Continuing Evolution - AP Biology

1. The use of herbicides to control weeds in agricultural fields often leads to the evolution of herbicide-resistant weeds. Which of the following is a necessary condition for the continued evolution of this resistance?
   Answer: There must be pre-existing heritable variation for resistance within the weed population. Natural selection cannot occur without heritable variation. For a weed population to evolve resistance, some individuals must already possess genes that confer some level of resistance. The herbicide then acts as a selective agent, favoring these individuals. Choice A is incorrect as mutations are random, not directed by the herbicide. Choice C would result in the entire population being wiped out, preventing any evolution. While horizontal gene transfer (Choice D) can happen, it is not a necessary condition for the evolution of resistance via standard natural selection.
2. This development of pesticide resistance in Colorado potato beetles is an example of continuing evolution because it demonstrates which of the following?
   Answer: The pesticides act as a selective agent, causing a shift in the allele frequencies of beetle populations toward genotypes that confer resistance. The continued application of pesticides creates a strong selective pressure on the beetle population. Beetles with pre-existing genetic variations that confer resistance are more likely to survive and reproduce, passing these traits to their offspring. This leads to a change in the genetic makeup of the population over generations, which is the definition of evolution. Choice A is incorrect; the populations are thriving due to resistance, not declining. Choice B is Lamarckian. Choice D is incorrect as the development of resistance is clear evidence that the population is not in equilibrium and is actively evolving.
3. Which of the following processes ensures that evolution continues to occur even in an organism with a highly streamlined genome like M. genitalium?
   Answer: The continuous introduction of random mutations during DNA replication and the presence of selective pressures from its environment. No matter how simple an organism is, the fundamental processes of evolution still apply. DNA replication is never perfectly accurate, so mutations will continue to arise randomly, creating new genetic variation. The organism's environment (e.g., the host's immune system, antibiotic presence) will continue to exert selective pressures. This combination of variation and selection ensures that evolution is an ongoing process. Choice A is not a known mechanism. Choice B states the opposite of what would drive evolution. Choice D is incorrect as there is no final 'optimal state' in evolution.
4. A weed population in soybean fields is exposed to the same herbicide each spring. Seeds are collected and genotyped yearly at a locus with alleles H (herbicide resistance) and h. Over 9 years, allele H increases from 0.08 to 0.61, while planting density and field size stay similar. Which observation best demonstrates evolution occurring in this weed population?
   Answer: Allele H rose from 0.08 to 0.61 in the population over multiple years of sampling. Continuing evolution involves populations developing genetic resistance to pressures like herbicides, as seen in these weeds. Choice C correctly shows evolution with allele H for resistance increasing from 0.08 to 0.61 over nine years, indicating weeds carrying H survived applications better and seeded more, changing the population genetically. Stable planting density and field size suggest consistent selection pressure drove this trend. The yearly sampling reveals a clear heritable shift toward resistance. Choice A is tempting, describing temporary growth slowdowns from herbicide, but this misconception mixes short-term physiological recovery with evolutionary genetic changes. To identify evolution, prioritize data on allele frequency trends over time, distinguishing them from individual responses.
5. This change in the squirrel population is best described as an instance of ongoing evolution by natural selection because...
   Answer: it represents a quantifiable shift in the allele frequencies of a population over multiple generations that is correlated with an environmental change. Evolution is defined as a change in allele frequencies in a population over time. The observation shows exactly this: a measurable change in the phenotype frequency (which reflects a change in allele frequency) over a 30-year period. The correlation with the changing environment (darker forest) strongly suggests that this is an adaptive change driven by natural selection (better camouflage for black squirrels). Choice B is Lamarckian. Choice C dismisses the strong correlation with the environment, making natural selection a more parsimonious explanation than drift. Choice D describes the opposite of what was observed, as the frequencies are actively changing, not stable.
