Evolutionary Mechanisms and Natural Selection (1C) - Bio & Biochem

1. A population of field mice shows variation in coat color due to a single gene with two alleles: dark (D) and light (d). After a wildfire, the habitat becomes covered in dark ash for several years. Predation by hawks increases during this period. Over 10 generations, the frequency of allele D rises from 0.40 to 0.72. Which statement best reflects the process of natural selection described?
   Answer: Allele D increased because dark-coated mice had higher survival and reproductive success in the ash-covered environment. This question tests understanding of natural selection in populations. Natural selection is the process where heritable traits that enhance survival and reproduction become more common in successive generations. In this scenario, the ash-covered habitat after the wildfire provides camouflage for dark-coated mice, reducing predation by hawks. Allele D increases because dark-coated mice have higher survival and reproductive success, passing on more copies of D to offspring, as stated in choice B. Choice A fails because it suggests acquired traits are inherited, which is Lamarckian and not supported by evidence. To verify natural selection, check if allele frequency changes correlate with fitness differences in the environment. This principle applies to any trait under selective pressure, distinguishing it from random processes like drift.
2. Two populations of the same frog species live on opposite sides of a newly formed lava field that prevents movement between them. In the western population, mating calls are low-frequency; in the eastern population, calls are high-frequency. Females preferentially mate with males whose call matches the local population's typical frequency. After many generations, individuals from the two sides brought into the same pond rarely mate with each other despite being physically capable of producing viable offspring. Which event would most likely lead to speciation in this scenario?
   Answer: The evolution of behavioral isolation via divergence in mating calls that reduces gene flow between the two populations. This question tests understanding of speciation through the evolution of reproductive isolation. Speciation occurs when populations accumulate differences that prevent interbreeding, with behavioral isolation being one important mechanism. The populations already show divergence in mating calls and female preferences, creating prezygotic isolation where individuals from different populations rarely mate despite being physically capable of producing viable offspring. This behavioral isolation reduces gene flow and allows further divergence, potentially leading to complete reproductive isolation. Answer B would increase gene flow (opposing speciation), C describes temporary drift without lasting effect, and D describes parallel evolution that would maintain similarity. To identify speciation mechanisms, look for: (1) reduced gene flow between populations, (2) evolution of mating barriers, and (3) divergence in traits affecting reproduction.
3. A cave population of insects becomes isolated from a surface population after a landslide blocks the cave entrance. In the cave, individuals that rely less on vision allocate more energy to reproduction. After many generations, cave insects show reduced eye size and rarely interbreed with surface insects due to different mating cues. Which event would most likely lead to speciation?
   Answer: Reproductive isolation increases as divergent mating cues reduce gene flow between cave and surface populations. This question tests understanding of allopatric speciation. Speciation arises from geographic isolation followed by divergence and reproductive barriers. The landslide isolates cave insects, leading to trait divergence and mating cue differences that reduce interbreeding. Reproductive isolation increases as cues diverge, promoting speciation as in choice A. Choice C fails by suggesting non-genetic transmission of acquired traits. To evaluate, check for barriers post-isolation. This model fits many cave or island speciations.
4. In a population of freshwater snails, shell thickness varies continuously. A new fish predator is introduced that can crush only thin shells. After several generations, the mean shell thickness increases and variance decreases. Which statement best reflects the process of natural selection described?
   Answer: Directional selection favored thicker shells because individuals with thicker shells survived predation and reproduced more. This question tests understanding of directional selection on quantitative traits. Directional selection shifts the population mean toward one extreme by favoring variants with higher fitness. The introduced fish predator selectively preys on thin-shelled snails, increasing the mean shell thickness over generations. Thicker shells are favored because they enhance survival against predation, leading to directional selection as in choice C. Choice D fails by invoking Lamarckian inheritance, where acquired traits are passed on, which is not genetically supported. To detect directional selection, monitor if trait means shift consistently with selective pressures. This applies to traits like antibiotic resistance or pesticide tolerance.
5. A freshwater fish species has two color morphs controlled by alleles G (green) and S (silver). In a clear lake with abundant aquatic plants, birds preferentially catch silver fish; in a nearby murky lake with little vegetation, birds preferentially catch green fish. A researcher transplants 200 fish from each lake into large enclosures in the other lake (no migration afterward). After 5 generations, allele frequencies shift toward the locally favored morph in each enclosure. Which statement best reflects the process of natural selection described?
