THE WAYS OF CHANGE: DRIFT AND SELECTION

Some forms of selection maintain diversity in populations:

1. Negative frequency-dependent selection 2. Heterozygote advantage

If Cavalli-Sforza and colleagues had measured allele frequencies as 0.869 for the A allele and 0.131 for the S allele, how many homozygous genotypes should they have expected to find? Would they have considered the population to be at equilibrium? 9354 AA and 29 5S. No, they would not have considered the population to be at equilibrium. 9354 AA and 211 SS. Yes, they would have considered the population to be close to equilibrium. 9354 AA and 211 S5. No, they would not have considered the population to be at equilibrium. 2811 AA and 2993 SS. No, they would not have considered the population to be at equilibrium.Show Answer

9354 AA and 211 S5. No, they would not have considered the population to be at equilibrium.

Which population would be most likely to have allele frequencies in Hardy-Weinberg equilibrium? A population in a rapidly changing environment. A population where immigration is common. A large population that currently is not evolving. A population that cycles between a very large and a very small number of individuals.

A large population that currently is not evolving.

How can a drastic reduction in population size lead to inbreeding depression?

A smaller population size makes it more likely that two closely related individuals will mate, which also makes it more likely that their offspring will get two copies of a rare recessive trait with harmful effects. As for the genes, there will be more harmful homozygous combinations; this will result in a in a reduction of the fitness of these individuals and a reduction in the average fitness of the population

A genetic bottleneck in a population often results in loss of alleles. an increase in inbreeding. an increase in genetic drift. All of the above.

All of the above.

What can measuring genetic distance, or Fs, tell scientists about a group of organisms? Whether groups have begun to diverge from each other. Whether genes are under strong selection. How barriers may be influencing gene flow. All of the above.

All of the above.

Natural selection in action

Alleles that lower fitness experience negative selection Alleles that increase fitness experience positive selection

Using the average excess of fitness of an allele, explain why when an allele is very rare (p = 0), the change in allele frequency from one generation to the next due to selection (Ap will be small even when the fitness effects of the allele are considerable

Ap is the frequency of the allele, p. times the average excess of fitness for that allele divided by the average fitness of the population. In other words, the change in frequency of an allele is a function of how common that allele is and how it affects fitness relative to other alleles in the population. When p is close to zero, Ap will be close to zero because multiplying any number by a number that is essentially zero yields a number that is essentially zero. On the other hand the average excess of fitness of an allele is p * (WI - 2)] + [g × (WI2 - w)] or P X (w12 - w)] + [g × (W22 - 20)] - - depending on the allele being described. As a result, a rare allele (even if it is only slightly greater than zero) can eventually become more common if it has a higher fitness relative to other alleles and it can become less common if it has a lower fitness relative to other alleles.

Gene flow can counteract the loss of alleles due to drift

Arrow thickness reflects intensity of gene flow between populations Amount of gene flow varies with the range of movement of the organism

Dominant & Recessive Alleles

Assume coat color in mice is controlled by a Mendelian locus where blond coats, aa, are recessive to brown coats, AA/Aa. Based on this information alone, how do we expect the frequency of a to change in a very large population?

Key Concept

Diploid individuals carry two alleles at every locus Populations contain mixtures of individuals, each with a unique genotype. Evolution: change in allele frequencies from one generation to the next

Which of these statements about inbreeding is false? Inbreeding is not a mechanism of evolution. Inbreeding can affect the fitness of individuals, but it does not necessarily alter allele frequencies within a population. Inbreeding increases the probability that two alleles at any locus will be identical because of a shared common ancestor. Inbreeding alters allele frequencies within a population but does not affect the fitness of individuals.

Inbreeding alters allele frequencies within a population but does not affect the fitness of individuals.

Why did the genetic variation of northern elephant seal populations remain low for generations after the bottleneck event? How could genetic drift have played a role in slowing the recovery of genetic diversity?

