Speciation
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AP Biology › Speciation
Two populations of mammals are separated by a newly formed canyon that prevents most movement between sides. After 3,000 generations, individuals from the two sides can mate in captivity and produce healthy F1 offspring, but F1 offspring produce few viable gametes, resulting in very low fertility. Genetic analyses show substantial divergence in allele frequencies between the canyon populations. Which reproductive isolation mechanism is most directly demonstrated by the captive breeding results?
Acquired infertility in individuals being inherited by their offspring
Habitat isolation because individuals occupy different ecosystems
Reinforcement occurring before any hybrids are produced
Temporal isolation caused by breeding at different times of day
Hybrid sterility as a postzygotic barrier reducing gene flow
Explanation
This question assesses the skill of analyzing speciation processes in AP Biology, focusing on postzygotic barriers after geographic separation. Hybrid sterility acts as a postzygotic barrier, with F1 offspring producing few viable gametes, reducing effective gene flow between the canyon-separated mammal populations. The canyon initiates allopatric isolation, allowing genetic divergence, and the low fertility of hybrids demonstrates intrinsic incompatibilities that evolved during separation. Reproductive isolation is evidenced by substantial allele differences and the inability of hybrids to contribute to future generations. A tempting distractor is option D, proposing reinforcement before hybrids, but this requires prior hybrid production, reflecting the misconception that reinforcement precedes rather than follows secondary contact. A transferable strategy is to analyze breeding experiment results to classify isolation as pre- or postzygotic in divergence scenarios.
A plant species has two populations that overlap geographically. Cross-pollination experiments show that pollen from population A lands on stigmas of population B and germinates, but pollen tubes usually fail to reach ovules; the reciprocal cross shows the same pattern. Within-population crosses produce seeds at high rates. Field observations show pollinators visit both populations, and flowering times overlap. Allele frequencies differ between populations at multiple loci. Which reproductive barrier most directly limits gene flow between the populations?
Behavioral isolation because pollinators avoid visiting the other population
Mechanical isolation because flower shapes prevent pollen transfer
Hybrid sterility because adult hybrids cannot produce functional gametes
Gametic isolation because pollen tube growth is unsuccessful between populations
Allopatric isolation because a river blocks pollinator movement entirely
Explanation
This question assesses the skill of analyzing speciation processes in AP Biology, particularly gametic barriers in plants. Gametic isolation limits gene flow because pollen tubes from one population fail to reach ovules in the other, preventing fertilization despite germination. This prezygotic barrier occurs even with overlapping ranges and pollinator visits, leading to divergent allele frequencies. Reproductive isolation logic involves incompatibility at the gamete level, reducing seed production in crosses while within-population crosses succeed. A tempting distractor is option D, suggesting hybrid sterility, but this is postzygotic and applies after fertilization, reflecting the misconception that barriers act on hybrids rather than gametes. To tackle similar questions, examine cross-pollination outcomes to pinpoint the stage of reproductive failure.
Two populations of birds were separated on different islands for 8,000 years. When storms later allow occasional migration between islands, hybrids are produced but have reduced fertility compared with offspring from within-island matings. Over subsequent generations, observations show fewer hybrid pairings as birds increasingly choose mates with local song patterns. Genetic data show continued divergence despite some contact. Which process most directly explains the decrease in hybridization after secondary contact?
Genetic drift making every individual identical across both islands
Reinforcement increasing prezygotic isolation due to selection against hybrids
Polyploidy instantly creating a new bird species in one generation
Use and disuse changing songs in individuals and then being inherited
Convergent evolution causing the two populations to merge into one
Explanation
This question assesses the skill of analyzing speciation processes in AP Biology, especially reinforcement after secondary contact. Reinforcement increases prezygotic isolation as selection against low-fertility hybrids favors birds that choose mates with local song patterns, reducing hybridization over generations. Initially separated allopatrically, the populations diverge, and upon contact, postzygotic barriers like hybrid infertility drive the evolution of stronger mate preferences. Reproductive isolation is thus enhanced by natural selection to avoid costly hybrids, maintaining genetic divergence despite some migration. A tempting distractor is option C, proposing convergent evolution merging populations, but this ignores the continued divergence, reflecting the misconception that contact always leads to fusion rather than reinforced isolation. A transferable strategy is to track changes in hybridization rates post-contact to identify reinforcement in speciation.
A bird species has two populations separated by a desert. During an unusually wet decade, a narrow corridor of shrubs connects the populations, and some interbreeding occurs, producing viable hybrids. Over subsequent generations, the corridor disappears and contact becomes rare again. When contact happens, individuals preferentially mate with their own population’s song type, and hybrid matings occur less often than expected from encounter rates. Which process most directly explains the increase in prezygotic isolation during intermittent contact?
