(A) Gene frequencies in the sunfish population remained stable and the population size

Transcription

1 1. Ecologists are studying how genetic and environmental factors can affect the masses of adult sunfish. The graph below and to the left shows the original size distribution for a population of sunfish in a lake. Northern pike fish, predators of sunfish, are introduced into the lake. The graph above and to the right shows the size distribution for the population of sunfish in the same lake many years after the introduction of northern pike. Which statement about the effect that the introduction of the northern pike had on the sunfish population is best supported by the data in the graphs? (A) Gene frequencies in the sunfish population remained stable and the population size decreased because the pike ate sunfish of all sizes, as shown by the even distribution of sizes and the decreased number of individuals in the second graph. This answer suggests the student may understand that the sunfish population decreased, but does not understand that the trend in the graphs suggests that gene frequencies did not remain stable because the graphs show that the size distribution shifted to include fewer small sunfish after the introduction of the northern pike. The graphs show a shift in the size distribution of the sunfish population to larger sunfish, which is due to a change in gene frequencies, resulting from northern pike preying on smaller sunfish and reducing the sunfish population size. (B) Gene frequencies changed in the sunfish population, but the population size was unchanged because the pike ate smaller sunfish and the larger sunfish survived to produce more offspring, as shown by the constant number of individuals and the shift to larger masses in the second graph. Page 1 of 9

2 This answer suggests the student may understand that sunfish gene frequencies changed over time, but does not understand that the graphs show that the overall population size of the sunfish decreased because there were fewer sunfish in each size group after the introduction of the northern pike. The graphs show a shift in the size distribution of the sunfish population to larger sunfish, which is due to a change in gene frequencies, resulting from northern pike preying on smaller sunfish and reducing the sunfish population size. (C) Population size decreased and gene frequencies changed in the sunfish population to favor genes that produce larger sunfish because the pike were eating more of the smaller sunfish in the population, as shown by the decreased number of individuals overall and the shift to larger masses in the second graph. Rationale: This answer suggests the student understands that the trend in the graphs suggests that there was a decrease in smaller fish in the sunfish population and that, over time, sunfish gene frequencies shifted to favor large-size alleles. The student also understands that an increase in the mean mass of the sunfish is evidence that would support the claim that there was a shift in the size distribution of the sunfish population to larger sunfish due to a change in gene frequencies. (D) Population size decreased and gene frequencies changed to favor medium-sized sunfish because the pike were eating only small and large sunfish, preventing the large sunfish from producing more offspring, as shown by the increase in the number of medium-sized fish in the second graph. This answer suggests the student may understand that the sunfish population decreased and that sunfish gene frequencies changed over time, but does not understand that the trend in the graphs suggests that the shift in the sunfish gene frequencies favors large fish (not medium-sized), because the graphs show that the size distribution shifted to include fewer small sunfish after the introduction of the northern pike. The student may not understand that the original size distribution showed that there had been a large number of medium-sized sunfish in the population, so a shift in the size distribution of the sunfish population to larger sunfish would still include a large number of medium-sized sunfish, but fewer small sunfish. Aligned to: LO 1.5 CA 1.5: Impact of Environment on Evolution Page 2 of 9

3 2. Ecologists are studying the effect of soil quality on the growth of alder trees and have identified a gene (R) in which trees with the recessive phenotype (rr) are better able to tolerate acidic soil. The table below shows the numbers of trees and the phenotypes for the R gene (trees tolerant of acidic soil and trees not tolerant of acidic soil) in a population of alder trees in a Canadian woodland. Which conclusion best explains what is happening to the alder tree population over time? (A) The data show that the alder tree population is decreasing and that evolution is not occurring, because the gene frequencies of the two alleles (R and r) remain unchanged. This answer suggests the student may understand that the alder tree population is decreasing, but does not understand that the gene frequencies did not remain the same because the population with the dominant phenotype (not tolerant of acidic soil) decreased and the number of trees with the recessive phenotype (tolerant of acidic soil) remains unchanged. (B) The data show that the alder tree population is decreasing and that evolution is occurring, because evolution always occurs when population sizes become reduced. This answer suggests the student may understand that the population is decreasing and that evolutionary changes occur more rapidly in small populations, but does not understand that the data show that there was an increase in the percentage of alder trees tolerant of acidic soil (recessive phenotype), which corresponds with a change in the gene frequencies of the two alleles (R and r), and that it is the change in gene frequencies in populations that results in evolutionary change. (C) The data show that the alder tree population is going through a genetic bottleneck (genetic drift), because the population is small and the gene frequencies are rapidly changing due to random events. Page 3 of 9

4 This answer suggests the student may understand that the decreasing population could result in changes in gene frequencies through a genetic bottleneck, but does not understand that there is no evidence to support a genetic bottleneck (genetic drift) and that changes in gene frequencies produce evolutionary change. The data show that there was an increase in the percentage of alder trees tolerant of acidic soil (recessive phenotype), which corresponds with a change in the gene frequencies of the two alleles (R and r), and that it is the change in gene frequencies in populations that results in evolutionary change. (D) The data show that the alder tree population is evolving, because the phenotypes are changing due to changes in the gene frequencies of the two alleles (R and r). Rationale: This answer suggests the student understands that the number of trees for each phenotype in the population in 2010, relative to the population numbers in 1800, suggests that gene frequencies are changing and that evolution is occurring, because a higher percentage of the total population is now tolerant of acidic soil, and that this change in phenotypic variation is a result of a change in the genetic makeup of the alder tree population over time. Aligned to: LO 1.5 CA 1.5: Impact of Environment on Evolution Page 4 of 9

