Science Practice Challenge Questions

19.1 Population Evolution


Consider a polymorphic gene with three alleles: A, B, and C.

A. If the frequencies of the alleles A and B are 0.2 and 0.3, the frequency of allele C is closest to ___.

  1. 0.25
  2. 0.5
  3. 0.2
  4. 0.3

Consider a gene with only two alleles: dominant A and recessive a. In a population of 1,000 organisms, the fraction expressing the homozygous recessive phenotype is 0.37.

B. The calculated allele frequencies p and q have values that are closest to ___.

  1. 0.69 and 0.31
  2. 0.31 and 0.69
  3. 0.37 and 0.63
  4. 0.63 and 0.37

C. The calculated number of individuals in this population that are heterozygotes is closest to ___.

  1. 240
  2. 230
  3. 430
  4. 476

Mountain pine beetles (Dendroctonus ponderosae) were collected from a one-acre tract of lodge pole pine trees (Pinus contorta) in a region of British Columbia where the forests are under temperature stress. The beetles were crushed, and a cellulase enzyme was extracted. Three polymorphs of the enzyme were observed when separated by gel electrophoresis. The three proteins observed correspond to alleles labeled C1, C2, and C3. The numbers of beetles with each allele are shown in the following table.

Genotype C1 C1 C2 C2 C3 C3 C1 C2 C1 C3 C2 C3 Total
Observed 120 230 112 175 198 165 1,000
Table 19.5

D. The calculated allelic frequencies pC1, pC2, and pC3 are closest to ___.

  1. pC1 = 0.57 pC2 = 0.57 pC3 = 0.59
  2. pC1 = 0.29 pC2 = 0.29 pC3 = 0.42
  3. pC1 = 0.61 pC2 = 0.80 pC3 = 0.59
  4. pC1 = 0.31 pC2 = 0.40 pC3 = 0.29

E. In order to investigate the presence of selection at the cellulase locus due to changing temperature, a biologist should:

  1. calculate the values of the sums pC1 + pC2 + pC3 and (pC1 + pC2 + pC3)2. If these numbers are not equal to 1, the gene is not in Hardy-Weinberg equilibrium, and the gene is evolving.
  2. return next year and repeat this examination of the enzyme, calculating frequencies of each allele each year. Then calculate the values of the sums pC1 + pC2 + pC3 and (pC1 + pC2 + pC3)2. If these numbers are not the same each year, the gene is not in Hardy-Weinberg equilibrium, and the gene is evolving.
  3. return each year for several years and repeat this examination of the enzyme, calculating frequencies of each allele each year. If the allele frequencies are changing, the gene is not in Hardy-Weinberg equilibrium, and temperature is exerting a selection pressure.
  4. return each year for several years and repeat this examination of the enzyme, calculating frequencies of each allele each year. If the allele frequencies are changing, the gene is not in Hardy-Weinberg equilibrium. Analysis of the dependence of allele frequencies on temperature could indicate selection.

19.2 Population Genetics


Calamus finmarchicus is the dominant copepod in the Gulf of Maine. The polymorphic aminopeptidase locus, Lap-1, has been shown to be useful for the genetic differentiation of populations of this organism. By examining the population dynamics of copepods, the dynamics of the fin fish on which they feed can be predicted. The aerial photograph shows a landmass separating two coastal estuarine habits, the mud flats of Egypt Bay and the Mount Desert Narrows. For the past 40 years, transport between the two habits has been hindered by a dam over the Carrying Place Inlet. However, small volumes of water occasionally crest the dam.