Many alleles were lost in the bottleneck event due to the death of the individuals that carried those alleles. The small number of sul vors had only a subset of the original genetic diversity. Because the recovering populations initially were also very small, and the mutation rate is relatively low, genetic drift had a much stronger effect, which could possibly eliminate any new alleles that arose or make it harder for genetic diversity to increase.

population subdivision

No subdivision Extreme Subdivision Intermediate Subdivision

How does pleiotropy affect selection on alleles?

Pleiotropy is the condition when a mutation in a single gene (like a regulatory gene) affects the expression of more than one phenotypic trait. Therefore, even if a mutation on an allele has a positive effect on the fitness of an organism under a specific selective pressure, there may be a negative effect on other genes that, overall, reduces the ability of the organism to reproduce and pass on that gene. Thus, selection for that allele would decrease, even if it has a positive effect in isolation. In essence, then, pleiotropy represents the interconnectedness of biology and emphasizes that one needs to consider the net effect of an alele on an organism before judging whether or not that allele (again. under specific selective conditions) positively or negatively impacts fitness

Pleiotropy may constrain evolution

Pleiotropy: mutation in a single gene affects more than one different phenotypic trait

Hardy-Weinberg equilibrium

Population allele frequencies do not change if: - Population is infinitely large - Genotypes do not differ in fitness - There is no mutation - Mating is random - There is no migration

The figure below shows how human-made barriers, such as highways, have affected standing genetic diversity in natural populations of bighorn sheep. Which of the following is not a valid conclusion from this graph? Because of highways, some populations that are physically very close to each other look, genetically, just as isolated as if they were far apart. Sheep in populations isolated by barriers like highways do not manage to migrate into other populations in their range. Inbreeding is likely to be more prevalent when the geographic distance illustrated in the graph is greater. In the absence of barriers, the probability that sheep migrate and interbreed decreases as the physical distance between populations increases.

Sheep in populations isolated by barriers like highways do not manage to migrate into other populations in their range.

. If a mutation produces a new deleterious recessive allele in a population, what is least likely to happen to the frequency of that allele? It will remain at a low frequency within the population for a very long time. Drift will determine whether it persists in the population. The allele will be rare enough that it almost never occurs in a homozygous state. The allele will quickly be purged from the population by selection.

The allele will quickly be purged from the population by selection.

What do population geneticists mean when they refer to the fitness of an allele? The ability of the allele to survive in a population. The contribution of an allele to the strength and overall health of a genotype. The contribution of an allele to a genotype's relative success at producing new individuals. Whether or not an allele is dominant.

The contribution of an allele to a genotype's relative success at producing new individuals.

Founder effect

The founder effect is a type of bottleneck resulting from a small number of individuals colonizing a new, isolated habitat

What would happen to the frequency of heterozygous carriers of sickle-cell anemia (with an 1S genotype) if mosquitoes were completely wiped out in a large region? Explain

The frequency of AS individuals would decrease without balancing selection. Without mosquitoes there would be no transmission of malaria, so the S allele would lose its advantage of protecting against death from malaria. It would not likely disappear right away because as the S allele became rarer and rarer its probability of occurring in a homozygous genotype that is, paired with another S allele) would be low. Once in this rare state, drift alone would determine whether it persisted in the population Selection could act on the S allele only when in the homozygous SS genotype, which causes sickle-cell anemia in the phenotype

Pesticide resistance and pleiotropy

The frequency of the Ester1 gene increased in response to the use of pesticides in coastal areas

If two individuals mate, one of them heterozygous at a locus and the other homozygous for a recessive allele at the same locus. what will be the outcome? The offspring will be either heterozygous or homozygous for the recessive allele. The offspring will be homozygous for the dominant allele, heterozygous, or homozygous for the recessive allele. The offspring will not evolve because they will carry the same alleles as the parents. The recessive allele eventually will become the dominant allele in the population.

The offspring will be either heterozygous or homozygous for the recessive allele.

Antagonistic pleiotropy

beneficial effects for one trait but detrimental effects for other traits Net effect on fitness determines outcome of selection

Genetic drift can cause the loss of an allele in a species. happens faster in large populations than in small ones. does not occur in large populations. is a function of Hardy-Weinberg equilibrium.

can cause the loss of an allele in a species.