6. A population of rabbits includes two alleles at a coat-color locus: W (white) and B (brown). In 2017, snow cover duration decreased due to warmer winters. Researchers sampled the same population each spring: 2016 $f(B)=0.12$, 2018 $f(B)=0.20$, 2020 $f(B)=0.29$, 2022 $f(B)=0.33$. Predators more easily detect white coats on snow-free ground. Which observation best demonstrates continuing evolution in this rabbit population?
   Answer: The frequency of allele B increases in the population over successive years. This question assesses the skill of understanding continuing evolution, which involves ongoing changes in allele frequencies within a population over generations due to selective pressures. The correct answer, choice B, demonstrates evolution because it shows the frequency of the B allele increasing from 0.12 in 2016 to 0.20 in 2018, 0.29 in 2020, and 0.33 in 2022, indicating a population trend toward brown coats as snow cover decreases. This change reflects natural selection favoring brown rabbits that are less detectable by predators on snow-free ground, enhancing survival and reproduction. Spring sampling of the same population tracks the genetic response to warmer winters. A tempting distractor is choice A, which describes individual rabbits shedding fur earlier in warm years, but this is wrong because it represents seasonal phenotypic plasticity, not genetic evolution across generations. To identify evidence of continuing evolution in similar questions, focus on data showing shifts in allele or genotype frequencies over multiple generations rather than short-term individual responses.
7. A bacterial population is grown in a flask with antibiotic X. At the start, 1% of cells carry a resistance allele (R) on the chromosome. After 24 hours (about 12 generations) in the same antibiotic concentration, 38% of cells carry R; after 48 hours, 72% carry R. The population size remains large, and the culture is well mixed. Which observation best demonstrates evolution occurring in this bacterial population in real time?
   Answer: The proportion of cells with the R allele increases over successive generations in the flask. This question assesses the skill of understanding continuing evolution, which involves ongoing changes in allele frequencies within a population over generations due to selective pressures. The correct answer, choice A, demonstrates evolution because it shows the proportion of cells with the R allele increasing from 1% at the start to 38% after 24 hours and 72% after 48 hours, indicating a population trend toward higher resistance under constant antibiotic exposure. This rapid shift across generations in a large, well-mixed culture reflects natural selection favoring resistant cells that survive and reproduce more effectively. The consistent antibiotic concentration and large population size support that this is a genetic change driven by selection rather than other factors. A tempting distractor is choice B, which describes individual cells reducing antibiotic uptake temporarily, but this is wrong because it represents a physiological response or acclimation in single cells, not a heritable shift in the population's genetic makeup. To identify evidence of continuing evolution in similar questions, focus on data showing shifts in allele or genotype frequencies over multiple generations rather than short-term individual responses.
8. A population of frogs breeds in ponds near a highway. Researchers measure allele frequencies at a locus with alleles N (noise-tolerant call) and n. Over 12 years, the frequency of allele N increases from 0.15 to 0.44, with similar numbers of breeding adults each year and no translocations. Which observation best demonstrates evolution in this frog population?
   Answer: Allele N increased in frequency from 0.15 to 0.44 across many breeding seasons. Continuing evolution encompasses genetic adaptations to ongoing environmental challenges, like noise in this frog population. Choice B is correct because allele N for noise-tolerant calls increased from 0.15 to 0.44 over 12 years, suggesting frogs with this allele communicated better amid traffic, leading to more successful breeding. Stable breeding adult numbers and no translocations indicate selection drove this genetic shift. The long-term trend shows heritable changes propagating in the population. Choice A is a tempting distractor, describing variable call volumes based on traffic, but this misconception equates behavioral flexibility with evolutionary genetic change. To detect evolution, always check for allele frequency alterations across multiple generations, not situational adjustments.
9. Which of the following statements provides the strongest support for the claim that all species have evolved and continue to evolve?
