Introduction to Natural Selection
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AP Biology › Introduction to Natural Selection
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?
Allele frequencies remain constant because food type changes do not affect reproductive output.
Birds with shallow beaks develop deeper beaks during life and transmit that trait to offspring.
Both alleles disappear because the population will stop reproducing until softer seeds return.
Allele D increases because birds with deeper beaks leave more offspring when hard seeds dominate.
Allele d increases because birds with shallow beaks can learn new feeding behaviors and pass them on genetically.
Explanation
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.
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?
Shallow-beak alleles will increase because shallow-beaked birds avoid competition for large seeds.
Deeper-beak alleles will increase in frequency because birds with deeper beaks leave more offspring.
Birds will develop deeper beaks from cracking seeds, and offspring will inherit the acquired depth.
The population will lose beak-depth variation because natural selection prevents genetic variation from arising.
Allele frequencies will not change because drought is a temporary environmental condition.
Explanation
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 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?
The o allele increases because low oxygen causes fish to switch on genes that create lower tolerance.
Allele frequencies stay constant because the bloom affects all fish equally regardless of genotype.
The frequency of allele O increases because more O-bearing fish survive to reproduce during low oxygen.
All fish become highly tolerant within one generation because exposure changes their DNA permanently.
Both alleles disappear because environmental stress prevents inheritance of oxygen tolerance.
Explanation
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.
A population of flowering plants shows heritable variation in flowering time controlled by alleles F (early) and f (late). A new mowing schedule cuts the field in mid-season each year, removing many late-flowering plants before they set seed, while early-flowering plants often produce seeds before mowing. Plants cross-pollinate within the same field, and offspring flowering time resembles parental genotypes. Other environmental factors remain similar across years. Which outcome is most likely over time?
Allele f increases because late flowering allows plants to avoid competition and always increases seed production.
Both alleles become equally frequent because mowing creates a balanced advantage for both flowering times.
Allele frequencies remain unchanged because mowing affects only adult plants, not the next generation.
Allele F increases because early-flowering plants contribute more seeds to the next generation under mowing.
Late-flowering plants begin flowering earlier after mowing and pass that acquired timing to offspring.
Explanation
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. Mowing removes late-flowering plants with allele f before seeding, while early-flowering plants with allele F produce seeds beforehand and contribute more to the next generation. This selective pressure increases the frequency of allele F, as early-flowering individuals pass on the trait more often. At the population level, the heritable variation in flowering time drives evolution under consistent mowing. A tempting distractor is choice D, which claims late-flowering plants change timing and inherit it, illustrating 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 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?
Allele frequencies remain unchanged because grazing affects only plant size, not reproduction.
Allele H increases because tall plants capture more sunlight and always outcompete short plants.
Both alleles become equally frequent because grazing creates new mutations at the height gene.
Allele h increases because short plants contribute more seeds to the next generation than tall plants.
Individual tall plants become short after being clipped and pass that new height to offspring.
Explanation
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.
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?
Allele C decreases because dense fur requires more energy and therefore reduces survival in winter.
Allele frequencies remain constant because winter temperatures do not affect reproductive success.
Allele c increases because rabbits with less dense coats can acclimate to cold and pass that to offspring.
The frequency of allele C increases because denser-coated rabbits leave more offspring after surviving winters.
Both alleles disappear because cold winters prevent inheritance of fur density.
Explanation
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.
A mouse population has variation in coat color, and the trait is heritable. After snowfall becomes rare, the ground remains dark for most winters. Owls capture light-colored mice more often than dark-colored mice, reducing reproduction of light-colored mice. Which outcome is most likely over generations as dark winters persist?
Alleles for light coat color will increase because owls remove dark mice that compete for food.
Coat-color alleles will change because the population must match the dark ground to persist.
Alleles for dark coat color will increase because dark-coated mice contribute more offspring.
Light-colored mice will darken their coats in response to winter conditions, preventing allele changes.
Allele frequencies will not change because predation changes only appearance, not heredity.