   Answer: Allele frequencies shifted because predators created different selective pressures in each environment, changing relative reproductive success of morphs. This question tests understanding of natural selection through predator-mediated differential survival. Natural selection occurs when environmental factors cause consistent differences in reproductive success among genetic variants. In each lake, bird predation created different selective pressures: green fish were more vulnerable in murky water, while silver fish were more vulnerable among plants. When fish were transplanted, local predation patterns caused allele frequencies to shift toward the locally camouflaged morph over 5 generations. Answer B incorrectly invokes intentional color change and inheritance of acquired traits, C misunderstands drift in large populations, and D wrongly suggests selection requires identical starting conditions. To identify predator-mediated selection, look for: (1) environment-specific survival advantages, (2) consistent directional changes matching local conditions, and (3) heritable traits affecting predation risk.
6. A population of insects experiences a sudden shift from green vegetation to brown vegetation after an invasive plant spreads. Body color is heritable and varies continuously. In field mark–recapture studies, darker individuals have higher survival on brown vegetation due to reduced predation, and the mean body color becomes darker over successive generations. Which statement best reflects the process of natural selection described?
   Answer: The environment selects among existing heritable variants, causing darker phenotypes to contribute more alleles to the next generation. This question tests understanding of natural selection on quantitative traits. Natural selection acts on heritable phenotypic variation when certain variants have higher fitness in a given environment. Body color shows continuous variation (quantitative trait) with heritable basis, and the environmental shift from green to brown vegetation creates new selection pressure. Darker insects survive better through reduced predation on brown backgrounds, causing them to leave more offspring. Over generations, this differential reproduction shifts the population mean toward darker coloration. Answer A correctly identifies selection acting on existing heritable variation. Answer B incorrectly invokes Lamarckian inheritance of acquired traits, while C suggests drift in large populations where it would be negligible. For selection on quantitative traits, look for: (1) heritable phenotypic variation, (2) fitness differences along the trait continuum, and (3) population mean shifting in the direction of higher fitness.
7. A marine snail population has two shell morphs controlled by alleles H (thick shell) and h (thin shell). A new crab predator is introduced; it more easily crushes thin shells, and thick-shelled snails survive attacks more often. After 12 generations, H rises from 0.20 to 0.65. What adaptive advantage is most likely to occur under this selection regime?
   Answer: An increased proportion of thick-shelled snails because individuals with allele H are less likely to be eaten and therefore leave more offspring. This question tests understanding of how natural selection leads to adaptive changes in populations facing new predation pressure. Natural selection favors traits that increase survival and reproduction, causing populations to become better adapted to their environment. The introduction of shell-crushing crabs creates strong selection for thick shells (H allele), as these snails survive attacks more often and thus leave more offspring. The increase in H from 0.20 to 0.65 over 12 generations demonstrates this adaptive response. Answer B incorrectly predicts selection for thin shells, C denies that predation can cause directional change, and D invokes Lamarckian inheritance. To identify adaptive advantages under selection, look for: (1) new selective pressure, (2) differential survival linked to heritable traits, and (3) population-level increases in advantageous traits.
8. In a large population of insects, a pesticide is applied annually. A resistance allele R exists at low frequency before application. After several years, R increases substantially. Resistance is costly in pesticide-free lab conditions. Which statement best reflects the process of natural selection described?
   Answer: R increased because resistant individuals had higher survival and left more offspring when pesticide was present. This question tests understanding of natural selection for resistance. Natural selection amplifies beneficial alleles under pressure, despite costs elsewhere. Pesticide application favors resistant insects, increasing allele R. R increased as resistant individuals survived and reproduced more, as in choice B. Choice A fails by suggesting induced mutations in adults. To confirm, observe frequency rises with application. This parallels insecticide resistance evolution.
9. A moth population rests on tree bark. A heritable allele D produces darker wing coloration. When industrial soot darkens the bark, birds more easily detect light moths. Over multiple generations in the soot-darkened area, D increases; in a nearby unpolluted area, D remains rare. Which statement best reflects the process of natural selection described?
