1. A pesticide that was rarely used in 1932 was used with increasing frequency until it was banned altogether by 1972. Fruit flies (Drosophila melanogaster) that are resistant to this pesticide carry the r allele. The table below shows the frequency of the r allele in a population of fruit flies over time. In 2011, 3% of fruit flies were homozygous for the r allele. Which statement best predicts how the frequency of the r allele will change in future generations of fruit flies? (A) The frequency of the r allele will increase until the population reaches genetic equilibrium because more individuals are heterozygous than homozygous recessive. This answer suggests the student may understand that the frequency of an allele will no longer change once a population reaches genetic equilibrium and that more individuals are heterozygous than homozygous recessive, but does not understand that the data show that the frequency of the r allele is decreasing over time, not increasing. (B) The r allele will become less common in the population because there is a selective advantage for the dominant and heterozygous phenotypes. This answer suggests the student understands that the decrease in the r allele frequency from 1968 to the present most likely indicates that there is a selective advantage for the R allele and that the frequencies of heterozygous and homozygous dominant phenotypes are increasing in the population. (C) The r allele will become more common in the population because fewer flies in the population than expected have the recessive phenotype. Page 1 of 9
This answer suggests the student may understand that fewer fruit flies are homozygous recessive than expected in the population, but does not understand that the prevalence of the recessive allele shows a decrease over time, not an increase. (D) The frequency of the r allele will remain stable because the population has already reached genetic equilibrium. This answer suggests the student may understand that the frequencies of alleles in a population that is in genetic equilibrium will not change, but does not understand that the population is not in genetic equilibrium since fewer individuals are homozygous recessive than expected based on the allelic frequency. 2. The majority of Eastern gray squirrels (Sciurus carolinensis) have gray fur, but a small percent of the population have black fur. The percent of squirrels with black fur is less in the deciduous forests of the United States, where trees lose their leaves in winter, and is greater in northern coniferous forests, where conifer trees keep their needles throughout the winter, producing a dark forest floor. Which statement best explains the higher percent of squirrels with black fur in northern coniferous forests than in deciduous forests? (A) Individual squirrels in coniferous forests adapted to the forest floor and gradually changed their fur to a slightly darker color. Over time, the darker squirrels passed on the darker trait, and the squirrels fur became black over many generations. This answer suggests the student may understand that variations are passed down and can increase in frequency over time, but does not understand that individual squirrels cannot adapt their phenotype to their environment, and that natural selection caused changes in the allele frequencies over time because squirrels in coniferous forests with the darker fur had a selective advantage and were able to survive and reproduce better than squirrels with lighter fur. (B) The coniferous forests contain small, isolated populations of squirrels, and the allele for black fur became prevalent due to a combination of limited genetic diversity and genetic drift. Page 2 of 9
This answer suggests the student may understand that a genetic bottleneck and genetic drift can produce changes in allele frequencies in small populations, but does not understand that there is no evidence to support the notion of limited genetic diversity or genetic drift, and that the evidence suggests that one trait (black fur) is favored over the other trait (grey fur), indicating natural selection as the cause in changes in allele frequencies over time. (C) Squirrels with black fur in coniferous forests have a selective advantage over squirrels with gray fur, and these squirrels were able to pass the trait for black fur on to more offspring than the black squirrels in the deciduous forests. This answer suggests the student understands that natural selection favors squirrels with black fur in coniferous forests, and because this trait has a selective advantage, more individuals with the trait survived and passed the trait on to offspring in coniferous forests than in deciduous forests. (D) The number of mutations creating the black fur allele is higher in coniferous forests than in deciduous forests. This has created a higher allelic frequency for the black fur allele, allowing for greater expression of the black phenotype. This answer suggests the student may understand that genetic mutations are the source for all variations and adaptations, but does not understand that natural selection causes changes in allele frequencies over time by favoring certain mutations over others. Page 3 of 9
3. Green crabs are native predators of the blue mussels that live along the coast of Maine. The blue mussels have acquired an adaptation that allows them to detect the unique waterborne chemicals produced by green crabs and produce thicker shells that are more difficult for the green crabs to break. In 1990, the Asian shore crab was introduced to the southern coast of Maine. As of today, Asian shore crabs have not migrated to the waters off northern Maine. Asian shore crabs are more aggressive than green crabs, and were able to break the thicker shells the mussels developed in response to the green crabs. Scientists were amazed to find that in just 15 years after the introduction of Asian shore crabs in the south, most of the blue mussels along the southern coast of Maine have developed even thicker shells that are difficult for the Asian shore crab to break. Scientists hypothesize that the southern mussels acquired the ability to detect and respond to the waterborne chemicals produced by the Asian shore crab. To test this hypothesis, scientists conducted an experiment to determine the effect of the waterborne chemicals produced by Asian shore crabs on southern and northern blue mussels. Southern and northern blue mussels were exposed to the unique waterborne chemicals produced by green crabs and Asian shore crabs for three months. The complete results of the green crab experiment and partial results of the Asian shore crab experiment are shown below. Which statement best predicts how the northern mussels will respond when they are exposed to the unique waterborne chemicals released by Asian shore crabs? (A) There will be no change in the shell thickness of northern mussels because the majority of individuals do not have an adaptation to detect and respond to the unique chemicals released by Asian shore crabs. This answer suggests the student understands that northern mussels are unable to immediately detect and respond to the unique chemicals released by the Asian shore crab, and that this adaptation due to natural selection will take more than three months to be observed in the northern population. Page 4 of 9