   Answer: Genomic analyses show that mutation rates are non-zero in all known organisms, providing a constant source of new variation. Evolution requires genetic variation, and mutation is the ultimate source of that variation. The fact that mutations occur continuously in all life forms means that the raw material for evolutionary change is always being generated, ensuring that evolution is an ongoing process. Choice A points to common ancestry, not continuing evolution. Choice B describes evolutionary stasis in some lineages, but doesn't negate the fact that evolution is ongoing in others and that even these lineages are still subject to mutation and selection. Choice D is incorrect; mitosis is for somatic cells, and errors can still occur, while meiosis and mutation provide variation for sexually reproducing organisms.
10. This relationship is an example of a coevolutionary arms race, which demonstrates that evolution is an ongoing process because...
    Answer: the selective pressures that each species exerts on the other are reciprocal and constantly changing, driving continuous adaptation. In a coevolutionary arms race, each species acts as a major selective force on the other. As snakes evolve more resistance, it selects for more toxic newts, which in turn selects for even more resistant snakes. This reciprocal selection drives continuous, ongoing evolution in both populations. Choice A is incorrect; there is no 'perfect' state in evolution. Choice C describes an incorrect, non-genetic mechanism. Choice D is a possible outcome but does not explain why the process itself demonstrates continuing evolution.
11. On an island, a lizard population includes two color morphs controlled largely by a single gene: dark (allele d) and light (allele l). After a wildfire in 2012, the ground surface became darker due to ash and char. Researchers estimated allele frequencies from random samples: in 2011, $f(d)=0.22$; in 2013, $f(d)=0.35$; in 2016, $f(d)=0.48$. Sampling methods and population boundaries were consistent across years. Which observation best demonstrates continuing evolution in this population?
    Answer: The frequency of allele d increases in the island population across multiple years. This question assesses the skill of understanding continuing evolution, which involves ongoing changes in allele frequencies within a population over generations due to selective pressures. The correct answer, choice B, demonstrates evolution because it shows the frequency of the d allele increasing from 0.22 in 2011 to 0.35 in 2013 and 0.48 in 2016, indicating a population trend toward darker coloration after the wildfire darkened the habitat. This change reflects natural selection favoring dark lizards that are better camouflaged on the ash-covered ground, leading to higher survival and reproduction rates. Consistent sampling methods and population boundaries ensure the trend is a genuine genetic shift in response to the environmental change. A tempting distractor is choice A, which describes individual lizards darkening after basking, but this is wrong because it represents phenotypic plasticity or temporary physiological adjustment, not a heritable genetic evolution across the population. To identify evidence of continuing evolution in similar questions, focus on data showing shifts in allele or genotype frequencies over multiple generations rather than short-term individual responses.
12. On an island, a population of lizards shows two color morphs controlled by alleles G (green) and B (brown). After a wildfire darkened the ground, researchers recorded allele frequencies each breeding season. The frequency of allele B rose from 0.30 in the first season after the fire to 0.67 five seasons later, with similar census sizes each year. Which observation best demonstrates continuing evolution in this lizard population?
    Answer: Allele B increased from 0.30 to 0.67 across multiple breeding seasons in the island population. Continuing evolution captures how populations adapt genetically to environmental changes, such as the lizards on this island post-wildfire. Choice C is correct because allele B, linked to brown coloration that likely provides better camouflage on darkened ground, increased from 0.30 to 0.67 over five breeding seasons, showing selective advantage and higher reproduction for brown lizards. Stable census sizes suggest this shift resulted from differential survival and breeding success in the altered habitat. The multi-season trend underscores a genetic response to predation pressure after the fire. Choice A tempts by describing seasonal color changes due to sun exposure, a misconception of phenotypic plasticity mistaken for evolution, as it does not involve heritable genetic alterations. For similar questions, focus on tracking allele frequency changes across generations to confirm evolutionary processes.
