Q. 104 The migratory desert locust, Schistocerca gregaria, exists in two mutually exclusive forms: a shortwinged, uniformly colored, solitary insect and a long-winged, brightly colored, gregarious morph. These
phenotypes depend on crowding. Such phenotypic plasticity is called
(A) reaction norm.
(B) polyphenism.
(C) Batesian mimicry.
(D) polymorphism.
This article dives into the fascinating world of phenotypic plasticity in the migratory desert locust (Schistocerca gregaria), tackling a key multiple-choice question from biology exams. If you’re studying genetics, entomology, or evolutionary biology, you’ll appreciate how crowding triggers dramatic changes in locust forms—from solitary short-winged individuals to gregarious long-winged swarms.
Locusts like Schistocerca gregaria showcase remarkable adaptability, switching between a solitary, short-winged, uniformly colored form and a long-winged, brightly colored gregarious phase based on population density. This crowding-dependent shift raises a classic question in phenotypic plasticity:
Correct Answer: (B) Polyphenism
Polyphenism perfectly describes this phenomenon. It refers to the development of discrete, alternative phenotypes from the same genotype in response to distinct environmental cues, like crowding in locusts. In S. gregaria, low-density conditions produce the solitary morph, optimized for independent living with camouflage coloring and shorter wings. High-density triggers serotonin release and behavioral changes, leading to the gregarious morph—longer wings for swarming, black patterns for group signaling, and increased appetite fueling plagues.
This isn’t gradual variation; it’s a sharp, binary switch, making polyphenism the precise term. Studies, such as those on phase polyphenism in locusts, highlight its evolutionary role in survival amid unpredictable desert resources.
Explanation of All Options
To clarify why the others don’t fit, here’s a breakdown:
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(A) Reaction Norm: This describes the continuous range of phenotypes a single genotype produces across a gradient of environmental conditions. Think of plant height varying smoothly with soil nutrients. Locust morphs are discrete (two distinct forms), not a spectrum, so reaction norm doesn’t apply. It’s broader plasticity, often quantitative.
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(B) Polyphenism: Correct. As noted, it captures environmentally induced, discrete morphs like locust phases, butterfly wing patterns under predation pressure, or caste systems in social insects (e.g., honeybee queens vs. workers). No genetic difference needed—just triggers like density.
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(C) Batesian Mimicry: This is mimicry where a harmless species imitates a toxic one to avoid predators, like the viceroy butterfly resembling the monarch. It’s anti-predator defense via resemblance, unrelated to crowding or morph switches in locusts. No plasticity here; it’s evolved appearance.
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(D) Polymorphism: Genetic-based variation within a population, producing multiple distinct forms maintained by selection (e.g., peppered moth color variants or blood types). Locust forms aren’t genetically fixed—they’re reversible and environmentally driven—so this misses the plasticity aspect.
| Option | Key Feature | Matches Locust Morphs? | Example |
|---|---|---|---|
| Reaction Norm | Continuous phenotypic range | No (discrete forms) | Height in response to light |
| Polyphenism | Discrete, environment-induced morphs | Yes | Locust solitary vs. gregarious |
| Batesian Mimicry | Harmless species mimics dangerous one | No (not mimicry) | Non-toxic snake resembling coral snake |
| Polymorphism | Genetic multiple forms | No (environment-driven) | Ladybug spot patterns |
Why This Matters in Biology and Research
Phenotypic plasticity like polyphenism helps organisms thrive in variable environments, a hot topic in climate change studies—denser locust swarms could worsen plagues. For students prepping for exams (NEET, CSIR NET, or entomology courses), mastering these terms builds a strong foundation in evolutionary developmental biology (evo-devo).
Understanding desert locust polyphenism also informs pest control: breaking the gregarious phase cycle via pheromones or RNAi targets serotonin pathways.
