Evolution Exam #2

Mendel's law of segregation

Each individual has two copies of a gene at a given locus, and two gene copies are separated with equal probability during gamete production

purple

Mendel also concluded that, since all F1 generation

offspring were -, - was dominant to white,

so you get - flowers with any - gene copy

(AA or Aa)

recessive

White flowers were __ and appear only when both genes are for white color

Mendel's law of independent assortment

alleles passed to offspring at one locus are independent from alleles passed to offspring at other loci (if unlinked)

Phenotype

may blend

Genotype

stays distinct

not lost

with medelian inhertiance, favorable mutuations are ___ ____

Two things Mendel still didn't know

1. The actual biological mechanism for gene transmission (DNA/ genome biology)

2. The factors producing inherited variation (mutation, [recombination- crossing-over], etc.)

mutation

a random change to the DNA sequence of an organism that is the ultimate source of genetic variation

beneficial, deleterious, neutral

3 types of mutations

No, they fuel natural selection but do not anticipate it

Example- Luria- Delbrück = evolution of bacteriophage resistance

Do beneficial mutations occur in response to selective pressures?

unpredicable, random with respect to effect on fitness

What is the fitness consequence of most mutations?

1. Dominance describes how an allele is expressed (trait shown, AA or Aa),

2. Frequency expresses the abundance and how many copies of the allele exist

- examples: polydactyly, achondroplasia

How can dominant traits be rare in populations (allele frequency vs. dominance)?

Population genetics

study of distribution and frequencies of alleles in population

Hardy-Weinberg model (HWM)

a mathematical model to predict the population-level consequences of Mendelian inheritance

What does HWM do?

establishes what will happen over time to allele frequencies in the absence of evolutionary processes

1. Large population

2. No mutation

3. Random Mating

4. No migration

5. Genotypes do not confer differences in fitness (no NS)

Assumptions of the HWM.

p = frequency of dominant allele

q = frequency of recessive allele

p^2= homozygous dominant

2pq= heterozygous

q^2 = homozygous recessive

Basic understanding of how to predict genotype frequencies based on allele frequencies under the HWM. p+q=1; p^2 + 2pq + q^2 = 1

allele frequency

p+q=1

genotype frequency

p^2 + 2pq + q^2 = 1

What is a null model?

used to provide specific quantitative conditions under which something is not expected; identify cases where assumptions are broken; compare what we see in nature to what we expect is assumptions are correct

Chi-Square test

- Do allele or genotype frequencies change between generations?

- Are the genotype frequncies are we would expect under random mating?

How do we test if alleles on a locus are at HW equilibrium?

What does HW equilibrium mean?

Allele and genotypic frequencies in a population remain constant

• If assumptions are met, then it goes to equilibrium in one generation

genetic drift

process of random fluctuations in allele frequencies due to the sampling effect

It violates the assumptions of a large population and causes random fluctuations in allele frequencies over generations

How can genetic drift impact the HW equilibrium?

reduction in genetic diversity within a population, loss or fixation of alleles, increased homozygosity, and populations diverge

What is the outcome of genetic drift? Loss of genetic diversity. Alleles are lost or fixed.

Basic understanding of the Wright-Fisher model.

A quantitative measure of allele frequency changes in small populations due to random genetic drift

- alleles in descendant generations are randomly sampled with replacement from the parental populations

- Due to the effects of random sampling with

replacement, different parental allele copies leave

different numbers of descendants.

Inversely proportional because drift is stronger in small populations

What is the effect of population size on genetic drift?

What is effective population size? (Ne)

The number of individuals of a population that would lose genetic diversity at the same rate as the actual, observed population

Bottleneck

populations will fluctuate in size over time, resulting in a non-representative set of alleles for subsequent populations, even after the population size rebounds

founder events

A type of bottleneck results from a small number of individuals colonizing a new, isolated habitat

Two types of genetic drift: reduce population size

-When the population size is small, genetic drift will have a larger effect

What are bottlenecks and founder events, and how do they impact genetic drift?

1. creates genetic distance between populations

   1. Rates of divergence are dependent on population size

2. Geography and speciation

How does genetic drift cause populations to diverge and form new species?

How selection affects allele frequencies

increasing the frequency of beneficial alleles that confer a survival or reproductive advantage and decreasing the frequency of deleterious (harmful) alleles.

selection coefficient

describes the fitness reduction of one phenotype relative to another

• as selections gets stronger, allele frequncies change faster and approaches fixation earlier

frequency-independent selection

fitness associated with a trait is not directly dependent on the frequency of that trait in a population

1. Direction selection

2. Overdominance

3. Underdominance

three kinds of frequency-independent selection

Directional selection

When one allele is consistently favored over the other, selection drives allele frequencies in a single direction (towards the favored allele);

"fixation" because allele is "fixed" (depends on allele dominance) ,

selection acts on phenotype, one allele is favored over another

overdominance

When a heterozygote (e.g. A1A2) has a higher fitness than either homozygote (A1A1 or A2A2); leads to balanced polymorphism

• (heterozgote advantage)

A1A1- no cost or benefit

A1A2- no cost, malaria resistant

A2A2- high cost, malaria resistant

what does overdominance do to allele frequencies in a population?

Underdominance

when a heterozygote (e.g. A1A2) has a lower fitness than either homozygote (A1A1 or A2A2); Extremely rare

• homozygote advantage

What is frequency-dependent selection?

When the costs and benefits of a trait depend on the frequency of that trait in a population.

balancing selection

A type of natural selection that maintains multiple versions

of a gene (alleles) at a stable frequency within a population over generations.

Negative frequency-dependent selection and overdominance (heterozygote advantage)

Cite two kinds of balancing selection

How does mutation impact allele frequencies?

Even with a high mutation rate, it takes thousands of generations for allele frequencies to reach equilibrium

Mutation-selection balance.

population reaches an equilibrium frequency of the A1 allele and the deleterious A2 allele, never be fixed