EHJ352H1 Lecture Notes - Lecture 9: Gamma Distribution, Intron, Genetic Load

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sapphireporpoise585

26 Jan 2013

School

UTSG

Department

Ecology & Evolutionary Biology

Course

EHJ352H1

Professor

Stephen Wright

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Document Summary

Mutations happen, most are deleterious and thus all populations contain deleterious mutations. Frequency of deleterious allele (q) = u/s. The mean fitness depend on mutation rate: equilibrium frequency of deleterious allele = u/hs. If mutation is really deleterious there will not be many at equilibrium, its average effect is rather small. If a minor deleterious allele will have more copies. Thus selection will either make the effect really big and amount of alleles really small, or the bigger effect on the mean fitness due to the large amount of copies but small individual effect. W=1-2u, w is almost equal to 1. Only 1 gene and if multiple by all genes then the result is rather big. Selection decreases the number of mutations and mutation increases the number of mutations. Haldane: weq = e^-u, u is the average number of new deleterious mutation rate each time the offspring is produced, per generation.

Related Questions

Need help with evolution questions:

1.) A haplotype is best defined as the ________________.

	haploid genotypes of all the gametes produced by a diploid individual

ABO blood type conferred by an individual gamete

genotype of either the paternal or maternal chromosomal complement

multilocus genotype of a chromosome or gamete

2.) Which of the following statements regarding linkage disequilibrium is true?

Exists when D is less than zero, but not when it is greater than zero.

Is reduced by sexual reproduction.

Is increased by crossing-over during meiosis.

Is increased by any random sampling error that happens to create or destroy certain chromosome genotypes but not others.

Is reduced by selection that favors certain combinations of genotypes.

3.) Selection on multilocus genotypes in random-mating populations leads to linkage disequilibrium when _______________.

it reduces mutation rates

it eliminates all haplotypes from the population

it prevents crossing-over during meiosis

some allele combinations confer greater fitness than do others

4.) When two loci are in linkage equilibrium, the allele carried on a chromosome at one locus is independent of the allele carried on the chromosome at another locus. When this is the case, _________________.

allele frequencies will not change over time

the two loci (or, more specifically, their protein products) do not interact

the frequency of each haplotype can be calculated by multiplying the frequency of its two alleles

all possible haplotypes occur at equal frequency

5.) Which of the following accurately characterize(s) Muller's ratchet model for the selective advantage of sexual reproduction?

Drift establishes linkage disequilibrium because multilocus genotypes with few mutations are eliminated by chance events.

In populations without sexual reproduction, deleterious mutations accumulate, eventually imposing such a burden that the population goes extinct.

Sex is favored selectively because it recreates the zero-mutation genotypes lost due to drift.

All of the above correctly characterize Muller's ratchet model for the evolution of sexual reproduction.

6.) The genetic signature of recent positive selection on a locus is _______________.

high frequency and low linkage disequilibrium

low frequency and low linkage disequilibrium

low frequency and high linkage disequilibrium

high frequency and high linkage disequilibrium

7.) Sexual reproduction is advantageous both in changing environments and in constant environments, compared to asexual reproduction.

True

False

8.) Sexual reproduction causes genetic recombination via crossing-over, which combines genotypes from different parents.

True

False

9.) One reason sex may be maintained in natural populations is proposed by the "Red Queen Hypothesis", which states that.....

genotypes created by sex are created by natural selection over and over

recombination creating novel genotypes that can resist infection by parasites

mutation load is reduced by novel adaptive mutations

the ability of sex to fix multiple

Evolution naturally produces better features: stronger muscles,teeth and minds. Killing the weakest, evolution wipes defectivegenes out of populations.

The mutations are necessary for advance. However, they arerandom and, thus, mostly negative. Right? How does the natureeliminate them from the population?

I see that one mechanism is polygamy: male have higher mutationrates. They tend to reproduce as much as possible. Female havelower mutation rates, they couple with only with best men andcontribute much more to their offsprings. So, men generate as muchrandom solutions as possible, whereas the role of female is toconserve the best of them. This means that the most of the male(bad mutations) die unreproduced. This eliminates the bad mutationsand favours the progress of good qualities. The higher animalsstarted to form harems and tournaments over their ownership. Thisfurther fosters the elimination of weak men and profileration ofstrong genes. However, human monogamy has disabled this selectionmechanism. Best men are dedicated to only one woman. Other womencannot mix their genes with other good genes from previousgenerations. They are forced to look at the low quality men, whocarry degenerate mutations. Might be it is monogamy that makes ushuman but it implies that all genes reproduce.