13. A population of yeast is grown in identical flasks, with one sugar source as the only carbon supply. At a gene influencing sugar transport, allele S is initially at frequency 0.50. After 300 generations, allele S is at 0.93 in the same continuously reproducing population. Which observation best demonstrates evolution occurring in this yeast population?
    Answer: Allele S increased in frequency from 0.50 to 0.93 over 300 generations in the population. Continuing evolution illustrates genetic changes in lab populations under controlled conditions, as with this yeast in sugar medium. Choice A correctly demonstrates evolution through allele S's rise from 0.50 to 0.93 over 300 generations, indicating better sugar transport conferred advantages, increasing reproduction and shifting population genetics. Identical flasks and continuous reproduction highlight selection's role in this trend. The generational scale confirms a heritable adaptation. Choice B tempts by noting increased transporter activity upon exposure, a misconception of induced physiological changes mistaken for evolution without genetic frequency shifts. A useful strategy is to analyze long-term allele data to verify evolution, separating it from immediate cellular responses.
14. Which of the following best explains, from an evolutionary perspective, why the second antibiotic treatment was ineffective?
    Answer: The initial antibiotic treatment acted as a selective pressure, allowing a few naturally resistant bacteria to survive and reproduce, leading to a population that is now largely resistant. This is a classic example of natural selection. The antibiotic kills susceptible bacteria, but individuals with pre-existing resistance survive and reproduce. This differential survival and reproduction lead to an increase in the frequency of the resistance allele in the population over time. Choice B describes an incorrect, Lamarckian view of acquired characteristics. Choice C offers a physiological explanation for the patient's condition but not an evolutionary explanation for the bacteria's resistance. Choice D is incorrect because mutations are random events, not directed responses to environmental pressures like antibiotics.
15. This observation of a change in beak depth is evidence of continuing evolution because it demonstrates that...
    Answer: a change in environmental conditions led to the differential survival and reproduction of individuals with certain heritable traits. This scenario describes directional selection. The drought changed the available food source, which acted as a selective pressure. Birds with genetically determined larger, deeper beaks were better able to crack the hard seeds, leading to higher survival and reproductive rates. This caused the average beak depth of the population to increase over generations. Choice A is a Lamarckian explanation. Choice B incorrectly implies that evolution has a goal or direction of 'progress'. Choice C is less likely because the change is clearly correlated with an environmental pressure, suggesting selection, not random drift.
16. A population of field mice includes two alleles at a coat-color gene: L (light) and D (dark). After a new predator arrives, biologists sample the population every 2 years. Over 10 years, allele D increases from 0.22 to 0.55, while the habitat and migration rates remain similar. Which observation best demonstrates evolution occurring in this mouse population?
    Answer: Allele D rose in frequency from 0.22 to 0.55 across multiple sampling intervals. Continuing evolution describes genetic adaptations in populations facing new selective pressures, like these field mice with a new predator. The correct answer, choice C, illustrates this through allele D's increase from 0.22 to 0.55 over 10 years, implying darker coats offered camouflage advantages, leading to higher survival and reproduction rates. Consistent habitat and migration rates indicate natural selection drove this population-level genetic shift. The decadal trend reflects ongoing heritable changes in response to predation. Choice A is a tempting distractor, portraying behavioral hiding as evolution, but this misconception confuses learned or plastic behaviors with genetic evolution requiring allele changes. To spot evolution, always verify shifts in genetic frequencies over generations, not just behavioral modifications.
17. How does this repeated pattern of pelvic spine reduction in freshwater sticklebacks illustrate continuing evolution?
    Answer: It demonstrates that evolution can be predictable when similar selective pressures act on different populations with similar genetic potential. The repeated, independent evolution of the same trait (pelvic reduction) in response to a similar environmental shift (from marine to freshwater) is a powerful example of ongoing, adaptive evolution. It shows that evolution is not entirely random in its outcome; similar selection pressures can drive populations down similar evolutionary paths. Choice A is incorrect; traits can be re-evolved. Choice C incorrectly implies directed mutation. Choice D uses the term 'de-evolving,' which is a misconception; evolution is not a ladder of progress, and loss of a structure can be adaptive.
