Rokas Second Exam

Microevolutionary studies

Examine how processes such as mutation, natural selection, recombination, migration, non random mating, and genetic drift determine as well as change the genetic composition of populations

Mutation

Generates new variants/new alleles

Recombinations

Responsible for introducing new combinations of alleles in the population (does not introduce new alleles.

Mutations

are changes in the nucleotide sequence of DNA and can involve changes of one (point mutations) or many nucleotides

can cause new genes or alleles to arise

How are mutations distinguished

synonymous: mutations that don’t change the encoded amino acid

non synonymous: changes in nucleotides that change amino acids

Mutations can be

Beneficial —> Increases fitness of population

harmful (deleterious) —> Decreases fitness of population or increases the population’s susceptibility to disease

neutral—> Neither good nor bad effects for the population

Mutation is a

weak evolutionary force because point mutation rates are generally low with one mutation every 100,000 genes per generation

Mutations that are known to be harmful or deleterious and why

Chromosomal mutations that delete disrupt or rearrange loci such as deletions, duplications, translocations and inversions are harmful and deleterious

When mutations leave genes left in tact they can be

neutral or beneficial

If a mutation is beneficial

it can be naturally selected to maintain the fitness of the population

Mutation is a

random process with no foresight

In humans the rate of point mutations is 5x higher in

sperm then eggs so dads contribute the most harmful mutations

why?

In sperm maturation there is more divisions and DNA replications so there is more of an opportunity for an error to occur.

Generation time in the variation in rates of mutation

Single celled organisms and viruses have many more generations per unit of time than larger ones

Environmental impact on variation in the rate of mutation

Many environmental conditions can directly increase the mutation rate. For example carcinogens, and other human made substances can increase mutation rate

Stress impact on the variation in rates of mutation

Under some conditions, stress increases rates of mutation to appear adaptive.

In some cases such as in microbial organisms, they notice when survival is low and stress is high so they increase their mutation rates hoping that particular mutations appear that will be adaptive and will be favored for individuals to survive until the next generation

When is mutation common?

On a per gamete or per genome

When is mutation rare?

Mutation is rare per gene

Mutations with phenotypic effects are usually

harmful (deleterious) and recessive (hidden)

Mutation on its own

has a very little effect on HW equilibrium because it takes many generations for one particular mutation to take rise in a population so in order for it to spread in a population and reach a proper frequency it takes a while

but it provides the ultimate raw material for selection and evolution

What are mutation rates affected by?

Stress, sex, generation time, the environment, and psychological stress

Assortative mating

(non-random mating) like genotypes mate with each other or avoid each other

Inbreeding

(Nonrandom mating) mating among individuals that are more closely related than individuals drawn by chance in the population

Positive assortative mating

Like genotypes mate with like genotypes

decreases heterozygosity for genes affecting that trait

example AAxAA or aaxaa or AaxAa

What happens to allele frequencies during positive assortative mating?

They remain the same because both the dominant and recessive allele are increased by the same fraction

Heterozygotes decrease if similar genotypes are pairing but only for

the loci affecting the trait

With complete positive assortative mating heterozygotes are

decreased by half each generation

Negative assortative mating

Mating is with unlike phenotypes

Example: AaxAA

Increase in heterozygosity in the population

On average heterozygotes increase if

dissimilar genotypes are pairing

Even though positive assortative mating and inbreding both involve like genotypes mating with like what is the difference between them.

Inbreeding affects all loci in the genome not just a singular locus like in positive assortative mating

Inbreeding changes genotype frequencies by

increasing the proportion of homozygotes and reducing the proportion of heterozygotes across the whole genome

Most extreme case of inbreding

Selfing

What happens to allele frequencies under inbreeding each generation?

Allele frequencies stay the same but the actual number of homozygotes increases while the number of heterozygotes decreases

The genotype changes caused by assortative mating

affect only the loci contributing to that phenotype

The genotype changes produced by inbreeding

affect all loci in the genome

Inbreeding results in a

genome wide loss of diversity due to the loss of heterozygosity

Why are the effects of inbreeding and associative mating able to be reversed with one generation of random mating?

