Study Questions 3

1. Explain why a medication "cocktail", a mixture of multiple treatments, works better than a simple

AZT treatment of the AIDS virus?

The virus may quickly develop a resistance to one drug due to their rapid generation time and increased likelihood of mutations within a short period of time. However, it is less likely that the virus will develop resistance to more than one drug within the same time period. Therefore, it is useful to use combination drug treatments, or periodically change drug treatments, to prevent the virus from developing drug resistance.

2. Draw a graph of directional, stabilizing, and disruptive selection. (1 for each) Also describe the role

of each of these three processes as part of a divergence between species over time.

Directional selection: Selection is acting to maximize or minimize a phenotypic trait, leading to an increase or a decrease in the magnitude of the trait within successive generations (if trait is heritable).

Stabilizing selection: Selection is acting to eliminate the extreme magnitudes of a particular phenotypic trait, favoring those individuals with intermediate trait values.

Disruptive Selection: Individuals intermediate in a certain phenotypic trait are being selected against, so that individuals with extreme values of trait will have a higher fitness. This results in a two-direction selection process, and can eventually lead to significant enough differences between phenotypic trait extremes to allow for speciation to occur.

3. Explain the variables in the equation "R = h2 * S", and draw a graph to illustrate the relationship

between them.

S is the selection differential of a certain trait; h2 is a measurement of the heritability of a trait; R is the response of the trait in the next generation as a result of the selection differential.

4. What is frequency dependent selection and give an example of it based on male-male competition for

access to females.

The fitness of an individual is dependent upon the frequency of that individual’s phenotype within the population. In most cases, an individual with the rarest phenotype is likely to have the greatest fitness, and over time that phenotype will increase in frequency, which then allows for a new rarer phenotype to be at an advantage. An example of frequency dependent selection in male-male competition is in species where males have two different mating strategies, to be either a Dominant male or a Sneaker male. If dominant males are very common in the population, then it may pay off for males to adopt a sneaker strategy, thereby avoiding the necessity of fighting with other dominants for access to females.

5. What are the three conditions necessary for a Fisherian Runaway of exaggerated male traits due to

sexual selection.

1) There is variation in the male trait and it is heritable.

2) Female preference exists and is heritable.

3). There is a large enough frequency of females in the pop. that have the preference.

During runaway the traits must stay heritable, and a piggy-back effect between the preference and the male trait (a genetic correlation) causes the trait to get more extreme and the preference more intense.

6. What force can move an individual off of an adaptive peak and what force then can move it up to the

top of another adaptive peak.

There are three possible forces that can move individuals off of adaptive peaks (assuming the adaptive landscape is relatively constant): Genetic Drift (most likely), mutation (not very likely as most mutations will be strongly selected against from the start), and Sexual Selection (only applicable to sexual organisms). Natural Selection is the process required to move individuals to the top of another adaptive peak.

7. What does inbreeding do over time to allele frequencies and what does it do to genotype frequencies?

Initially inbreeding will not change allele frequencies but will lead to a decrease in genotype frequencies. This results if the inbreeding is a result of positive assortative mating, where individuals are mating with other individuals of the same genotype. If there are initially two alleles in a population, heterozygotes will decrease in frequency, whereas both of the homozygous phenotypes will increase. However, if one of the alleles is deleterious in the homozygous form, then this will lead to more exposure of the allele in the deleterious phenotype, and the frequency of this allele will eventually decline to zero. So, over time inbreeding is likely to lead to a decrease in both allele and genotype frequencies.

8. Are reproductive barriers necessarily geographical? And if not what other mechanisms are there?

No, there are many other reproductive barriers that can develop among different species. Other mechanisms include both premating and postmating isolating mechanisms (See next question).

9. Give an example of each of the three premating isolating mechanisms, and the four postmating

isolating mechanisms between species.

Premating:

1). Seasonal separation; spatial separation. (Plants which flower in different seasons; terrestrial organisms seperated by large water masses)

2). Behavioural differences; mechanical differences. (Different mating calls or displays; different sized genitalia)

Postmating:

1). Sperm transfer, but sperm dies before fertilization.

2). Fertilization occurs by zygote dies.

3). Fertiliation occurs, F1 hybrids produce which are sterile; or fertile F1 hybrids are produced, but subsequent F2 hybrids are sterile (hybrid breakdown). (Donkeys are an example of an infertile F1 hybrid)

10. Explain the differences between the three modes of speciation.

Allopatric Speciation: A geographical barrier physically separates species into two subpopulations. Subpopulations then undergo different process of Genetic Drift and mutation accumulations, leading to different selection pressures within both subpopulations. This process will eventually lead to speciation over many generations if there is little or no gene flow.

Parapatric Speciation (adjacent): A difference in selection pressure along a cline can lead to speciation over many generations, particularly if there is a sharp change in phenotype success across one section of the cline. If this leads to a relatively small zone of hybridization between the two subpopulations, then speciation is more likely to occur.

Sympatric Speciation: Individuals are found within the same population and there is no geographical barrier separating the populations into subgroups. This process is very unlikely to occur in natural settings; however, if it does occur there are two mutations required: 1) Unfitness of hybrid between individuals with different traits, and 2). Assortative mating so that individuals mate with other individuals of the same character trait. See Drosophila example in your notes.

11. Reprise: You head up to Ano Nuevo with Dan Costa to sample a population of elephant seals. Dan

teaches you how to isolate the allele for flipper length and instructs you that it is coded at one locus by

two alleles. You find seals 90 with short flippers, 420 with medium flippers, and 490 with long

flippers. Give the allele and genotype frequencies for this generation. Is this population in

Hardy-Weinberg Equilibrium?

L allele codes for long flippers, l allele codes for short flippers. Heterozygous individuals, Ll, show intermediate flipper lengths. Since we are not certain if the population is in HW Equilibrium, we must first approach this problem by calculating the allele frequencies based on the information given to us. This can be done by actually counting how many times each allele is represented in the population out of the total number of alleles.

Frequency (L), or p, can be calculated as follows:

There are 490 individuals with the LL genotype, and 420 individuals with the Ll genotype, out of a total of 1000 individuals in the population. This means that L is represented 2*490+420, or 1400 times in the population. So p, or the frequency of L, is equal to 1400 divided by the total number of alleles in the population, 2*1000. The same procedure can be followed to calculate the frequency (l), or q. Calculations are demonstrated below.

p= f(L)= (2*490+420)/2000= 0.7

q= f(l)= (2*90+420)/2000= 0.3

From these values we can then calculate what the genotype frequencies should be if the population is in HW Equilibrium.

f(LL)= p2= 0.7*0.7= 0.49

f(ll)= q2= 0.3*0.3 = 0.09

f(Ll)= 2pq= 2*0.7*0.3= 0.42

From the estimated genotype frequencies we can then calculate what the number of individuals in the population with each phenotype should be if the populations were in HW Equilibrium:

f(LL)*population total= 0.49*1000= 490 individuals with long flipper phenotype

f(Ll)* population total= 0.42*1000= 420 individuals with medium flipper phenotype

f(ll)* population total= 0.09*1000= 90 individuals with short flipper phenotype

Since these phenotype estimates match up with the actual numbers of elephant seals that we counted with the given phenotypes, then we can conclude that Yes, the population is in HW Equilibrium. A job well done!