18. A population of field mice includes two hemoglobin alleles, H1 and H2. In 2000, $f(H2)=0.10$ in a lowland population. A drought from 2001–2004 reduced available water, and the same population was sampled each year: 2002 $f(H2)=0.16$, 2004 $f(H2)=0.23$, 2006 $f(H2)=0.24$. H2 is associated with improved oxygen delivery during dehydration stress. Which observation best demonstrates evolution occurring in this mouse population?
    Answer: The population's allele frequency for H2 increases across multiple sampling years. This question assesses the skill of understanding continuing evolution, which involves ongoing changes in allele frequencies within a population over generations due to selective pressures. The correct answer, choice B, demonstrates evolution because it shows the frequency of the H2 allele increasing from 0.10 in 2000 to 0.16 in 2002, 0.23 in 2004, and 0.24 in 2006, indicating a population trend toward better oxygen delivery during drought stress. This shift reflects natural selection favoring mice with the H2 allele that cope better with dehydration, resulting in higher survival and reproductive success. The consistent sampling during and after the drought period highlights how environmental pressure drives genetic change in the population. A tempting distractor is choice A, which describes individual mice drinking less during droughts, but this is wrong because it represents behavioral adaptation or physiological response within a lifetime, not heritable genetic evolution. To identify evidence of continuing evolution in similar questions, focus on data showing shifts in allele or genotype frequencies over multiple generations rather than short-term individual responses.
19. Which of the following best explains how a tumor can become resistant to chemotherapy over time, demonstrating continuing evolution on a cellular level?
    Answer: The chemotherapy drug acts as a selective pressure, favoring the survival and proliferation of pre-existing cancer cells with resistance-conferring mutations. This is an example of natural selection at the cellular level. Due to genetic instability, a tumor contains a diverse population of cells. Some of these cells may randomly have mutations that make them resistant to a chemotherapy agent. The drug then kills susceptible cells, leaving the resistant ones to multiply, eventually leading to a fully resistant tumor. Choice A is incorrect because adaptation is not a uniform, simultaneous process. Choice C is not a known biological mechanism. Choice D describes an acquired ability in a Lamarckian sense, which is incorrect.
20. In a coastal marsh, a population of killifish lives near a factory outflow containing PCBs. In 2005, researchers sampled 200 fish and found the allele frequency of a detox-enzyme variant (D) was 0.18. The same site was sampled again in 2010 (200 fish), and the D allele frequency was 0.31; in 2015 (200 fish), it was 0.44. Fish were randomly sampled each year from the same breeding area, and the D allele is associated with higher survival in PCB-contaminated water. Which observation best demonstrates evolution occurring in this killifish population over time?
    Answer: The frequency of the D allele increases across years in the same population samples. This question assesses the skill of understanding continuing evolution, which involves ongoing changes in allele frequencies within a population over generations due to selective pressures. The correct answer, choice B, demonstrates evolution because it shows the frequency of the D allele increasing from 0.18 in 2005 to 0.31 in 2010 and 0.44 in 2015, indicating a population trend toward higher prevalence of the detox-enzyme variant associated with PCB survival. This shift in allele frequency across years reflects natural selection favoring individuals with the D allele in the contaminated environment, leading to more of them surviving and reproducing. Consistent sampling from the same breeding area ensures this trend represents a genetic change in the population rather than sampling artifacts. A tempting distractor is choice A, which describes individual fish expressing higher enzyme activity after exposure, but this is wrong because it represents phenotypic plasticity or acclimation within an individual's lifetime, not heritable genetic change across generations. To identify evidence of continuing evolution in similar questions, focus on data showing shifts in allele or genotype frequencies over multiple generations rather than short-term individual responses.

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