- A lizard population shows variation in toe-pad width, a heritable trait. After frequent storms, vegetation becomes smoother and less textured. Lizards with wider toe pads cling better and produce more offspring than lizards with narrower pads. Which outcome is most likely in the population after several generations in the smoother habitat?
Answer: Alleles for wider toe pads will increase because wider-toe individuals leave more offspring.
This question assesses understanding of natural selection in lizards adapting to habitat changes. The correct answer is C because natural selection operates when individuals with certain heritable traits (wider toe pads) have greater reproductive success than others under specific environmental conditions (smoother vegetation after storms). The stimulus explicitly states that lizards with wider toe pads cling better to smooth surfaces and produce more offspring, creating the differential reproduction necessary for natural selection. Over generations, alleles for wider toe pads will be passed on more frequently, increasing their frequency in the population. Answer B incorrectly suggests that individual lizards can widen their toe pads through use during their lifetime (acquired characteristics), which would not change allele frequencies since genetic makeup remains unchanged. When solving natural selection problems, focus on heritable traits that affect reproductive success, not on changes that occur within an individual's lifetime.
- A grass population includes heritable variation in flowering time: allele E causes earlier flowering than allele e. A mowing schedule removes most plants that have not yet produced seeds by mid-season; earlier-flowering plants set seed before mowing more often. The mowing schedule stays the same for many years. Which outcome is most likely over generations?
Answer: Allele E will increase in frequency because earlier-flowering plants contribute more seeds before mowing.
This question examines natural selection through timing of reproduction. The correct answer is C because plants with the E allele flower earlier and successfully produce seeds before mowing, while later-flowering plants (e allele) are often mowed before setting seed. This difference in reproductive success causes earlier-flowering plants to contribute more offspring to subsequent generations, increasing the E allele frequency over time. Answer A incorrectly suggests that later-flowering plants can compensate after mowing, but the key is that they miss their reproductive opportunity entirely. When analyzing selection scenarios, focus on which variants successfully reproduce under the environmental conditions.
- In a bird population, beak depth varies and is heritable, influenced by alleles D (deeper beak) and d (shallower beak). A shift in available seeds occurs: most remaining seeds are large and hard, and birds with shallower beaks crack them less efficiently and produce fewer offspring. Birds with deeper beaks more often obtain enough food to reproduce. The population continues to interbreed in the same region, with minimal immigration. Which outcome is most likely after several generations?
Answer: Allele D increases because birds with deeper beaks leave more offspring when hard seeds dominate.
This question tests understanding of natural selection, the process where heritable traits that improve survival and reproduction become more common in a population over generations. The shift to hard seeds disadvantages birds with shallower beaks (allele d), who feed less efficiently and produce fewer offspring, while deeper-beaked birds (allele D) succeed and reproduce more. Consequently, the frequency of allele D increases, as deeper-beaked birds pass on the trait to more descendants. This change happens at the population level through differential reproductive success under the new food availability pressure. A tempting distractor is choice E, which wrongly suggests shallow-beaked birds develop deeper beaks and transmit them, reflecting the misconception of inheritance of acquired traits. For natural selection questions, always identify the environmental pressure, the heritable variation it acts on, and how it leads to changes in allele frequencies through differential reproduction.
- A rabbit population varies in fur density due to heritable alleles C (denser coat) and c (less dense coat). Following several unusually cold winters, rabbits with less dense coats experience lower survival before breeding, while rabbits with denser coats more often survive to reproduce. The population remains in the same region with continued interbreeding and little migration. No additional selective pressures are described. Which outcome is most likely after multiple cold winters?
Answer: The frequency of allele C increases because denser-coated rabbits leave more offspring after surviving winters.
This question tests understanding of natural selection, the process where heritable traits that improve survival and reproduction become more common in a population over generations. Cold winters reduce survival of rabbits with less dense coats (allele c) before breeding, while denser-coated rabbits (allele C) survive better and reproduce more. This leads to an increase in the frequency of allele C, as denser-coated individuals contribute more offspring inheriting the trait. The population-level shift results from the selective pressure of cold favoring the insulating fur density. A tempting distractor is choice C, which incorrectly states rabbits acclimate to cold and pass it on, representing the misconception of acquired trait inheritance. For natural selection questions, always identify the environmental pressure, the heritable variation it acts on, and how it leads to changes in allele frequencies through differential reproduction.
