Hardy-Weinberg and Agents of Evolution (3)

Genotype frequency

the % of individuals in a population with a specific genotype. It shows the distribution of allele combinations in a population (e.g. AA, Aa, aa)

Allele frequency

% of all copies of a certain gene in a population that carry a specific allele. Shows the distribution of genetic variation in a population (e.g. A or a)

What is evolution measured as?

as changes in allele frequencies in populations from generation to generation

How are gene alleles shown in chromosomes?

What is used to represent the frequency of alleles identified?

• for gene loci with two alleles

• ‘p’ for frequency of one allele

• ‘q’ for frequency of other allele

How do you find genotype frequency?

• # of individuals with that genotype/ total # of individuals

• Genotype: C^rC^w, C^rC^w, C^w, C^r

How to find the allele frequency?

• # of occurrences of that allele/ 2n (2 times the # of individuals)

• Each individual has two alleles

What does finding the allele frequency assume?

the commonness or rarity of each allele in the gene pool assuming individuals are diploid, and both gametes contribute to the production of offspring

What does the sum of allele frequencies always have to add to?

p+q=1

What is one way to find genotype and allele frequency?

• Count # of individuals with variant forms of trait

• can only work purely on observable traits when one phenotype is encoded by one genotype

Why does counting based on observable traits not work for allele/gentype freq?

• many traits encoded by large # of genes —> difficult to make inferences of underlying genotype through phenotype

• phenotype is byproduct of genotype and environment

What is the modern way of measuring genotype and allele frequency?

Gel electrophoresis

How does the gel electrophoresis process work?

• segments of DNA seperated according to size as they migrate through gel when electrical charge applied

• rate of proteins move from ends of gel= determined by charge and size

• individual that is heterozygote for a mutation changes amino acid ina. protein —> effects migration of protein in gel —> produces two distinct bands

What does DNA sequencing allow for?

• unambiguous way to detect all genetic variation

• variation studied through differences in DNA sequence (A instead of G at specified nucletiodie position in particular gene)

  • polymorphisms

What are polymorphisms?

• variable nucleotide positions

How do you calculate allele frequency with DNA sequencing? 

• collect population sample and count # of occurrences of given mutation

What equation is used to predict genotype frequencies in the next generation?

p²+2pq+q²

What do the variables in p²+2pq+q^{2 represent?}

• p²= aa or AA

• q²= AA or aa

• 2pq= Aa (heterozygote)

What only situation does Hardy-Weinberg principle describe?

When allele and genotype frequencies do not change

What does the Hardy-Weinberg Principle show about evolution?

• When no allele or genotype frequencies change —> absence of evolution

• Data between Hardy-Weinberg to real world data= how much evolution has occurred

What model is used for the Hardy-Weinberg Principle?

The null model

What does the null model show?

the relationship between allele and gene frequency in HDP

What is a Hardy-Weinberg Equilibrium?

• A population where frequencies of alleles and genotypes in a population will remain constant from generation to generation

• population only Mendelian segregation and recombination of alleles are at work, and no other evolutionary agents (e.g., natural selection).

What conditions have to be met for the HDP?

• No mutations are occurring

• No gene flow (the population is closed to migration from other populations)

• The population is infinite in size (very large)

  • sufficiently large to preventing sampling error

• No natural selection (all genotypes survive and reproduce equally)

• Individuals mate randomly with respect to genotypes

What happens when large population conditions are not met?

• Large population: chance plays bigger role in small population

  • Ex: genetic drift more impactful in small population

What happens with HDE doesn’t meet natural selection condition?

• Natural selection: harmful allele declines and helpful allele increases

What happens with HDE doesn’t meet immigration condition?

• allele frequency will decline/increase in proportio to # of immigrants

What happens with HDE doesn’t meet mutation condition?

• Mutations so rare —> small effect on allele frequencies on timescale studied by population genecisits

What happens with HDE doesn’t meet random mating condition?

• affects genotype frequencies from generation to generation, but not allele frequency

What are the agents of microevolutionary change?

• mutation

• gene flow

• genetic drift

• natural selection

• nonrandom mating

What are mutations?

• Spontaneous heritable variation in DNA

• Rare event, significant over long time scales

What is mutation a major source of?

• Major source of heritable variation

What is the relationship between neutral mutations and NS?

• not governed by NS

• mutation has no effect on fitness

• Ex: mutation m has no effect on fittness —> presents in single individual heterozygote—> fails to reproduce —> m dies out —> random because nothing selects against m

What happens to lethal mutations in populations?

• lethal mutations = death

• may persist if recessive

What causes gene flow?

Migration

What is gene flow?

movement of alleles from one population to another

What can enhance gene flow?

• life history

• Behavior

• Dispersal agents (ex pollinators) can enhance and facilitate gene flow

What can disrupt gene flow?

• life history

• behavior

• Physical barrier that prevents populations from breeding

• Chemical barriers (ex: pest control)

What is the impact of gene flow

• homogenization

What is homogenization

makes both population more similar and decreases genetic variation between the population

Genetic Drift

Change in allele frequency due to random effects of small population size

Which populations are most impacted by genetic drift?