Polyploidy producing a new species during the wet decade in the bird populations
Mechanical isolation because beak shapes prevent copulation between populations
Reinforcement increasing assortative mating because hybrids reduce overall reproductive success
Genetic drift eliminating mate choice alleles in both populations due to high gene flow
Mutation in single individuals creating new songs that spread without selection
Explanation
This question requires analyzing speciation through reinforcement of reproductive barriers. The populations were separated by desert, had intermittent contact during a wet period producing viable hybrids, then showed increased prezygotic isolation (preferential mating with own song type) after contact ceased. This pattern indicates reinforcement - natural selection favoring individuals who avoid hybridization because hybrids reduce reproductive success, strengthening behavioral barriers over time. Choice B is incorrect because genetic drift would act randomly, not systematically increase assortative mating; this misconception attributes directed evolutionary change to random processes. Reinforcement is recognizable when prezygotic barriers strengthen after periods of hybridization, preventing future costly hybrid production.
A grass species includes a small population in which some individuals have a sudden chromosome doubling event, producing $4n$ individuals, while the rest of the species remains $2n$. The $4n$ individuals occur in the same field as $2n$ individuals. Crosses between $4n$ and $2n$ plants produce seeds that rarely develop into fertile adults, while $4n \times 4n$ crosses produce fertile offspring. Over time, allele frequencies at many loci diverge between $2n$ and $4n$ groups. Which process most directly caused reproductive isolation to arise?
Polyploidy causing immediate postzygotic isolation between $2n$ and $4n$ groups
Behavioral isolation because pollinators learned to avoid $4n$ flowers
Inheritance of acquired traits because chromosome number changes with soil nutrients
Gene flow increasing because $2n$ and $4n$ individuals interbreed freely
Allopatric isolation because a physical barrier separated $2n$ and $4n$ populations
Explanation
This question requires analyzing speciation through polyploidy, a mechanism creating instant reproductive isolation. When chromosome doubling produces 4n individuals in a 2n population, crosses between different ploidy levels (4n × 2n) produce triploid offspring that rarely develop into fertile adults due to meiotic problems, while 4n × 4n crosses produce viable offspring. This immediate postzygotic barrier allows the 4n lineage to diverge as a separate species despite living alongside 2n individuals. Choice C is incorrect because behavioral isolation involves mate choice, but the barrier here is chromosomal incompatibility after fertilization; this misconception confuses prezygotic and postzygotic mechanisms. Polyploidy is unique in creating instant reproductive isolation through chromosomal incompatibility rather than gradual divergence.
Two populations of birds were separated on different islands for 8,000 years. When storms later allow occasional migration between islands, hybrids are produced but have reduced fertility compared with offspring from within-island matings. Over subsequent generations, observations show fewer hybrid pairings as birds increasingly choose mates with local song patterns. Genetic data show continued divergence despite some contact. Which process most directly explains the decrease in hybridization after secondary contact?
Polyploidy instantly creating a new bird species in one generation
Reinforcement increasing prezygotic isolation due to selection against hybrids
Use and disuse changing songs in individuals and then being inherited
Genetic drift making every individual identical across both islands
Convergent evolution causing the two populations to merge into one
Explanation
This question assesses the skill of analyzing speciation processes in AP Biology, especially reinforcement after secondary contact. Reinforcement increases prezygotic isolation as selection against low-fertility hybrids favors birds that choose mates with local song patterns, reducing hybridization over generations. Initially separated allopatrically, the populations diverge, and upon contact, postzygotic barriers like hybrid infertility drive the evolution of stronger mate preferences. Reproductive isolation is thus enhanced by natural selection to avoid costly hybrids, maintaining genetic divergence despite some migration. A tempting distractor is option C, proposing convergent evolution merging populations, but this ignores the continued divergence, reflecting the misconception that contact always leads to fusion rather than reinforced isolation. A transferable strategy is to track changes in hybridization rates post-contact to identify reinforcement in speciation.
A marine invertebrate releases eggs and sperm into the water. Population X lives in a bay with low salinity; population Y lives in a nearby offshore area with higher salinity and no physical barrier between them. Spawning occurs at the same time, and currents mix gametes from both populations. Laboratory assays show that X sperm rarely bind to Y eggs (and vice versa), even when mixed at equal concentrations; within-population fertilization rates remain high. Adults from both populations survive equally well in either salinity when moved. Which mechanism most directly causes reproductive isolation between X and Y?