- In a bacterial population, some cells carry an allele that produces an enzyme that degrades a specific antibiotic; the allele is heritable and passed during cell division. When the antibiotic is added to the environment, cells with the enzyme survive and divide more than cells lacking it. Which outcome is most likely after many generations under antibiotic exposure?
Answer: Alleles for antibiotic degradation will increase because carriers leave more descendant cells.
This question tests understanding of natural selection in bacterial populations exposed to antibiotics. The correct answer is C because natural selection occurs when individuals (bacterial cells) with advantageous heritable traits (antibiotic-degrading enzyme allele) have higher reproductive success (survive and divide more) than others. The stimulus states that cells with the enzyme survive and divide more under antibiotic exposure, while cells lacking it presumably die or divide less, creating differential reproduction. Since the allele is heritable and passed during cell division, cells carrying it will become increasingly common in the population over generations. Answer A incorrectly suggests that cells can acquire new alleles during their lifetime in response to antibiotics, which confuses natural selection with mutation or horizontal gene transfer. To correctly analyze natural selection, identify pre-existing heritable variation, determine which variants reproduce more successfully, and predict that successful variants will increase in frequency.
- In a grass population, plants vary in height due to a heritable gene with alleles H (tall) and h (short). A herd of grazing mammals feeds by clipping vegetation at a consistent height, removing a larger fraction of tall plants before they produce seeds. Short plants are less likely to be clipped and more often set seed. All plants release pollen and seeds within the same field, and no new alleles enter the population. Which outcome is most likely after many generations of grazing?
Answer: Allele h increases because short plants contribute more seeds to the next generation than tall plants.
This question tests understanding of natural selection, the process where heritable traits that improve survival and reproduction become more common in a population over generations. Grazing mammals clip tall plants with allele H more often before they seed, while short plants with allele h are less affected and produce more seeds, leading to higher reproductive success for short plants. Consequently, the frequency of allele h increases in the population as short plants contribute more offspring that inherit the short height trait. This evolutionary shift happens at the population level due to the selective pressure of grazing favoring the short stature. A tempting distractor is choice D, which erroneously claims that clipped plants change height and pass it on, representing the misconception of Lamarckian evolution. For natural selection questions, always identify the environmental pressure, the heritable variation it acts on, and how it leads to changes in allele frequencies through differential reproduction.
- In a bird population, beak depth varies and is heritable. A drought reduces small, soft seeds, leaving mostly large, hard seeds. Birds with deeper beaks crack hard seeds more effectively and produce more offspring than birds with shallow beaks. Which outcome is most likely after several generations of drought conditions?
Answer: Alleles for deeper beaks will increase because birds with deeper beaks leave more offspring.
This question examines natural selection in birds responding to food availability changes. The correct answer is A because natural selection occurs when individuals with advantageous heritable traits (deeper beaks) produce more offspring than others under specific environmental conditions (abundance of hard seeds during drought). The stimulus clearly states that birds with deeper beaks crack hard seeds more effectively and produce more offspring, creating differential reproduction based on beak depth. Since beak depth is heritable, alleles for deeper beaks will be passed on more frequently to offspring, causing their frequency to increase over generations. Answer B incorrectly suggests that all birds can develop deeper beaks during drought (acquired characteristics), which confuses evolutionary change with phenotypic plasticity and would not alter allele frequencies. To correctly identify natural selection, look for heritable variation that affects reproductive success and predict that alleles associated with higher reproduction will increase in frequency.
- A beetle population includes green and brown individuals. Color is heritable and controlled by alleles at a single locus. After a wildfire, the ground becomes darker, and birds more easily spot green beetles than brown beetles, reducing the number of green beetles that reproduce. Which explanation best accounts for how allele frequencies will change over generations following the fire?
Answer: Bird predation will cause brown-color alleles to increase because brown beetles contribute more offspring.
This question assesses understanding of natural selection driven by predation in a beetle population. The correct answer is B because natural selection operates when individuals with certain heritable traits (brown color) have higher reproductive success than others due to environmental pressures (bird predation on visible green beetles). The stimulus indicates that birds more easily spot and eat green beetles on dark ground, reducing their survival to reproduction, which means brown beetles contribute proportionally more offspring to the next generation. Over multiple generations, this differential reproduction causes brown-color alleles to increase in frequency in the population. Answer C incorrectly implies that populations evolve traits because they "need" them (teleological thinking), rather than understanding that natural selection is a mechanistic process based on differential reproduction. When analyzing natural selection scenarios, focus on which individuals actually reproduce more successfully, not on what seems beneficial for the population.
