Speciation
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AP Biology › Speciation
Two closely related frog populations occupy the same forest. Males call from similar locations, and females approach calling males. Playback experiments show that females from population 1 approach only the call frequency pattern typical of population 1, while females from population 2 approach only the pattern typical of population 2. When researchers place males and females together in an enclosure, mating occurs mostly within populations even though breeding seasons overlap and hybrids, when produced, develop into viable and fertile adults. Genetic analyses show increasing divergence in allele frequencies over time. Which prezygotic barrier most directly maintains reproductive isolation?
Polyploidy in frogs producing immediate isolation in a single generation
Mechanical isolation because gametes cannot be transferred between individuals
Postzygotic isolation because hybrids die before reaching adulthood in the forest
Geographic isolation because a river prevents contact between the two populations
Behavioral isolation based on differences in mating calls that limit interpopulation mating
Explanation
This question tests identification of behavioral isolation through mate choice based on acoustic signals. Female frogs show strong preferences for their own population's call frequency pattern, approaching only males with matching calls despite physical proximity and overlapping breeding seasons. This prezygotic barrier operates through female choice before mating attempts, with "mating occurs mostly within populations" due to call recognition preferences. The populations maintain genetic divergence through assortative mating based on acoustic communication. Choice B incorrectly suggests postzygotic isolation, but the passage states hybrids are "viable and fertile," ruling out hybrid inviability or sterility. To recognize behavioral isolation, look for differences in courtship signals (visual, acoustic, chemical) that influence mate choice and reduce interpopulation mating despite opportunity for contact.
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?
Mechanical isolation because flower shapes prevent pollen transfer
Hybrid sterility because adult hybrids cannot produce functional gametes
Behavioral isolation because pollinators avoid visiting the other population
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
Use and disuse changing songs in individuals and then being inherited
Convergent evolution causing the two populations to merge into one
Polyploidy instantly creating a new bird species in one generation
Reinforcement increasing prezygotic isolation due to selection against hybrids
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.
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
Convergent evolution causing the two populations to merge into one
Use and disuse changing songs in individuals and then being inherited
Genetic drift making every individual identical across both islands
Reinforcement increasing prezygotic isolation due to selection against hybrids
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 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?
Polyploidy causing immediate reproductive isolation through chromosome number differences
Temporal isolation because the groups flower in different months across the growing season
Genetic drift because individuals intentionally avoid mating with different chromosome types
Behavioral isolation because pollinators prefer different flower colors in the two groups
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.
Two populations of a flowering plant occur on opposite sides of a mountain ridge. Wind can carry pollen across the ridge, but field observations show that flowering begins about 6 weeks earlier on the east side than on the west side because of consistent temperature differences. Genetic data indicate divergence in allele frequencies, and controlled crosses show that if pollen is transferred by hand at the same time, fertilization and seed development occur normally. In natural conditions, however, pollen transfer rarely overlaps with receptive flowers across the ridge. Which isolating mechanism most directly reduces gene flow between the populations?
Temporal isolation due to nonoverlapping flowering times between the two populations
Genetic drift in individuals that changes their DNA in response to colder temperatures
Gametic isolation because pollen cannot fertilize ovules of the other population
Mechanical isolation because flower structures prevent pollen transfer between populations
Postzygotic isolation because hybrid seeds fail to develop after fertilization
Explanation
This question tests recognition of temporal isolation as a prezygotic barrier to gene flow. The plant populations flower at different times due to temperature differences on opposite sides of the mountain - a 6-week gap prevents pollen from one population from reaching receptive flowers in the other. Despite wind carrying pollen across the ridge and normal fertilization when hand-pollinated simultaneously, natural reproduction fails because flowering periods don't overlap. This temporal mismatch directly prevents fertilization opportunities, maintaining genetic divergence between populations. Choice B incorrectly suggests gametic incompatibility, but the passage states fertilization occurs normally when timing is controlled. To identify temporal isolation, look for time-based mismatches in reproduction (different seasons, times of day, or phenological shifts) that prevent mating or fertilization despite geographic proximity.
Two populations of frogs occupy adjacent wetlands with no physical barrier between them. Population 1 males call at 800 Hz and females preferentially approach 800 Hz calls; population 2 males call at 1200 Hz and females preferentially approach 1200 Hz calls. When researchers move adults from both populations into the same wetland, males continue their typical calls and females rarely approach males from the other population. Genetic analyses show reduced gene flow and increasing divergence between populations. Which isolating mechanism most directly reduces interbreeding between the frog populations?
Behavioral isolation due to divergence in mating calls and female preferences
Polyploidy producing instant isolation through mismatched chromosome numbers
Habitat isolation caused by use of different continents for breeding
Hybrid inviability because adults die immediately after mating occurs
Gametic isolation because sperm cannot penetrate eggs of the other population
Explanation
This question examines speciation through behavioral reproductive isolation. The two frog populations have evolved different mating call frequencies (800 Hz vs 1200 Hz) and corresponding female preferences, creating a prezygotic barrier to reproduction. Even when placed in the same wetland with no physical barriers, females rarely approach males from the other population because they don't recognize the different frequency calls as mating signals. This behavioral isolation maintains reproductive separation and allows continued genetic divergence between populations. Choice C incorrectly identifies habitat isolation, but the frogs occupy adjacent wetlands with no barriers, not different continents. To identify behavioral isolation, look for differences in courtship signals, mating rituals, or mate recognition that prevent interbreeding despite physical proximity.