   Answer: D increased in the polluted area because darker moths were less likely to be eaten and therefore reproduced more. This question tests understanding of spatially varying selection. Natural selection drives local adaptations when environments differ, as in industrial melanism. Soot darkens bark, favoring dark moths via reduced predation, increasing D in polluted areas. D increased because darker moths reproduced more, as in choice A. Choice B is wrong, invoking Lamarckian change. To check, compare frequencies across environments. This classic example illustrates selection gradients.
10. Two populations of the same frog species occupy opposite sides of a newly formed canyon that prevents adult movement and strongly limits gene flow. Over 500 generations, males from the north population produce calls with a higher dominant frequency than males from the south population. In mate-choice trials, females preferentially approach calls from their own population, even when reared in identical laboratory conditions. Which event would most likely lead to speciation in this scenario?
    Answer: Evolution of prezygotic reproductive isolation via assortative mating based on divergent call characteristics. This question tests understanding of speciation through reproductive isolation mechanisms. Speciation occurs when populations evolve reproductive barriers that prevent gene flow, and prezygotic barriers like behavioral differences in mate choice are particularly effective. The scenario describes divergent evolution of call characteristics and female preference for local calls, creating assortative mating where individuals preferentially mate within their population. The correct answer B accurately identifies this as prezygotic reproductive isolation through behavioral differences. Answer D incorrectly suggests populations would converge despite the geographic barrier and evolved preferences. To recognize incipient speciation, look for: (1) reduced gene flow, (2) divergent traits affecting reproduction, and (3) preferential mating within groups rather than between them.
11. A flowering plant species occupies a continuous habitat. A new highway creates a physical barrier that prevents pollen and seed dispersal between north and south sides. Over 200 generations, the two sides experience different pollinator communities, and flowering time becomes heritably shifted earlier in the north and later in the south. Crosses between north and south plants produce viable seeds, but F1 hybrids have greatly reduced fertility due to mismatched flowering time with local pollinators. Which event would most likely lead to speciation?
    Answer: Reinforcement strengthens prezygotic isolation as selection disfavors low-fertility hybrids, reducing interbreeding. This question tests understanding of speciation through reproductive isolation mechanisms. Speciation occurs when populations evolve reproductive barriers that prevent gene flow, even if they come back into contact. The highway creates initial geographic isolation (allopatry), allowing the populations to diverge in flowering time due to different pollinator communities. The key is that hybrids have reduced fertility - a postzygotic reproductive barrier. Answer B correctly identifies reinforcement, where selection against low-fitness hybrids strengthens prezygotic barriers (like flowering time differences), completing reproductive isolation. Answer A would prevent speciation by restoring gene flow, while C describes neutral evolution unrelated to reproductive barriers. To recognize speciation scenarios, look for: (1) initial isolation allowing divergence, (2) evolution of reproductive barriers, and (3) selection maintaining or strengthening those barriers when populations meet again.
12. A desert plant population contains alleles at a locus affecting stomatal density: H (high) and L (low). During a decade-long drought, plants with low stomatal density have higher seed set due to reduced water loss. Allele L rises from 0.30 to 0.65. What adaptive advantage is most likely to occur?
    Answer: Increased water-use efficiency becomes more common because L carriers leave more offspring under drought conditions. This question tests understanding of adaptive advantages under natural selection. Natural selection promotes alleles that confer survival or reproductive benefits in challenging environments. During the drought, low stomatal density (allele L) reduces water loss, leading to higher seed set and allele frequency increase. Increased water-use efficiency becomes common as L carriers reproduce more, providing an adaptive advantage as in choice D. Choice B is incorrect because it suggests drought induces mutations, but selection acts on existing variation. To evaluate adaptation, assess if trait frequencies change with environmental fitness benefits. This principle extends to drought resistance in crops or wild plants.
13. In a fish population, a single gene influences tolerance to low dissolved oxygen: allele T increases tolerance, allele t decreases tolerance. A lake undergoes eutrophication, causing frequent hypoxic events. After several years, population genetic sampling shows that $p(T)$ rises from 0.20 to 0.55, while genotype frequencies each year deviate from Hardy–Weinberg expectations due to consistently lower survival of tt juveniles during hypoxic weeks. Which statement best reflects the process of natural selection described?