In this map, number 1, which is labeled Mount Desert Narrows, is located at the mouth the river, which empties into Egypt Bay. Number 2, labeled carrying place inlet, is located south of number 1, near a body of water that empties into a different bay. The dam is between number 1 and number 2.
Figure 19.19

To evaluate the geographic isolation of invertebrate populations in these two habitats, copepods are sampled at the points labeled 1 and 2 on the photograph. These points lie at either ends of the Carrying Place Inlet. Enzymes encoded by three alleles, labeled A, B, and C, were determined by gel electrophoresis of equal numbers of the organisms collected at the two sites. Numbers of each genotype are given in the following table:

Site AA AB AC BB BC CC Total
1 82 114 102 74 98 30 500
2 96 108 92 54 110 40 500
Table 19.6

A. Calculate the frequencies, f, of each allele and complete the following table:

Site f(A) f(B) f(C)
Table 19.7

B. Using a χ 2 χ 2 test, evaluate these data to determine if the aminopeptidase gene in these two populations is evolving. State your conclusion as claims supported by evidence at both the 95% and 99% confidence levels. The formula for the χ 2 χ 2 test is provided on the AP Biology Exam.

χ 2 = (oe) 2 e χ 2 = (oe) 2 e

This table of critical p values is also provided on the AP Biology Exam.

Degrees of Freedom
p 1 2 3 4 5 6 7 8
0.05 3.84 5.99 7.82 9.49 11.07 12.59 14.07 15.51
0.01 6.64 9.32 11.34 13.28 15.09 16.81 18.48 20.09
Table 19.8

C. Based on these data, predict, with justification, changes over time in the aminopeptidase enzyme for these populations.

D. The B form of this aminopeptidase is slightly more efficient at extracting nutritional leucine from a protein than the A and C forms but slightly less efficient at extracting valine and serine. Describe an investigation of the two habitats that could suggest a causal relationship between changes in allele frequency and characteristics of the environment.

E. Single-nucleotide mutations are neutral when they encode changes in proteins that result in no significant differential selection. If differences in environmental factors between sites 1 and 2 are not observed, predict what other factors could result in departures from Hardy-Weinberg equilibrium for aminopeptidase.


Bioluminescence is an example of convergent evolution; 30 distinct lineages have acquired this characteristic, and all involve some form of a class of molecules called luciferins. Sexual selection pressures are strong for light-emitting organisms. Ellis and Oakley (Curr Biol, 2016) examined the number of species that lack luminosity in groups of closest evolutionary relation (sister linear) with those species that are luminous. Similarly, scientists made the same comparison between groups that use luminosity for concealment (counter-illumination) and their sister lineages. The graphs summarize their results, comparing the natural logarithm of the number of species in each lineage.

Two graphs are shown. In each graph, the number of luminous species is plotted on the y-axis on a natural log scale. The x-axis shows the number of nonluminous and luminous sister lineages. The graph on the left is a comparison between 10 species that are luminous and their sister lineages that lack luminosity. In each case, the trend is a linear increase from nonluminous to luminous lineages. The graph on the right is a comparison between species that are luminous for purposes of concealment and their si
Figure 19.20

Based on the data shown in the graphs, describe a model that can account for the increased speciation of bioluminescent lineages, including the mechanism of speciation.

A biologist is using a simulation to model populations of African hornbills (Bycanistes spp. and Ceratogymna spp.), a keystone species of the savanna. Populations of the birds are declining due to habitat loss. The hornbill’s diet consists primarily of termites and fruit. A critical component of termite digestion is chitin deacetylase, an enzyme whose mutation rate is a model parameter. The other model parameter is population size, N. In the results of the simulation study shown in the graphs, there is no selection, and the mutation rate is fixed. Although both population size and mutation rate are fixed, randomness results in the five different outcomes shown in each graph.
Two graphs show simulations to model populations of African hornbills. In each graph, the chitin deacetylase allele frequency over 20 generations is calculated. In graph a, N equals 16. The trend is an increase in the frequency of two alleles and a decrease in the frequency of the other three alleles. In graph b, N equals 524,288. The trend is that the frequency of each of the five alleles remains relatively constant.
Figure 19.21

A. Select the graph displaying the results that are closer to Hardy-Weinberg equilibrium. Justify the selection of the graph.

B. Based on these simulations, predict the future heterozygosity, 2pq, of the smaller populations, as shown in graph A.

C. Justify the use of a simulation study with no selection under environmental conditions in which the availability of both termites and fruit is high.

D. If a change in the environment occurs suddenly, such as an increase in average temperature, where fruit production declines, analyze the effect of the change on allele frequency in the large and small populations.