5 3. Anolis is a small lizard that lives throughout the southeastern United States. The color of an Anolis varies from light green to dark brown and is governed by four sets of alleles. Due to a fire, an area where a population of Anolis live that was once covered with light-colored vegetation is now mostly covered with dark soil. Which graph most accurately shows the likely change in the pigmentation distribution in the Anolis population after the fire? (A) This answer suggests the student may understand that the environmental change will cause a shift in phenotypic distribution, but does not understand that this will result in a distribution resembling one bell curve shifting toward darker pigmentation because individuals with more pigmentation will have a better chance of surviving in the new environment. The student may mistakenly assume that two discrete phenotypes will result, but a distribution resembling two bell curves would only result if selective pressures favored extremes and not intermediate phenotypes. (B) Rationale: This answer suggests the student understands that the environmental change will favor darker phenotypes because individuals with greater pigmentation will have the greatest chance of survival and reproduction and that a pattern resembling a bell curve will result because a continuous distribution of phenotypes is likely, due to the numerous combinations of alleles that can occur to create many intermediate colors. Page 5 of 9

6 (C) This answer suggests the student may understand that the environmental change will cause a shift in phenotypic distribution, but does not understand that this will result in a pattern resembling a single bell curve shifting toward darker pigmentation because individuals with more pigmentation will survive, resulting in a continuous distribution of darker phenotypes due to the numerous combinations of alleles that can occur to create many intermediate colors. The student may not understand that phenotypes would most likely be in a continuum, instead of discrete. (D) This answer suggests the student may understand that the environmental change will cause a shift in phenotypic distribution, but does not understand that this will result in a distribution resembling a single bell curve shifting toward darker pigmentation because individuals with more pigmentation will survive, resulting in a continuous distribution of darker phenotypes due to the numerous combinations of alleles that can occur to create many intermediate colors. The student may mistakenly assume that discrete phenotypes will be present, because he or she may not understand that phenotypes would most likely be in a continuum due to the numerous combinations of alleles that can occur to create many intermediate colors. Aligned to: LO 1.5 CA 1.5: Impact of Environment on Evolution Page 6 of 9

7 4. Increases in atmospheric carbon dioxide levels may be lowering the ph of oceans in a process known as ocean acidification. Researchers set up an experiment to test whether a population of algae that are normally harmed by a low ph could adapt to the acidification. Algae were exposed, for a period of one year (about 500 generations), to different degrees of ocean acidification, as shown in the table below. To test for adaptation, the researchers then transferred the algae from phase 1 of the experiment into environments with different CO 2 levels, to examine how the algae from phase 1 performed when exposed to varying CO 2 levels. In phase 2, algae from each of the groups in phase 1 were grown in current atmospheric conditions, and algae from groups B and C were grown in the same conditions as in phase 1, alongside algae grown in current-atmosphere conditions during phase 1 (group A). The graphs below summarize the results for phase 2 of the experiment. Page 7 of 9

8 Based on the experiment, which conclusion best explains how algae may respond to increasing levels of atmospheric carbon dioxide? (A) The data show that algae have lower growth rates in acidified oceans, but that algae that are exposed to increased levels of CO 2 will be favored through natural selection and be better adapted to lower ph than present-day algae. Rationale: This answer suggests the student understands that the algae show some evolutionary change after a year of exposure to higher CO 2 levels, because the trends shown in the graphs indicate that algae grown in extreme acidification displayed a faster growth rate in the extreme conditions than the algae grown in normal CO 2 levels. (B) The data show that algae that are exposed to high levels of CO 2 are least fit and, therefore, that these algae will be unable to adapt to extreme increases in CO 2 but may be able to handle less extreme changes in CO 2 levels. This answer suggests the student may understand that the algae grown in the higher CO 2 levels were least fit under normal CO 2 levels, but does not understand that these algae had actually adapted through artificial selection to grow in those conditions, because the algae grown in the higher CO 2 levels thrived, in comparison to the algae grown in the current CO 2 levels, when exposed to high levels of CO 2. (C) The data show that algae are successful in adapting to changes in ocean ph if the changes are slow and very gradual, as evolutionary changes require long periods of time. This answer suggests the student may understand that large evolutionary change requires a long period of time, but does not understand that the data suggest that significant changes can occur within populations relatively quickly, because the algae exposed to higher CO 2 levels showed a significant change after only one year. (D) The data show that algae will be unable to respond to changes in ocean ph, because the algae that were exposed to high CO 2 levels for a year show a significant decrease in growth rate. Page 8 of 9

9 This answer suggests the student may understand that all of the treated algae show decreased growth rates in the high-co 2 environments, but does not understand that this is not evidence that the algae will not be able to survive in the changed environment, because algae exposed to higher levels of CO 2 did adapt. Aligned to: LO 1.5 CA 1.5: Impact of Environment on Evolution Page 9 of 9