    Answer: Allele T increases because individuals with TT and Tt leave more offspring after differential juvenile survival. This question tests understanding of natural selection through differential survival based on genotype. Natural selection acts when genetic variants affect fitness in ways that change allele frequencies across generations. The hypoxic events create strong selection pressure where tt juveniles die more frequently, while TT and Tt individuals survive better due to their oxygen tolerance. This differential survival means T-bearing individuals contribute more offspring, increasing T frequency from 0.20 to 0.55. Answer B correctly identifies this mechanism of differential juvenile survival leading to increased reproductive contribution. The deviation from Hardy-Weinberg equilibrium specifically during hypoxic events confirms ongoing selection. Answer A incorrectly suggests direct mutation induction, while D contradicts the evidence of unequal fitness. To identify selection, look for: (1) consistent fitness differences among genotypes, (2) directional change in allele frequencies, and (3) deviations from random mating expectations.
14. A bacterial population contains two heritable variants at a gene affecting an efflux pump: variant E (high efflux) and variant e (low efflux). When a low dose of antibiotic is introduced, cultures started with equal frequencies of E and e show that, after 24 hours, E composes 90% of the population. In antibiotic-free media, E and e remain near 50/50. What adaptive advantage is most likely to occur in the antibiotic environment?
    Answer: Cells with E reduce intracellular antibiotic concentration and leave more viable offspring than cells with e. This question tests understanding of natural selection through differential survival in response to environmental challenges. Natural selection requires heritable variation that affects fitness in a given environment. The efflux pump variants show clear fitness differences: variant E pumps out antibiotics more effectively, allowing those bacterial cells to survive and reproduce in the presence of the drug. The shift from 50% to 90% E in just 24 hours demonstrates strong selection, as E-bearing cells leave more offspring. Answer A correctly identifies the mechanism - reduced intracellular antibiotic concentration leads to higher survival and reproduction. Answer B incorrectly suggests Lamarckian conversion of alleles, while C invokes drift which cannot explain such rapid, directional change. The control condition (no change in antibiotic-free media) confirms this is environment-specific selection. To identify selection in microbes, look for: (1) rapid frequency changes, (2) environment-specific fitness differences, and (3) the trait must be heritable, not induced.
15. A plant population on a mountain has two alleles at a flowering-time locus: E (early) and L (late). A drought shortens the growing season, and plants that flower earlier set more seed. Over 6 generations, E increases from 0.30 to 0.55 while overall population size remains large and stable. Which statement best reflects the process of natural selection described?
    Answer: E increased because early flowering increased reproductive success under drought, causing a directional change in allele frequency across generations. This question tests understanding of natural selection favoring traits that increase reproductive success under environmental stress. Natural selection occurs when heritable phenotypic differences lead to differential reproduction, causing allele frequency changes across generations. The drought shortened the growing season, giving early-flowering plants (E allele) a reproductive advantage because they could set seed before conditions deteriorated. The increase in E from 0.30 to 0.55 over 6 generations reflects this fitness advantage. Answer B incorrectly describes drift (which would be random, not directional), C invokes impossible genome switching, and D misrepresents allele dynamics. To identify selection for life-history traits, look for: (1) environmental change affecting reproductive timing, (2) consistent advantage for one phenotype, and (3) directional allele frequency change in large populations.
16. A freshwater fish has two hemoglobin variants, H1 and H2. In warm water, H2 has lower oxygen affinity and supports higher sustained swimming performance; in cold water, H1 supports higher performance. A region experiences a long-term increase in average water temperature over 30 years, and the frequency of H2 rises from 0.30 to 0.70. What adaptive advantage is most likely to occur as temperatures increase?
    Answer: H2 will increase in frequency because individuals carrying H2 leave more offspring in warm water due to improved performance. This question tests understanding of natural selection favoring alleles that enhance fitness in changing environments. Natural selection increases the frequency of alleles that confer higher reproductive success under prevailing conditions, here involving temperature-dependent hemoglobin function. As water warms over 30 years, fish carrying H2 have better swimming performance in the new conditions, allowing them to feed more effectively, escape predators, or secure mates, thus leaving more offspring. The correct answer B accurately describes this fitness advantage leading to increased H2 frequency. Answer D incorrectly suggests acquired characteristics during lifetime, violating principles of genetic inheritance. To identify environmental selection, look for: (1) environmental change, (2) genetic variants with different performance, and (3) frequency shifts matching the fitness effects.