Inbreeding and associative mating only affect genotype frequencies not allelic ones so their effects are quite ephemeral/temporary

Why does inbreeding decrease fitness?

It generates offspring that are homozygous for deleterious alleles at times. This causes an inbreeding depression.

Genetic Drift

Sampling error in the production of offspring genotypes from the parental gene pool that results in random changes in allele frequencies.

What are the consequences of genetic drift?

-Genetic drift leads to a loss of variation in a population if the population is finite

-Genetic drift has a bigger effect on small populations

-Genetic drift leads to differentiation

Long term patterns of genetic drift are sensitive to periods of

small population size

Population Bottleneck

When a population goes through a period of unusually small population size

Founder Effect

When a population goes through this period of unusually small population size at a time when it arrives to a new uncolonized area.

Founder effect

When a colony is started by a few members of the original population, the small population size of the colony means that it may have

-A non random sample of the genes present in the original population

-Reduced genetic variation from the original population

Founder effect by definition

The establishment of a new population by a few original founders (in an extreme case, by a single fertilized female) which carry only a small fraction of the total genetic variation of the parental population

Why does genetic drift have such a substantial effect on bottle neck populations?

because allele frequencies in the population are likely to change just by random chance and many genes may be lost from the population, reducing the population’s genetic variation

Effects of genetic drift

-Genetic drift is significant in small populations

-Genetic drift can cause allele frequencies to change at random (causes microevolution)

-Genetic drift can lead to a loss of genetic variation within population s

-Genetic drift can lead to differentiation among populations

-Genetic drift can cause harmful alleles to become fixed in the population

Gene flow

the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes and gene flow can also change allele frequencies and it can do so quite dramatically

Gene flow can

change allele frequencies quite dramatically

What can gene flow do within a population?

It can introduce or reintroduce alleles which increases its genetic variation

By moving genes around what can gene flow do?

It can make distant populations genetically similar to one another which reduces the chance of divergence

Gene flow has the opposite effect of

genetic drift. Gene flow reduces the chance of divergence to create homogenized populations and it reduces variation among populations over time.

What causes deviations from HWE?

Non random mating

Mutation and sexual recombination produce

the variation in gene pools that contributes to differences among individuals

Selection is a blend of

chance and sorting

Sorting

Beneficial alleles are favored by natural selection whereas deleterious one are removed

Natural Selection is not

random

Adaptive evolution is generated from

natural selection choosing alleles to increase the frequency of (these alleles provide a reproductive advantage.

Adaptations

traits that have evolved through the mechanism of natural selection

ADH^F allele

breaks down ethanol in food in fruit flies

Natural Selection increases

the frequencies of alleles that enhance survival and reproduction

Adaptive evolution occurs as

the match between an organism and its environment increases

Adaptive evolution is a

continuous process because the environment can change

Selection sees and favors specific phneotypes which results in

changes of the frequencies of their corresponding genotypes

Directional Selection/Mode of Selection

favors individuals at one end of the phenotypic range

Disruptive Selection

favors individuals at both extremes of the phenotypic range.

Stabilizing Selection

Favors intermediate variants and acts against extreme phenotypes

Directional selection causes

change in the mean value of a character in a population that is either higher or lower than its current mean value which results in a shift in the plot of trait frequency

Disruptive selection

increases variation by favoring extreme phenotypic values even if the mean of the distribution does not change; two peaks

Stabilizing selection

decreases variation and stabilizes the mean of trait in a population around a particular usually optimal value

1977 drought causes

large hard seeds in Galapagos islands and as a result individuals with longer and harder beaks to survive and reproduce. EXAMPLE OF DIRECTIONAL SELECTION

Example of Stabilizing Selection

Lowest death rate of fetuses occurs at exactly at 40 weeks of gestation which is when birth of child usually occurs.

Struggle for existence and survival of the fittest

are commonly used t describe natural selection but can be misleading.