Even brief bottlenecks

can lead to a drastic reduction in genetic diversity that can persist for generations

Relative fitness (w):

contribution of individuals with one genotype compared with the average contribution of all individuals in the population

Average excess fitness:

difference between relative contribution of individuals with one genotype and the average fitness of the population as a whole Change in allele frequency due to selection Δp = p x (aA1/ϖ)

Many organisms occupy ranges that are

discontinuous

Subdivided populations shows

distinct genetic structure

Bottlenecks reduce

genetic variation

The Hardy-Weinberg theorem demonstrates that dominant alleles are more common than recessive alleles. in the absence of outside forces, allele frequencies of a population will not change from one generation to the next. a locus can have only one of two alleles. evolution is occurring.

in the absence of outside forces, allele frequencies of a population will not change from one generation to the next.

Homozygous

individual carries two copies of the same allele

Heterozygous

individuals carries different alleles

genetic locus

location of a specific gene or sequence of DNA on a chromosome

Key Concepts • Hardy-Weinberg theorem ++++++++++++++++ • Mechanisms of evolution+++++++++++++++++

predicts that allele frequencies do not change in the absence of drift, selection, mutation, and migration are forces that change allele frequencies

Genetic drift is the ________________________________-- • Drift is an ________________________________________ • Alleles either become fixed or are lost more rapidly in small populations

random, non-representative sampling of alleles from a population during breeding important mechanism of evolution because it alters allele frequencies over time

Genetic Drift

results from random sampling error --Sampling error is higher in smaller sample

Bottlenecks

results in a nonrepresentative set of alleles for subsequent populations, even after the population size rebounds

Fitness

the survival and reproductive success of an individual with a particular phenotype • Components of fitness : • Survival to reproductive age • Mating success • Fecundity

Predictions from Hardy-Weinberg

• Allele frequencies (p,q) predict genotype frequencies (f) • p = frequency of a, q = frequency of A → p + q = 1 (=100%) p^2 + 2pq + q^2 = 1

Natural selection more powerful in large populations

• Effects of drift weaker in large populations • Small advantages in fitness can lead to large changes over the long term

Mutation-selection balance

• Equilibrium frequency reached through "tug-of war" between negative selection on deleterious alleles and new mutation • Explains persistence of rare deleterious mutations in populations

Populations evolve through a variety of mechanisms

• Hardy-Weinberg serves as the fundamental null model in population genetics • Hardy-Weinberg is useful because it provides mathematical proof that evolution will not occur in the absence of selection, drift, migration, or mutation

Concepts

• Inbreeding increases percentage of loci that are homozygous for alleles identical by descent • Inbreeding is not a direct mechanism of evolution, but can bring about selection against rare recessive alleles normally masked in a heterozygous state • Genetic bottlenecks often go hand in hand with inbreeding and selection • Recessive alleles exposed to selection

Mutations generate variations in populations

• Mutation rates for any given gene are low • Per genome and population, many new mutations rise each generation —Estimate in humans: 7.9 billion new mutations per year • Source of variation upon which selection and drift act

Concepts

• Mutations are the source of new genetic variation in populations • Many mutations occur in a large population • Balancing selection maintains multiple alleles in populations ---- Negative frequency-dependent ----Heterozygote advantage

Concepts

• Population subdivision enhances the effects of genetic drift, which reduces genetic variation in local populations and leads to divergence between populations . • Gene flow counteracts the effects of subdivision by homogenizing allele frequencies

Inbreeding and the Hapsburg Dynasty

• Rare deleterious alleles more likely to combine as homozygotes in inbred organism

Drift reduces genetic variation in a population

• Small populations experience strong drift • Some alleles become fixed in the population, while others disappear

population genetics

• The study of the distribution and frequencies of alleles in populations, and • How and why allele frequencies change -Natural Selection, Genetic Drift, Gene flow ---Population: a group of interacting and potentially interbreeding individuals of the same species