17. A mosquito population has standing genetic variation at a sodium channel gene: allele R reduces binding of a pyrethroid insecticide, allele S does not. Before insecticide use, $p(R)=0.05$. After widespread spraying for 3 years, $p(R)=0.62$, and field assays show higher survival of RR and RS mosquitoes after exposure compared with SS. Which statement best reflects the process of natural selection described?
    Answer: Allele R increases because resistant genotypes survive exposure and contribute disproportionately to the next generation. This question tests understanding of natural selection for insecticide resistance. Natural selection acts on pre-existing genetic variation when environmental changes create new fitness differences. The R allele was already present at low frequency (0.05) before insecticide use, representing standing genetic variation. When pyrethroid spraying begins, mosquitoes with R alleles survive exposure better because the altered sodium channel reduces insecticide binding. These resistant individuals reproduce more successfully, increasing R frequency to 0.62 over three years. Answer C correctly identifies this as differential survival and reproduction based on genotype. Answer A incorrectly suggests acquired resistance, while D invokes random drift which cannot explain the consistent directional change correlated with insecticide exposure. To identify selection for resistance, look for: (1) pre-existing variation, (2) environmental change creating fitness differences, and (3) allele frequency changes matching survival patterns.
18. A small, isolated population of 25 alpine flowers is sampled at a neutral microsatellite locus with alleles M1 and M2. In year 0, $p(M1)=0.52$. Over 10 years, allele frequency fluctuates irregularly (0.52, 0.44, 0.60, 0.36, 0.48, ...), with no measured differences in seed set or survival between genotypes, and the environment is stable. Based on the scenario, which outcome is most consistent with genetic drift?
    Answer: M1 frequency will show random walk-like changes due to sampling effects in a small population, potentially reaching fixation or loss. This question tests understanding of genetic drift in small populations at neutral loci. Genetic drift causes random fluctuations in allele frequencies, with magnitude inversely related to population size. With only 25 individuals, this alpine flower population experiences strong drift effects. The irregular fluctuation pattern (0.52→0.44→0.60→0.36→0.48) shows the random walk characteristic of drift - no directional trend, just stochastic variation. The neutral microsatellite locus means no fitness differences exist among genotypes, confirmed by equal seed set and survival. Answer B correctly identifies this as random sampling effects potentially leading to fixation or loss. Answer C incorrectly claims neutral alleles cannot change frequency, ignoring drift. In small populations, drift can overpower even weak selection. To recognize drift patterns, look for: (1) small population size, (2) irregular fluctuations without trends, and (3) changes at neutral loci or in traits without fitness effects.
19. In a coastal marsh, mosquitoes vary in salt tolerance. A seawall failure increases salinity, and larvae with higher salt tolerance have higher survival to adulthood. After several generations, the distribution shifts toward higher tolerance. What adaptive advantage is most likely to occur?
    Answer: Higher larval survival in salty water becomes more common because tolerant genotypes contribute more offspring. This question tests understanding of adaptation to environmental change. Natural selection favors heritable traits enhancing fitness in altered conditions. Increased salinity selects for tolerant mosquito larvae, shifting the population distribution. Higher larval survival in salty water becomes common via tolerant genotypes' reproduction, as in choice D. Choice C is wrong, suggesting Lamarckian inheritance. To assess, correlate trait shifts with survival benefits. This applies to salinity or pollution adaptations.
20. In a coastal lizard population, dorsal color is controlled by two alleles at one locus: D (dark) and d (light). After several years of increased predation by birds hunting on pale sand, researchers genotype adults each season. The frequency of allele D rises from 0.40 to 0.62 over 8 generations, and dark individuals are observed to survive to reproduction more often than light individuals in the same habitat. Which statement best reflects the process of natural selection described?
    Answer: Differential survival of dark individuals increases the contribution of allele D to the next generation, raising its frequency. This question tests understanding of natural selection as differential survival and reproduction based on heritable traits. Natural selection occurs when individuals with certain alleles survive and reproduce more successfully than others in a given environment, causing those alleles to increase in frequency over generations. In this scenario, dark lizards (with allele D) survive predation better than light lizards on pale sand, leading to increased reproduction of D-carriers. The correct answer B accurately describes this process: differential survival of dark individuals increases their genetic contribution to future generations. Answer C incorrectly invokes Lamarckian inheritance of acquired characteristics, which is not how evolution works. To identify natural selection, look for: (1) variation in traits, (2) differential survival/reproduction, and (3) resulting changes in allele frequencies across generations.

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