• small populations

• Ex: rare allele A is 1/1—→ destruction reduces population to one pair of individuals —> A in new population is ¼ (each indivi. has two alleles) —> loss of genetic variation

What is genetic drift equivalent of?

• genetic drift=sampling error

• small sample: extreme departures from expected outcome are common

What is the relationship between adaptation and genetic drift?

• does not cause useful traits (adaptation) because allele frequencies changing does not affect indivudal ability to survive or reproduce

• adaption: favors individuals with beneficial traits that increase their survival and reproduction in environment

• genetic drfit: genetic drift doesn't consider an allele's fitness, meaning it can lose beneficial alleles or fix harmful ones

Impact of genetic drift

• reduces genetic variability

• population bottleneck

• founder effect

What is a population bottleneck?

• Part of genetic drift

• Reduction in alleles due to population reduction

• Leads to a drastic, but often temporary, reduction in population size

What is the founder event?

• few individuals starting a new population

• part of genetic drift

Impact of the founder effect

• less genetic variety in smaller new population can lead to extinction

• Deleterious allele can become dominant because of lack of genetic variation

• relative to parent population —> allele frequencies randomly changed and genetic information lost

What is the impact of a bottleneck?

• leads to smaller genetic variation —> individuals may not be able to survive new environment

• can lead to fixation of alleles that are not best fit for environment

What is fixation?

• process by which one allele replaces all other in a population

• population shows only one allele at a particular gene → pop. is fixed to that allele

What happen to rebound populations after bottleneck?

Has smaller genetic variation

What often causes a population bottleneck?

Often caused by catastrophic factors: disease, starvation, drought

What is natural selection?

A process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than others because of those traits

How does NS impact allele frequency?

causes allele frequency to change generation to generation according to allele’s impact on survival and reproduction of individuals

Adaptation defintion

process where population progressively becomes better fitted to environment through natural selection

What claims (observations) does NS rely on?

1. variation among individuals of species

2. some of this variation is heritable

3. individuals compete for resources

4. genetic variation among individuals result in some individuals that are more likley to survive and reproduce —> pass genetic material to net generation —> has higher proportion of advantageous cells

What did Malthus say about population?

Natural populations have potential to increase in size geometrically (larger at an ever increasing rate) but competition for resources stop that

How is natural selection measured?

relative fitness

What is relative fitness?

• measure of ability of an individual to survive and reproduce in an environment

• relative to individuals in same species

• depends on environment

What is competitive advantage?

how well an organsim is adapted to its environment

What does natural selection drive a variation in for reproduction?

Reproductive succsess

What is reproductive fitness?

baed on reproductive effort (number of offspring produced)

What is Mendelian Genetics?

1. units are unitary/discrete (ex: purple vs. white —> no light purple)

2. Genetic characters have alternate forms, each inherited from each parent (alleles)

3. One allele dominant over another —> the phenotype shows dominant allele

4. Gametes are created created by random segregation. Heterozygote individual produce gametes with equal frequency of two alleles

5. Genes are unlinked. Different traits have indenpendent assortment

6. Video:

Why did people question Mendelian genetics?

• the pee’s were Discrete

• most variation occurs on a spectrum and are not discrete

Discrete meaning?

Clear alternative states (yellow and green)

What is Modern Synthesis and combination of?

Darwins theory of evolution and Mendelian genetics —> completed by Ronald Fisher

What did Ronald Fishes discover?

• several genes could contribute to one trait

What did Ronald Fisher add to Mendelian genetics?

• extends Mendel’s theory to include multiple genes per trait that could explain patterns of continued variation

What is positive selection?

NS that increases frequency of advantageous alleles

What is negative selection?

NS reduces frequency of deleterious allele

How does negative selection impact deleterious alleles?

• most mutations to genes are deleterious

• if deleterious mutation is recessive —> NS inefficient to remove it

  • homozygotes will be rare

What are the types of natural selection?

• Directional selection

• stabilizing selection

• Disruptive selection

Directional selection

• Favors individuals near one end of phenotypic spectrum

• Ex: pushing for larger tail feathers

Stabilizing selection

• Favours individuals with intermediate phenotypes

Disruptive selection

• Favours individuals with extreme phenotypes over intermediate forms

• Selection towards one type → major change → pushes for both extremes

What is artificial selection?

• humans select for traits

What is intrasexual selection? 

• one sex (usually names) compete for acsess of other sex (usually femals)

• focuses on competition between individuals of one sex

What is intersexual selection?

• males (typically) compete for attention with bright colors or advertisment displays

• females choose mate

What is random mating?

Individuals select mate w/out care for genotype

nonrandom mating?

individual preferentially choosing mates according to genotype

How does nonrandom mating impact alleles?

• just redistributes alleles already in the gene pool and, unlike migration or mutation, does not add new alleles to the population.

• genotype frequencies change in nonrandom mating, whereas allele frequencies do not.

Summary of agents of evolution