Yet 100 years ago it was not a big deal since we still hadanother filter: our grandfathers were born in families that hadnormally 10 children (I am speaking about Russia) and only 2-3survived until reproduction (the population increased reallyslowly). I bet that those who survived had really good health:thanks to the bad health and immense selection pressure, thenegative mutants are eliminated immediately and populationmaintains a perfect genome (aka biologic health). Killing moreweakest in the current generation improves the health of nextgenerations.

However everything has changed during the 20th century. Theadvances in economy and medicine have almost eliminated theselection. The selection pressure has relaxed to essentially zeroso that everybody survives and leaves the same amount of offspring(and some scientists are even concerned that degradatoryindividuals leave more offsprings than the prosperous ones). Wehave even eliminated the infant mortality. We use baby incubatorsso that the people with the weakest health could survive andreproduce as normal and we are proud of it. It may sound strangebut the mutation rate has not changed at all. Does it mean that thehuman genome is in danger?

I see the situation like you have inherited a perfect mechanismand decided not recovering it from the inevitably adverse action ofentropy, that degrades it constantly. There is a constant chance,d, that every good gene is affected by a mutation. It is afraction of healthy genes that will fail passing to the nextgeneration. This will leave only (1-d) genes healthy inthe next generation. The fraction of healthy genes will melt like1, (1-d), (1-d)

Q1. What is the founder effect?

Sampling error that occurs during the establishment of a newpopulation by a small number of migrants.

Strong natural selection acting on the founders of a newpopulation because the environment they are now living in is sodifferent from the environment they came from.

A phenomenon named after William Founder, who observed that toppredators generally have low genetic diversity.

Rapid population growth following a bottleneck.

Q2. The effects of genetic drift are more pronounced in largerpopulations.

True

False

Q3. Inbreeding tends to increase the proportion of homozygousindividuals in a population (i.e. individuals that have two copiesof the same allele).

True

False

Q4. The effective size of a population is:

The size of an idealized randomly-mating population that is notunder selection and has the same heterozygosity as the actualpopulation.

The size of an idealized randomly-mating population that has thesame heterozygosity as the actual population, but does not loseheterozygosity over time.

The size of an idealized randomly-mating population losinghomozygosity at the same rate as the actual population.

The size of an idealized randomly-mating population losingheterozygosity at the same rate as the actual population.

Q5. Which of the following tends to reduce the effective size ofa population?

Check all that apply:

Increasing the population size.

A biased sex ratio.

Non-random mating.

Q6. If gametes from a gene pool combine randomly to make only asmall number of zygotes, the allele frequencies among the zygotesmay be different than they were in the gene pool because:

The effects of natural selection are more pronounced in smallpopulations.

The effects of genetic drift over several generations are morepronounced with small numbers of gametes.

The effects of differences in frequencies for different allelesare more pronounced with small numbers of zygotes.

The effects of sampling error are more pronounced with smallsamples.

Q7. Genetic drift is different from natural selectionbecause:

In natural selection allele frequencies change because somealleles confer higher fitness, whereas in genetic drift allelefrequencies change because of chance sampling error.

Natural selection acts primarily in large populations, whereasgenetic drift acts primarily in small ones.

Natural selection is a mechanism of evolution, whereas geneticdrift is an outcome of evolution.

Natural selection tends to cause rapid evolution, whereasgenetic drift tends to cause slow evolution.

Q8. Imagine a population evolving by genetic drift in which thefrequency of allele K is 0.65. What is the probability that at somepoint in the future allele K will drift to a frequency of 1?

Q9. A dwindling population of 1000 frogs occupies an isolatedwatershed in Costa Rica. To help preserve the species, scientistscaught 20 frogs to start a new population in a nearby watershed.This species has a gene that affects eye shape. The 1000-memberwild population has two alleles for this gene: R and r, withfrequencies 0.7 and 0.3, respectively. What will be the allelefrequencies of R and r in the 20-member founder population?

The frequencies will be 0.7 for R and 0.3 for r.

The frequencies will be 1.0 for R and 0 for r.

The expected frequencies are 0.7 for R and 0.3 for r. The actualfrequencies could be different.

The founder populations's allele frequencies will necessarily bedifferent than the source population's frequencies.

Q10. For a population containing 90 females and 10 males, whatis the effective population size, N_e ?

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EHJ352H1 Lecture Notes - Lecture 9: Gamma Distribution, Intron, Genetic Load

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