles will combine and be inherited independently of each other, completely randomly. Basically when there is no linkage between alleles

• Occurs most often when two alleles are located on separate chromosomes or far apart on the same chromosome

• Disequilibrium is when alleles ARE linked

Genetic drift

Loss of allelic diversity

H_g+1 = H_g (1 - 1 / 2N)

• G refers to generation, and H refers to heterozygosity

• Heterozygosity formula

• As the population (N) goes to infinity, H_g+1 gets closer to H_g

• As the population (N) get smaller, H_g+1 gets increasingly smaller than H_g, meaning the frequency of heterozygotes is decreasing each generation

• Explains why smaller populations are so affected by drift (alleles eventually become fixed or lost)