A freshwater fish species occupies a single lake. Over time, some individuals feed mostly in open water and others feed near the shoreline. Genetic sampling shows two clusters of allele frequencies within the same lake, and tagging reveals that adults from the two clusters rarely enter the other habitat. During the spawning season, open-water fish spawn in deeper water while shoreline fish spawn in shallow, vegetated areas; fertilization occurs externally, and eggs develop normally when mixed in the lab. No geographic barrier separates the habitats, but gene flow between clusters is low. Which process most directly led to reduced gene flow between the two populations?
Polyploidy creating an instant reproductive barrier in a small subset of fish
Hybrid sterility that prevents gene flow after fertilization occurs in nature
Habitat isolation within the same lake, promoting sympatric divergence between subpopulations
Behavioral isolation caused by differences in courtship songs after a land barrier formed
Convergent evolution making the two groups genetically more similar over time
Explanation
This question requires identifying speciation occurring within a single geographic area without physical barriers. The fish populations show habitat isolation through ecological specialization - open-water feeders spawn in deep water while shoreline feeders spawn in shallow areas, reducing encounters during reproduction. This represents sympatric speciation where reproductive isolation evolves within the same lake through habitat preferences and spawning site fidelity. The populations maintain genetic distinctiveness despite no geographic barrier, with "gene flow between clusters is low" due to spatial separation of spawning sites. Choice A incorrectly invokes behavioral differences in courtship, but the passage emphasizes spawning location differences, not courtship behaviors. When analyzing sympatric speciation, look for mechanisms that reduce mating opportunities within the same geographic area, such as microhabitat preferences or resource specialization.
Two populations of the same frog species occupy neighboring valleys separated by a low ridge. Adults can cross the ridge, but they rarely do. In valley A, males call at 900 Hz; in valley B, males call at 1,200 Hz. Females show strong preference for the local call frequency, and cross-valley matings are uncommon even when adults are placed together at a breeding pond. Genetic markers show increasing divergence between valleys. Which isolating mechanism most directly reduces gene flow between the populations?
Gametic isolation because eggs and sperm cannot fuse between valleys
Gene flow increasing because adults are placed together at the breeding pond
Behavioral isolation based on divergence in mating calls and female preference
Mechanical isolation because ridge height prevents all movement of adults
Postzygotic isolation because hybrids are always inviable at the tadpole stage
Explanation
This question tests your ability to analyze speciation by identifying prezygotic reproductive barriers between populations. The populations are separated by a crossable ridge but show genetic divergence, with males producing different call frequencies (900 Hz vs 1,200 Hz) and females strongly preferring local calls. The key evidence is that cross-valley matings are uncommon even when frogs are placed together, indicating behavioral isolation based on acoustic communication differences. Choice B is incorrect because gametic isolation would prevent fertilization at the cellular level, not reduce mating attempts; this misconception confuses behavioral and cellular incompatibilities. When analyzing animal speciation, behavioral differences in courtship signals often create the first reproductive barriers between diverging populations.
A river fish species occupies both upstream and downstream habitats with no physical barrier between them. Upstream water is clear and fast; downstream water is turbid and slow. Over many generations, females upstream mate mostly with males showing bright flank coloration, while females downstream mate mostly with males showing dull coloration that is less visible in turbid water. Mark-recapture studies show fish move between regions, but genetic data reveal strong allele-frequency differences at loci linked to coloration and mate preference, with fewer mixed-genotype offspring than expected under random mating. Which mechanism most directly explains divergence despite ongoing gene flow?
Inheritance of acquired traits from individuals adjusting coloration to water
Sympatric divergence driven by assortative mating and disruptive selection
Postzygotic isolation from sterile hybrids that never survive to adulthood
Genetic drift alone producing identical allele frequencies in both regions
Allopatric speciation caused by complete geographic separation of subpopulations
Explanation
This question assesses the skill of analyzing speciation processes by evaluating divergence mechanisms in connected habitats. Sympatric divergence occurs through assortative mating, where females prefer males with coloration suited to local water conditions, combined with disruptive selection favoring extremes in clear versus turbid environments. This reduces gene flow despite movement, leading to reproductive isolation via prezygotic behavioral barriers and allele-frequency differences at relevant loci. Fewer mixed-genotype offspring than expected confirm isolation without a physical barrier. A tempting distractor is choice A, allopatric speciation, but this is wrong as no complete geographic separation exists, highlighting the misconception that divergence requires barriers rather than selection in sympatry. To approach similar problems, assess gene flow and selection pressures to differentiate sympatric from allopatric speciation.