Physiological costs of expressing sexually selected traits (for example, elevated thermal loads) can oppose natural selection that would otherwise favor smaller or less ornamented phenotypes, especially under environmental stress.
NOT BSTOTAL BS
NOT BS — Verdict: True
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The Short Version
The evidence strongly supports this evolutionary trade-off. Multiple peer-reviewed studies show that sexually selected traits can impose thermal, metabolic, immune, or fertility costs, and these costs often become more important under environmental stress. Cases where species evolve compensatory heat tolerance do not refute the point; they confirm that the traits can carry real physiological burdens.
Caveats
The claim is conditional, not universal: some species evolve compensatory mechanisms that reduce or offset these costs.
Most evidence comes from particular taxa and experimental systems, so the strength and form of the trade-off can vary across species and environments.
“Oppose natural selection” here means creating survival or stress-resistance trade-offs, not that sexual selection always loses or that ornamented traits are always maladaptive.