Q.104 Like any other trait, animal behavior also evolves by natural selection. Which one of the following
examples is NOT true with regard to the evolution of behavior by natural selection?
(A) The behavioral trait is determined only by genes
(B) The behavioral trait varies within the population of that species
(C) The reproductive success partly dependent upon the behavioral trait
(D) The behavioral trait is influenced by the genotype
Animal behavior evolves natural selection through heritable variation affecting fitness, as in Darwin’s finch song or spider web-building. This GATE ecology question tests core evolutionary principles: behavior isn’t “only genes” due to environment/learning.
Option Analysis
(A) The behavioral trait is determined only by genes
Incorrect (NOT true). Behaviors have genotype + environment (G×E) basis—e.g., identical twin songbirds develop different songs if reared apart. Instincts (maturation) mix with learning (conditioning); pure genetic determinism violates modern synthesis.
(B) The behavioral trait varies within the population of that species
Correct requirement. Polymorphism essential—e.g., hawk-dove strategies in game theory, where aggressive vs. passive males show frequency-dependent selection.
(C) The reproductive success partly dependent upon the behavioral trait
Correct. Fitness link mandatory—e.g., peacock tail displays boost mating success; poor dancers get fewer offspring.
(D) The behavioral trait is influenced by the genotype
Correct (narrow-sense heritability). Genes underlie variation—e.g., foraging path in Drosophila mutants; selection acts on this genetic component despite plasticity.
Four Conditions for Evolution by NS
From conversation context (prior ecology Q93), natural selection needs:
| Condition | Example | Fulfilled? |
|---|---|---|
| Variation | Spider web styles | ✓ (B) |
| Heritability | Offspring mimic parents | ✓ (D) |
| Differential Fitness | Better webs = more prey | ✓ (C) |
| Genetic Basis Only | ❌ Learning ignored | ✗ (A) |
Why A Fails: Nature-Nurture Reality
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Fixed Action Patterns (egg-rolling geese): Genetic, but triggered by sign stimuli.
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Imprinting (ducklings): Genetic window, but environmental target.
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Tool use (chimps): Innate predisposition + cultural transmission.
Modern ethology (Tinbergen) dissects causation, development, evolution, function—none claim “only genes.”
GATE Life Sciences Strategy
Recall Tinbergen’s 4 Whys + Darwin’s prerequisites. PYQs pair with kin selection (Hamilton), game theory (Maynard Smith). Trap: confusing innate with genetic-only—behavior evolves via h² (narrow heritability), not h=1.
Answer: (A) – Modern evolutionary biology rejects genetic determinism for complex behaviors.