Mechanical isolation because reproductive organs do not align during copulation
Reinforcement because hybrids are more fit than either parent population
Hybrid breakdown because F2 offspring are sterile in the offshore environment
Gametic isolation due to incompatibility between sperm and egg recognition proteins
Geographic isolation because the bay and offshore areas are separated by land
Explanation
This question examines gametic isolation in marine organisms with external fertilization. The key evidence is that "X sperm rarely bind to Y eggs (and vice versa)" despite gametes mixing in the water column, indicating molecular incompatibility at the sperm-egg recognition level. This prezygotic barrier operates after gamete release but before fertilization, with proteins on sperm surfaces failing to recognize or bind to egg receptors from the other population. The populations spawn simultaneously and currents mix gametes, eliminating temporal or geographic barriers. Choice A incorrectly suggests mechanical isolation, which involves physical incompatibility of reproductive structures, not relevant for broadcast spawners. When analyzing marine speciation, focus on molecular mechanisms like gametic incompatibility that can maintain isolation even when gametes physically encounter each other in the water.
A plant population contains a rare mutation that causes some individuals to produce unreduced (2n) gametes. When 2n gametes fuse with normal n gametes, resulting offspring have 3n and are largely sterile. When two 2n gametes fuse, resulting offspring have 4n and can produce fertile 2n gametes with each other but not with the original n plants. Over several generations, the 4n plants increase in frequency in one area of the population. Which process most directly generated reproductive isolation between the 4n plants and the original plants?
Temporal isolation because the groups flower in different months across the growing season
Behavioral isolation because pollinators prefer different flower colors in the two groups
Polyploidy causing immediate reproductive isolation through chromosome number differences
Genetic drift because individuals intentionally avoid mating with different chromosome types
Allopatric speciation due to a river forming a barrier that stops pollen flow between areas
Explanation
This question examines speciation through polyploidy, a common mechanism in plants. The mutation causing unreduced (2n) gametes leads to polyploid offspring - when two 2n gametes fuse, they create 4n individuals. These tetraploid (4n) plants can reproduce with each other but not with the original diploid (2n) plants due to chromosome number mismatch during meiosis. This creates instant reproductive isolation without geographic separation or gradual divergence. The 3n offspring from 2n × n crosses are largely sterile, further preventing gene flow. Choice E incorrectly suggests genetic drift involves intentional avoidance, but drift is random change in allele frequencies, not directed behavior. To recognize polyploid speciation, look for chromosome number changes that create immediate reproductive barriers through meiotic incompatibility.
A lizard species occupies a chain of desert oases. Adjacent oases exchange migrants each generation, but the two oases at the ends of the chain rarely exchange migrants directly. Genetic distance increases steadily with geographic distance along the chain. When lizards from neighboring oases are paired, they mate and produce fertile offspring, but lizards from the two end oases show low mating success and produce few viable offspring. No single physical barrier separates the end oases; instead, gene flow occurs mainly between neighboring sites. Which pattern most directly accounts for the divergence observed?
Lamarckian inheritance causing individuals to acquire desert traits and stop mating
Immediate reproductive isolation caused by a sudden chromosome number decrease
Sympatric speciation caused by polyploidy in a single oasis population
Ring species pattern with isolation by distance leading to end-population incompatibility
Uniform gene flow across all oases maintaining identical allele frequencies everywhere
Explanation
This question assesses the skill of analyzing speciation processes by recognizing patterns of gradual divergence. The ring species pattern creates isolation by distance, where incremental genetic changes along the chain lead to incompatibility between end populations despite gene flow between neighbors. This results in reproductive isolation without a single barrier, as end lizards show low mating success and few viable offspring. Adjacent interbreeding but cumulative divergence explains the pattern. A tempting distractor is choice D, uniform gene flow, but this is incorrect as it would prevent divergence, reflecting the misconception that limited migration homogenizes rather than allows stepwise isolation. For a transferable strategy, map genetic distance against geography to identify ring species and isolation by distance.
A marine invertebrate releases sperm and eggs into seawater for external fertilization. Two populations spawn in the same bay on the same nights, and currents mix gametes. Laboratory assays show that sperm from each population fertilizes eggs from its own population at a much higher rate than it fertilizes eggs from the other population, even when equal sperm concentrations are used. When cross-fertilization does occur, resulting larvae develop normally. Which reproductive isolating mechanism most directly reduces gene flow between the populations?
Gametic isolation caused by reduced compatibility between sperm and eggs
Geographic isolation caused by a land barrier separating the bay into two parts
Temporal isolation caused by spawning in different seasons of the year
Sexual selection within individuals changing gamete recognition proteins during life
Hybrid inviability causing larvae to die before reaching adulthood
Explanation
This question assesses the skill of analyzing speciation processes by pinpointing barriers in external fertilization. Gametic isolation reduces gene flow as sperm-egg compatibility is population-specific, preventing fertilization despite mixed gametes and shared spawning times. This prezygotic mechanism acts at the molecular level, with lab assays showing lower cross-fertilization rates but normal development when it occurs. No geographic or temporal barriers apply in the same bay. A tempting distractor is choice C, hybrid inviability, but this is incorrect since larvae develop normally post-fertilization, reflecting the misconception that isolation happens after zygote formation rather than before. For a transferable strategy, test fertilization success in controlled settings to identify gametic barriers in broadcast spawners.