(B) The majority of northern mussels will have a slight increase in shell thickness, but they will not produce shells as thick as those of the southern mussels because they have had less time to learn how to respond to the new predator. This answer suggests the student may understand that populations respond to environmental changes, but does not understand that evolution is a population-level process rather than an individual (Lamarckian) process. (C) The majority of northern mussels will have a rapid increase in thickness because these individuals have the same genetic information as southern mussels and they will be able to detect the unique chemicals released by Asian shore crabs. This answer suggests the student may understand that the northern and southern mussels share genetic information, but does not understand that it took 15 years for the southern mussel population to be able to detect and respond to the unique chemicals released by the Asian shore crab and that the northern mussels would not be able to detect and respond in just three months. (D) There will be little to no change in the shell thickness of northern mussels because the mussels already have thicker shells due to the presence of the unique chemicals released by green crabs. This answer suggests the student may understand that northern mussels develop thicker shells in response to the green crab, but does not understand that the northern mussels have not yet been exposed to the Asian shore crab and many will not have a mutation to detect the unique chemicals released by this crab. Page 5 of 9
4. Lactase is an enzyme needed to digest lactose, the sugar found in milk. Almost all humans carry the gene to produce lactase. Until about 20,000 years ago, the lactase gene was turned off by regulatory genes after infancy. Today, many people have the dominant allele (R) of a regulatory gene, which keeps the lactase gene turned on in adulthood. The table below shows the allelic frequencies for the regulatory gene in four countries. In Africa, the R allele is not present in the Tutsi population, but 90% of the individuals have three other mutations that keep the lactase gene functional in adulthood. Which statement best supports the data shown? (A) Over the past 20,000 years, the German and Tutsi populations had a selective advantage favoring lactase expression in adults and Chinese populations had a selective advantage favoring alleles that suppress lactase expression in adults. This answer suggests the student understands that certain mutations will be favored by natural selection, but does not understand that the lack of a mutation does not mean the normal allele provides a selective advantage. (B) Over the past 20,000 years, there has been a strong selective pressure to be able to digest lactose within the German and Tutsi populations, but this selective pressure was not as strong in Spain and France, and is relatively nonexistent in China. Page 6 of 9
This answer suggests the student understands that individuals in the German and Tutsi populations who were able to digest lactose in adulthood had a significant selective advantage over those who could not, and passed the dominant allele on to their offspring, that the selective advantage was not as strong in France and Spain, and that in China, there was no selective advantage for individuals who were able to digest lactose in adulthood and so the allele frequency did not change in the Chinese population. (C) Over the past 20,000 years, individuals in the Chinese population did not drink enough milk to cause the mutations to the regulatory gene that allows for the production of lactase into adulthood. This answer suggests the student may understand that populations under the same selective pressures will favor similar adaptations, but does not understand that these similar phenotypic changes may be due to differing genetic mutations and that exposure to certain foods will not cause mutations to develop. (D) Over the past 20,000 years, the German, French, Spanish, and Tutsi populations developed lactose tolerance due to the occurrence of simultaneous random mutations in most of the individuals in the population. This answer suggests the student may understand that genetic variation and mutation are required for evolutionary change, but does not understand that natural selection favors individuals with a trait and those individuals are more likely to survive and pass the trait on to their offspring, or that many individuals within a population are not likely to spontaneously and simultaneously develop the same mutation. Page 7 of 9
5. Fire ants (Solenopsis invicta) were first introduced into the southern United States in the 1930s and spread quickly throughout the southern states. At that time, nearly all fire ants were found in colonies with a single queen, called monogyne colonies. In the 1970s, scientists discovered that some colonies, called polygyne colonies, had multiple queens. Monogyne colonies produce more offspring in areas with few colonies, but have a reduced chance of survival in areas with many competing colonies. Polygyne colonies, on the other hand, have an increased rate of survival in areas with competing colonies. The ability to form polygyne colonies is due to a mutation in a single gene: Gp-9. There are two versions of Gp-9, B and b, which produce different responses in ants. The chart below shows how genotype affects behavior. Which statement best predicts how the allele frequencies (B and b) of the Gp-9 gene have changed among the growing fire ant population in the United States since the 1930s? (A) Since queens with genotype bb are weak and usually die, the B allele will increase in frequency compared to the weaker b allele due to natural selection. This answer suggests the student may understand that lethal alleles will be selected against over time, but does not understand that the heterozygous condition (Bb) has a selective advantage in areas with multiple colonies and will result in a higher frequency of the potentially lethal (b) allele in those areas. (B) Since workers with genotype BB kill all invading queens, the B allele will increase in frequency compared to the b allele. This answer suggests the student may understand that the homozygous dominant (BB) genotype produces monogyne colonies, but does not understand that polygyne colonies have a selective advantage in competitive areas, making the heterozygous (Bb) genotype more advantageous and leading to an increase in the b allele as the population grows. Page 8 of 9
(C) Because polygyne colonies have a greater survival rate in areas with multiple colonies than monogyne colonies do, the Bb genotype is favored and the b allele will increase in frequency. This answer suggests the student understands that the change in population density creates a selective advantage for polygyne colonies and the heterozygous (Bb) genotype because these colonies can outcompete monogyne colonies for limited resources, and that this will cause an increase in the frequency of the b allele. (D) Because monogyne colonies produce more offspring than polygyne colonies do, the B allele will increase in frequency as more BB queens are produced and establish more colonies. This answer suggests the student may understand that populations that produce more offspring are usually favored, but does not understand that the heterozygous (Bb) genotype has a selective advantage because it increases the ants ability to outcompete those that are homozygous dominant (BB), and the b allele frequency will increase as a result. Page 9 of 9