Fitness is

the contribution an individual makes to the gene pool of the next generation

Relative fitness (W)

The contribution of a genotype to the next generation compared with contributions of alternative genotypes for the same locus. Basically the most fit genotype in a population W=1

Selection coefficient (s)

a measure of the relative intensity of selection against a given genotype

s=1-W

In the case where the A1 allele is dominant

natural selection will quickly increase its frequency but then start there for a long time

In case where the A1 allele is recessive

and the A2 deleterious allele is dominant, then selection will take a long time to increase the frequency of the A1 allele in the population because most of the copies of the A1 allele are hiding in heterozygotes and since heterozygotes have the deleterious phenotype they are removed by selection so it takes a very long time for selection to act but once it does it does quickly and fixes the frequency of the A1 allele

Directional Selection Fitness

W₁₁=W₁₂> W₂₂

Stabilizing Selection Fitness

W₁₁<W₁₂> W₂₂

Disruptive Selection Fitness

W₁₁>W₁₂<W₂₂

Heterozygote Advantage (over dominance)

Heterozygotes have greater fitness than homozygotes and results in a balanced polymorphism

Heterozygote disadvantage (under dominance)

heterozygote has lower fitness than either homozygote

What only results in adaptive evolution?

Natural Selection of Evolutionary forces

Industrial Melanism

The peppered moth- classic study in the development of the theory of natural selection and evolution

typica → light, peppered, recessive

carbonaria → dark, melanic, dominant

How do you arrive at the genotype frequencies of the next generation?

We divide by the mean fitness of the population ^—-w

What are the effects of natural selection on HWE?

-Selection can cause allele frequencies to change from one generation to the next

-Selection can cause deviation from HW genotype frequencies

-Selection is deterministic (predictable). Selection will attempt to remove the deleterious allele from the population

-Selection can be a strong evolutionary force (compared to mutation) by changing allele frequencies from generation to generation

What were the postulates of Darwin in evolution by Natural Selection?

-Individuals vary

-At least some variation is heritable

-Some individuals leave more progeny than others due to the fit of some variants being better than others in the environment t

-The variation in survival and reproduction is not random but depends on heritable trait variation '

Outcome: Genetic variants with greater survival/reproduction increase in frequency in the population

What preserves genetic variation in natural populations?

-Much of the variation observed in generations is due to the combined action of mutation and genetic drift NOT selection

-Diploidy maintains genetic variation in the form of recessive alleles hidden from selection in heterozygotes. Diploidy masks deleterious alleles

Balancing selection examples

Heterozygote advantage

Frequency dependent selection

Balancing selection

occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population leading to a state called balanced polymorphism

Heterozygote Advanatge

Individuals who are heterozygotes at a particular locus have a greater fitness than homozygotes.

Natural selection will maintain two or more alleles at that locus when heterozygote advantage occurs.

Frequency Dependent Selection

The fitness of any morph declines if it becomes too common in th population

Example: prey species come to expect attacks in the direction that the majority of the scale eaters attack from at any given time

Natural Selection summary

-Natural Selection is differential success in reproduction from interaction between individuals that vary in heritable traits and their environments

-Natural Selection produces an increase over time in adaption of organisms to their environment

-If an environment changes over time then natural selection may result in adaptations to these new conditions

Natural Selection does not imply perfection

1.Selection can only act on existing variation

2.Evolution is limited by historical constraints

3. Adaptations are often comprises for instance the sickle cell allele confers resistance to malaria but then causes a weak immune system

4.Chance, natural selection and enviornwmnt interact and give rise to the variation that we observe

Sexual Selection

Natural Selection for mating success

What does sexual selection cause?

Sexual Dimorphism

Sexual Dimorphism

Marked Differences between the sexes in secondary sexual characteristics

What are the two types of sexual selection?

Intrasexual Selection

Intersexual Selection

Intrasexual Selection

direct competition among individuals of one sex (often males) for mates of the opposite sex

Example: male southern elephant seals fighting for the right to mate

Male stalk eyed flies face off for female attention

Intersexual Selection

often called mate choice; occurs when individuals of one sex (often females) are choosy in selecting their mates

Example: which male has the fanciest tail in peacocks will be chosen by females