Explanation
This question tests understanding of natural selection through predation on a mouse population. The correct answer is B because natural selection operates when individuals with certain heritable traits (dark coat color) have higher reproductive success due to environmental pressures (owl predation on visible light-colored mice). The stimulus indicates that owls capture light-colored mice more often on dark ground, reducing their reproduction, which means dark-colored mice contribute proportionally more offspring to future generations. Over multiple generations, this differential reproduction causes dark coat color alleles to increase in frequency in the population. Answer A incorrectly suggests that individual mice can change their coat color in response to environmental conditions (acquired characteristics), which would not change allele frequencies since the genetic makeup remains unchanged. When analyzing natural selection, focus on differential reproduction based on heritable traits, not on what individuals might do to survive.
In a coastal plant population, leaf waxiness varies due to heritable alleles W (high wax) and w (low wax). During frequent salt-spray events, plants with low wax lose more water and produce fewer seeds. Salt spray intensity remains high for decades. Which explanation best accounts for the expected change in allele frequencies?
Allele w will be preserved because the population benefits from maintaining low waxiness.
Allele W will rise because high-wax plants contribute a larger share of the next generation.
Allele W will rise because salt spray directly changes w alleles into W alleles in leaves.
Allele frequencies will stay constant because waxiness is determined only by salt conditions.
Allele w will rise because low-wax plants compensate by producing more seeds after exposure.
Explanation
This question tests natural selection through differential reproduction based on water retention. The correct answer is B because plants with the W allele (high wax) lose less water during salt spray events and therefore produce more seeds than plants with the w allele. Since seed production directly determines reproductive success, high-wax plants contribute a larger proportion of offspring to the next generation, causing the W allele frequency to increase over time. Answer D incorrectly suggests that environmental conditions can directly change one allele into another, which confuses mutation with selection. Natural selection changes allele frequencies by differential reproduction, not by transforming existing alleles.
A frog population has heritable variation in skin peptide composition: allele P produces peptides that inhibit a fungal pathogen, while allele p produces peptides with weaker inhibition. During repeated fungal outbreaks, frogs with weaker inhibition have lower survival to reproduction. If outbreaks continue for many generations, which outcome is most likely?
Both alleles will change equally because fungal exposure causes random changes in peptide genes.
Allele frequencies will remain stable because pathogens do not influence reproductive success.
Allele p will increase because infected frogs develop stronger peptides during their lifetimes.
Allele P will increase because frogs with stronger inhibition leave more offspring after outbreaks.
Allele P will decrease because selection always favors alleles that were common before outbreaks.
Explanation
This question demonstrates natural selection acting on disease resistance. The correct answer is A because frogs with the P allele produce peptides that better inhibit the fungal pathogen, leading to higher survival rates and more offspring compared to frogs with the p allele. During repeated outbreaks over many generations, this reproductive advantage causes the P allele to increase in frequency. Answer B incorrectly suggests that infected frogs can develop stronger peptides during their lifetime and pass this trait on, confusing acquired characteristics with inherited ones. Natural selection acts only on heritable variation that affects reproductive success.
A fish population shows heritable variation in body coloration controlled by alleles G (green) and S (silver). In a lake that becomes covered by dense green algae, predatory birds capture silver fish more often than green fish, reducing silver fish reproductive output. If algae cover persists, which outcome is most likely over generations?
Allele G will increase because algae exposure causes fish to switch from S to G alleles.
The population will become greener because fish coloration changes to match algae when needed.
Allele G will increase because green fish contribute more offspring under sustained bird predation.
Allele S will increase because silver fish learn to hide better in algae during their lives.
Allele frequencies will not change because predation removes individuals randomly from the gene pool.
Explanation
This question demonstrates natural selection through camouflage and predation pressure. The correct answer is C because green fish blend with the algae-covered environment, making them less visible to predatory birds and allowing them to survive and reproduce at higher rates than silver fish. Over generations, this differential reproductive success causes the G allele frequency to increase in the population. Answer A incorrectly suggests that individual fish can learn to hide better, confusing behavioral changes within a lifetime with heritable traits. To solve natural selection problems, identify which heritable variant has higher reproductive success in the given environment.