Topic 6

Question 1

Natural selection

Answer 1

• Differences in the survival and reproduction of phenotypes, leading to differences in their contribution to the next generation, resulting in a change in the frequency of heritable phenotypic variations in populations over time
• Selection acts on phenotypes, which can be controlled by single genes, multiple genes (polygenic), or genes interacting with the environment

Question 2

Darwins 4 postulates

Answer 2

1. Individuals within species are variable
2. Some of these variations are heritable
3. In every generation, more offspring are produced than survive, some are more successful at survival and reproduction than others
4. The survival and reproduction of individuals is not random. Those who reproduce, or reproduce the most are those with the most favourable variations and are naturally selected

• Those who survive and contribute the most to the next generation are the most fit

Question 3

the measure of fitness

Answer 3

• Fitness is a measure of reproductive success, which is affected by three different elements of natural selection:
  1. viability or mortality selection
  2. sexual selection
  3. fecundity selection

Question 4

viability or mortality selection

Answer 4

• an individual’s ability to survive and reach reproductive age

Question 5

sexual selection

Answer 5

• an individual’s ability to procure a mate (mating success)

Question 6

fecundity selection

Answer 6

• Family size, which is usually measured as the number of female gametes (eggs) produced

Question 7

adaptive evolution

Answer 7

• Natural selection is not the only means by which a population can evolve.
• Genetic drift, mutation and gene flow can also change allele frequencies, but natural selection is the primary means of adaptive evolution, by which the average individual becomes better adapted to their environments, or the average fitness of a population increases.
• An adaptation is a characteristic or trait that increases an individual’s fitness relative to individuals that do not possess it

Question 8

fitness

Answer 8

• The term Fitness refers to an individual’s reproductive success,
  and depends on survival, fecundity and its ability to obtain a
  mate.
• If phenotypes have a genetic basis, and have different fitnesses, selection will act on these phenotypes, resulting in genotype and allele frequency changes in the next generation

Question 9

absolute fitness

Answer 9

• The absolute fitness of a genotype refers to the average number of offspring (zygotes) contributed to generation t+1 by an individual (zygote) in generation t.
• We denote absolute fitness as capital W. The absolute fitness of genotypes AA, AB and BB are denoted WAA, WAB and WBB

Question 10

relative fitness

Answer 10

• It is convenient to express fitness as relative fitnesses (lower case w), which provides a measure of a genotype’s contribution to the next generation relative to that of other genotypes in the population.
• Relative fitnesses are normally scaled to the genotype with the largest absolute fitness.

Question 11

population fitness

Answer 11

• The mean absolute fitness of a population, denoted W gives the average reproductive rate of all individuals in the population. It is equivalent to λ in population ecology, which is the per capita reproductive rate of the population.
  W = PAA(WAA) + PAB(WAB) + PBB (WBB)
  
  • Where P denotes the genotype frequency
• If the mean absolute fitness of a population is
  greater than 1, the population will increase in
  size, if it is less than one, the population will
  decrease in size.

Question 12

relative population fitness

Answer 12

• We can also define the mean relative fitness of a population w, which is the average fitness of the entire population relative to those individuals within it that have the highest fitness.w = PAA(wAA) + PAB(wAB) + PBB (wBB)
  - Where P denotes the genotype frequency

Question 13

a single locus model of natural selection assumptions

Answer 13

1. Fitness differences are only due to differences in survival (viability)
2. Fitness differences are attributed to a single locus
3. Mating is random
4. No mutation, drift or gene flow occurs
5. Fitnesses are constant and independent of both allele frequencies and population size

Question 14

outcomes of single locus natural selection

Answer 14

• Selection will increase or decrease genotype frequencies as a function of their relative fitness, resulting in a change in allele frequencies.
• We have also seen that the change in allele frequencies resulted in an increase in the relative population fitness. Selection will act to maximize the relative population fitness
• Where will selection take a population under different circumstances of genotype fitness? We must consider different equilibria.
• An equilibrium point is a point where allele frequencies will remain constant and can be of two major types in the case of selection at a single locus (viability model)

Question 15

stable equilibrium

Answer 15

• Allele frequencies will approach a point of stable equilibrium from a short distance away (e.g., it is a point of attraction), and then stay in the vicinity of the equilibrium.
• A stable equilibrium is globally stable if it is always approached, regardless of the starting allele frequencies

Question 16

unstable equilibrium

Answer 16

• an unstable equilibrium is a point of repulsion.
• Allele frequencies will move away from the point of unstable equilibrium unless they are precisely on it.