- A coastal snail population shows variation in shell thickness controlled largely by a heritable allele T (thick) versus t (thin). A crab species preys on these snails and is more successful at crushing thin-shelled individuals, reducing their survival to reproduction. Thick- and thin-shelled snails occur in the same interbreeding population, and offspring shell thickness resembles that of their parents. Over multiple years, crab abundance remains high and other conditions are similar. Which outcome is most likely in the snail population over time?
Answer: The frequency of allele T increases because thick-shelled snails survive and reproduce more often.
This question tests understanding of natural selection, the process where heritable traits that improve survival and reproduction become more common in a population over generations. The presence of crabs that preferentially crush thin-shelled snails with allele t creates a selective pressure favoring thick-shelled snails with allele T, as they survive predation better and reproduce more. Over time, this leads to an increase in the frequency of allele T in the population, as thick-shelled individuals contribute a larger proportion of offspring that inherit the trait. Population-level change occurs because the heritable variation in shell thickness directly affects reproductive success under consistent predation. A tempting distractor is choice E, which wrongly implies that snails acquire thicker shells during life and pass them on, embodying the misconception of inheritance of acquired characteristics. For natural selection questions, always identify the environmental pressure, the heritable variation it acts on, and how it leads to changes in allele frequencies through differential reproduction.
- In a bird population, beak depth varies and is heritable. After several years of drought, only hard, large seeds remain abundant. Birds with deeper beaks crack these seeds more efficiently and produce more offspring than birds with shallower beaks. Both beak depths continue to occur among nestlings. Which outcome is most likely over generations?
Answer: Deeper-beak alleles will increase in frequency because birds with deeper beaks leave more offspring.
This question tests understanding of natural selection, the process where heritable traits that enhance survival and reproduction become more common in a population over generations. Drought leaves only hard, large seeds, which deeper-beaked birds crack more efficiently and use to produce more offspring. As a result, deeper-beaked birds contribute more to subsequent generations, increasing deeper-beak alleles in the population. This adaptation occurs through selection for effective foraging, with both beak depths persisting among nestlings. A tempting distractor is choice C, which incorrectly assumes birds develop deeper beaks from use and pass this on, representing the misconception of inheritance of acquired traits. For natural selection questions, always identify the selective pressure, the heritable trait, and how it affects reproductive success at the population level.
- A snail population includes individuals with thin and thick shells; shell thickness is heritable. A new crab predator is introduced and more easily crushes thin-shelled snails, reducing their survival to reproduction. Thick-shelled snails survive more often and produce more offspring. Which outcome is most likely after many generations with the crab present?
Answer: Alleles for thicker shells will increase because thick-shelled snails leave more offspring.
This question examines natural selection through predation pressure on a snail population. The correct answer is B because natural selection operates when individuals with advantageous heritable traits (thicker shells) survive to reproduce more successfully than others. The stimulus clearly states that the crab predator more easily crushes thin-shelled snails, reducing their survival to reproduction, while thick-shelled snails survive more often and produce more offspring. This differential reproduction means that alleles for thicker shells are passed on more frequently to the next generation, causing their frequency to increase over time. Answer E incorrectly claims that predation affecting mortality doesn't influence reproduction, but the stimulus explicitly states that thin-shelled snails have reduced "survival to reproduction," meaning they die before reproducing. When analyzing natural selection scenarios, remember that any factor affecting survival to reproductive age directly impacts reproductive success and thus drives evolutionary change.
- A bacterial population includes heritable variation in a membrane protein that affects antibiotic entry: allele X reduces entry, allele Y allows more entry. When an antibiotic is added to the environment, cells with allele X survive and divide more than cells with allele Y. Both alleles are still detected after treatment. Which outcome is most likely after repeated antibiotic exposure?
Answer: Allele X will increase in frequency because cells carrying it contribute more descendants.
