Two species of a plant cannot interbreed, because their flowers differ in size and shape and require pollination by different species of bee. This is a/an ___ reproductive barrier.

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In this question, we’re asked to classify the type of reproductive barrier preventing interbreeding between two plant species based on how their flowers differ.
Option a: gametic. This barrier occurs when gametes are incompatible at the molecular level, such as sperm and egg receptor mismatches, or pollen cannot fertilize ovules due to biochemical differences. Here, the issue is not at the gamete recognition stage but at the physical interaction between flowers and pollinators, so gametic isolation does not fit.
Option b: temporal. Temporal isolation happens when species breed at different times (days, seasons, or years), so even if they encounter each other, reproduction does not occur. The scenario describes differences in flower size and shape affecting pollination agents, not timing of reproduction, so temporal is not appropriate.
Option c: ecological. Ecological isolation involves differences in habitat or microhabitats that reduce encounters, such as one species living in a different environment. The description focuses on pollinator compatibility with floral morphology, not on habitat separation, so ecological isolation is not the best match.
Option d: mechanical. Mechanical isolation arises when physical differences prevent successful mating or pollination—for plants, the floral structures may be incompatible with the pollinators or with the reproductive organs of the other species. Since the flowers differ in size and shape and require different bee species for pollination, the mechanical barrier best explains the situation.
Option e: behavioral. Behavioral isolation involves differences in mating signals or behaviors that prevent interbreeding, such as courtship rituals. In plants, pollinator preferences are a factor, but the key detail here is the mechanical fit between flower morphology and pollinator, not behavioral cues of the plants or pollinators during mating attempts.
Thus, evaluating each option, the description aligns most closely with a mechanical reproductive barrier, where physical floral differences impede effective pollination by the appropriate pollinator species. The other options fail to capture the essential mechanical incompatibility emphasized by flower size/shape and pollinator specialization.

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Two species of a plant cannot interbreed, because their flowers differ in size and shape and require pollination by different species of bee. This is a/an ___ reproductive barrier.

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