Question 17

Balancing selection

Answer 17

• refers to any type of selection that acts to maintain genetic variation. Heterozygote superiority is one mechanism that acts to maintain variation, and is one explanation (among others), for the large amount of variability in natural populations

Question 18

directional selection

Answer 18

• Fitnesses Example: wAA = 1, wAB = 0.95, wBB = 0.90
• Note that these fitness values would be expected if A and B
  are codominant
• In this case, wAA > wAB > wBB and the system will eventually be pushed to fixation for allele A (e.g., p(A) = 1). This is a stable equilibrium which is also globally stable, unless the initial frequency of A is equal to 0. There is also an unstable equilibrium at p(B) = 1, (e.g., p(A) = 0) from which the system will always move away unless it is exactly on it. The opposite, where BB has the highest fitness will also result in directional selection

Question 19

heterozygote superiority

Answer 19

• Fitnesses Example: wAA = 0.90, wAB = 1, wBB = 0.95
• In this case, wAA < wAB > wBB and the heterozygote has the highest fitness. This is referred to as heterozygote superiority, overdominance or heterosis. This has two unstable equilibrium points at p(A) = 1, and p(B) = 1, and a third stable equilibrium point that can be defined as follows:
  p^(A) = (1-wBB)/((1-wAA)+(1-wBB))= 0.33333; this is the equilibrium frequency of allele A

Question 20

selection and deleterious alleles

Answer 20

• We might expect selection to eliminate rare deleterious recessive alleles, but this is not the case
• The rare deleterious recessive allele will “hide” in heterozygotes, and never be eliminated by selection. In fact, virtually all the rare recessive alleles will occur in heterozygotes

Question 21

marginal fitness of alleles

Answer 21

• In general, selection will act to increase the frequency of an allele if its marginal fitness is greater than the relative population fitness, and decrease its frequency if its marginal fitness is less than the relative population fitness
• This is useful to help understand what the fate of a new mutation will be (if a trait is controlled by a single locus) because of selection alone.
• Marginal fitnesses of alleles are different from relative genotype fitnesses because they are not constant and depend on the frequencies of alleles.
• In cases of heterozygote superiority, as an allele increases in frequency toward its stable internal equilibrium, its marginal fitness will decrease, and when an allele decreases in frequency toward its stable internal equilibrium, its marginal fitness will increase.
• In the cases of heterozygote superiority and inferiority, the marginal fitnesses of both alleles will be equal when at the internal stable equilibrium (superiority) or unstable equilibrium (inferiority), as well as equal to the relative population fitness.

Question 22

adaptive radiation

Answer 22

• the diversification of multiple descendent lineages from a single ancestral lineage. Adaptive radiations typically result in ecological specialization, and morphological innovations, and are driven by natural selection, often in concert with sexual selection.
• Adaptive radiations typically involve rapid ecological and morphological innovation, followed by long periods of morphological (and ecological) stasis.
• This pattern has been observed in the fossil record and resulted in the theory of punctuated equilibrium proposed by Niles Eldridge and Stephen J. Gould. Punctuated equilibrium refers to rapid morphological and ecological diversification, followed by long periods of stasis

Question 23

frequency dependant selection

Answer 23

• Frequency dependent selection occurs when the fitness of a phenotype (Genotype) depends on its frequency.
• Positive frequency dependent selection is when the fitness of a genotype decreases as its frequency becomes lower.
• Loci responsible for warning coloration are often under positive frequency dependent selection

Question 24

inverse frequency dependant selection

Answer 24

• With inverse frequency dependent selection, the lower the frequency of a phenotype (genotype) the greater its relative fitness.
• Inverse frequency dependent selection is often called negative frequency dependent selection.

Question 25

balancing selection (heterozygote superiority, inverse frequency dependant selection)

Answer 25

- Balancing selection refers to any type of selection that acts to maintain genetic variation. Heterozygote superiority is one mechanism that acts to maintain variation, and is one possible explanation (among others), for the large amount of variability in natural populations. - Inverse frequency dependent selection is another form of balancing selection

Question 26

selection and HW disequilibrium

Answer 26

- When phenotypic traits are controlled by a single locus, selection alone will often not result in statistically significant (e.g., detectable) HW disequilibrium. - Why: Differences in the fitnesses of genotypes are usually subtle, and one round of random mating will restore HW equilibrium each generation

Question 27

selection and linked loci

Answer 27

- Suppose we have a population in linkage disequilibrium (D= Dmax) consisting of the multilocus genotypes AABB and aabb. Also suppose that r = 0.0001, and the relative fitnesses for genotypes at the two loci are as follows: Locus A: wAA= 1 waa= 1 Locus B: wBB= 1, wbb= 0.3 - Selection will eliminate bb genotypes at locus B, but because of linkage disequilibrium, aa genotypes will also be inadvertently eliminated, and selection will push the multilocus genotype AABB to fixation. Genotype AA will get “dragged” to fixation with genotype BB in a process referred to as Hitchhiking

Question 28

points on selection

Answer 28

- The fitness of a genotype can vary spatially - The fitness of a genotype can vary temporally - The fitness of a genotype can vary with population size - The fitness of a genotype can be different in males and females - The fitness of a genotype can depend on genotypes at other loci

Question 29

epistasis

Answer 29

- An effect of the interaction of two or more gene loci on phenotypes and/ or fitness, where the joint effect of the loci is different than the sum of the individual loci taken separately

Question 30

pleiotropy

Answer 30

- a phenotypic affect of a single gene on more than one character. - The same gene can affect different traits in different life stages

Question 31

sexually antagonistic genes

Answer 31

- In some cases, alleles at an autosomal locus can have opposing fitness effects in males and females, which is referred to as sexually antagonistic selection.