This question tests understanding of natural selection, the process where heritable traits that enhance survival and reproduction become more common in a population over generations. Antibiotic exposure kills cells with allele Y more readily, while cells with allele X survive and divide more, contributing more descendants. Therefore, allele X increases in frequency across the bacterial population with repeated exposure. This selection favors resistance through reduced antibiotic entry, even as both alleles persist post-treatment. A tempting distractor is choice D, which falsely proposes all cells acquire allele X during exposure, misunderstanding that selection acts on pre-existing variation rather than inducing changes. For natural selection questions, always identify the selective pressure, the heritable trait, and how it affects reproductive success at the population level.
- A beetle population contains two heritable color morphs: green and brown. After an invasive plant spreads, the ground becomes covered with brown leaf litter year-round. Birds capture green beetles more often than brown beetles, and brown beetles produce more offspring each season. Both colors still occur among hatchlings. Which outcome is most likely in this beetle population over time?
Answer: Brown-color alleles will increase in frequency because brown beetles contribute more offspring.
This question tests understanding of natural selection, the process where heritable traits that enhance survival and reproduction become more common in a population over generations. The spread of brown leaf litter makes green beetles more visible to birds, while brown beetles blend in and produce more offspring. Therefore, brown beetles contribute disproportionately to the next generations, increasing the frequency of brown-color alleles in the population. This directional selection favors camouflage in the altered environment, even as both colors persist among hatchlings. A tempting distractor is choice A, which erroneously suggests that green beetles change color during development and pass this on, illustrating the misconception of acquired traits being inherited. For natural selection questions, always identify the selective pressure, the heritable trait, and how it affects reproductive success at the population level.
- In a lizard population, some individuals have a heritable allele for longer hind limbs, while others have shorter limbs. After a hurricane increases open sandy habitat, longer-limbed lizards run faster on sand and produce more offspring than shorter-limbed lizards. Limb-length variation remains among hatchlings. Which outcome is most likely over time in this population?
Answer: Alleles for longer hind limbs will increase in frequency because those individuals contribute more offspring.
This question tests understanding of natural selection, the process where heritable traits that enhance survival and reproduction become more common in a population over generations. The hurricane creates more open sandy habitat where longer-limbed lizards run faster and produce more offspring than shorter-limbed ones. Thus, longer-limbed lizards contribute more descendants, increasing the frequency of long-limb alleles in the population. This directional shift favors speed in the new habitat, with variation remaining among hatchlings. A tempting distractor is choice B, which incorrectly suggests short-limbed lizards grow longer limbs from exercise and pass this on, embodying the misconception of acquired characteristics being heritable. For natural selection questions, always identify the selective pressure, the heritable trait, and how it affects reproductive success at the population level.
- A population of freshwater fish shows variation in gill surface area, a heritable trait. A factory begins releasing warm water, lowering dissolved oxygen in the river. Fish with larger gill surface area obtain more oxygen and produce more offspring than fish with smaller gills. Which outcome is most likely after several generations in the warmed river?
Answer: Alleles for larger gill surface area will increase because those fish contribute more offspring.
This question tests understanding of natural selection in fish responding to environmental change. The correct answer is C because natural selection operates when individuals with certain heritable traits (larger gill surface area) produce more offspring than others under specific environmental conditions (low oxygen from warm water). The stimulus explicitly states that fish with larger gill surface area obtain more oxygen and produce more offspring in the warmed river, creating the differential reproduction necessary for natural selection. Over generations, this means alleles for larger gill surface area will be passed on more frequently, increasing their frequency in the population. Answer B incorrectly suggests that all individuals can change their traits in response to environmental conditions (phenotypic plasticity), which would not change allele frequencies since the genetic makeup remains unchanged. When solving natural selection problems, distinguish between plastic responses within individuals and evolutionary changes in populations through differential reproduction.
- In a snail population, shell thickness varies and is heritable: allele T produces thicker shells than allele t. A crab predator is introduced and more easily crushes thin-shelled snails, reducing their reproductive output. Crab predation remains high for many generations. Which outcome is most likely in the population?
Answer: Allele T will increase in frequency because thicker-shelled snails contribute more offspring under predation.
This question tests natural selection through predator-prey interactions. The correct answer is B because snails with thicker shells (T allele) resist crab predation better, allowing them to survive and reproduce at higher rates than thin-shelled snails. Over many generations of high crab predation, this differential reproductive success causes the T allele frequency to increase in the population. Answer D incorrectly suggests that predation can cause one allele to mutate into another, confusing the source of variation with the process of selection. Remember that natural selection sorts among existing heritable variants based on their reproductive success.
- A bacterial population contains heritable variation in antibiotic resistance due to alleles R (resistant) and r (nonresistant) at a locus. When an antibiotic is added to the environment, cells with allele r leave far fewer descendants than cells with allele R. Antibiotic exposure continues across many bacterial generations. Which outcome is most likely?
Answer: Allele R will increase in frequency because resistant cells contribute more offspring under antibiotic exposure.
This question examines natural selection in the context of antibiotic resistance. The correct answer is C because bacterial cells with the R allele survive antibiotic exposure and continue reproducing, while cells with the r allele die before reproducing. This differential reproduction causes the R allele to increase dramatically in frequency over many generations of antibiotic exposure. Answer A incorrectly suggests that antibiotics trigger bacteria to become resistant during their lifetime, confusing induced changes with pre-existing heritable variation. When analyzing selection problems, remember that environmental factors select among existing variants rather than creating new ones.
- A grass population includes individuals with either high or low waxy leaf coating. The coating level is heritable. During a multi-year drought, plants with higher waxy coating lose less water and produce more seeds than plants with low coating. Seedlings each year show both coating types. Which outcome is most likely after several drought years?
Answer: High-coating alleles will become more common because plants with that trait contribute more seeds.
This question tests understanding of natural selection, the process where heritable traits that enhance survival and reproduction become more common in a population over generations. During the drought, plants with higher waxy leaf coating lose less water and produce more seeds, giving them a reproductive advantage over low-coating plants. Consequently, high-coating plants contribute more offspring to subsequent generations, increasing the frequency of high-coating alleles in the population. This shift occurs because natural selection favors the trait that improves fitness under drought conditions, with variation persisting among seedlings. A tempting distractor is choice A, which wrongly implies that individual plants adapt by increasing wax production during their lifetimes and pass this on, embodying the misconception of inheritance of acquired characteristics. For natural selection questions, always identify the selective pressure, the heritable trait, and how it affects reproductive success at the population level.
- A freshwater fish population contains heritable variation in tolerance to low dissolved oxygen, influenced by alleles O (higher tolerance) and o (lower tolerance). During summer, algal blooms repeatedly reduce dissolved oxygen for several weeks, and fish with lower tolerance die at higher rates before spawning. Survivors reproduce within the same lake, and offspring oxygen tolerance resembles parental genotypes. Migration into the lake is rare. Which outcome is most likely after several bloom seasons?
Answer: The frequency of allele O increases because more O-bearing fish survive to reproduce during low oxygen.
This question tests understanding of natural selection, the process where heritable traits that improve survival and reproduction become more common in a population over generations. Repeated algal blooms reduce oxygen, causing higher mortality in fish with low-tolerance allele o before spawning, while high-tolerance fish with allele O survive and reproduce more often. This differential survival leads to an increase in the frequency of allele O, as surviving fish pass on the trait to a larger share of the next generation. At the population level, the heritable variation in oxygen tolerance drives this change under the consistent selective pressure of low oxygen. A tempting distractor is choice D, which incorrectly states that exposure permanently changes DNA in all fish, reflecting the misconception that environments directly alter genes within a generation. For natural selection questions, always identify the environmental pressure, the heritable variation it acts on, and how it leads to changes in allele frequencies through differential reproduction.
- In a fish population, some individuals have a heritable allele that produces a larger tail fin, while others have smaller fins. A new predator is introduced that more easily catches fish with larger fins, and small-finned fish survive and reproduce more often. Tail-fin size variation persists among juveniles. Which outcome is most likely after many generations?
Answer: The allele for smaller fins will increase in frequency because those fish leave more offspring.
This question tests understanding of natural selection, the process where heritable traits that enhance survival and reproduction become more common in a population over generations. The new predator catches large-finned fish more easily, allowing small-finned fish to survive and reproduce more often. Consequently, small-finned fish pass on more alleles for smaller fins, increasing their frequency in the population over generations. This occurs through differential survival and reproduction favoring the small-fin trait, with variation persisting among juveniles. A tempting distractor is choice B, which incorrectly assumes fish learn to avoid predators and pass this behavior genetically, confusing learned behaviors with heritable traits. For natural selection questions, always identify the selective pressure, the heritable trait, and how it affects reproductive success at the population level.
