Weapon size in animal species is under stabilizing selection, not open-ended directional selection

Evidence

Surprisingly few studies have actually measured the strength and nature of selection on extreme weapons—it’s incredibly labor-intensive to do. But those that have often find that as weapons get bigger and bigger the success of males rises up to a point, beyond which success begins to drop. Males with the very largest weapons tend to do worse than males with slightly smaller weapons. If males with the biggest weapons of all fared the best, selection would be “open-ended” and directional. The fact that the very biggest males did slightly worse shows us that selection in these populations is stabilizing, and it suggests these populations may be at or close to their balance point. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 850

Categorization

• XREF: Connects to the book’s broader balance-point/equilibrium theme about weapon costs, and to the evolutionary argument in previous chapters about escalation and coevolution. — connects to: The same three ingredients trigger weapon evolution across wildly divergent animal clades; Fortification-artillery escalation mirrors predator-prey arms races, not rival contests; Oscillating directional selection can produce net evolutionary stasis (+25 more)
• SEED: Angle for an essay: the biggest, most extreme traits often perform worse than slightly smaller ones — a counterintuitive principle that applies from beetle antennae to human overreach.

Related

• Extreme weapon size faces stabilizing, not open-ended, selection in real populations (similarity 0.95)
• Balanced selection keeps most animal weapons modest rather than extreme. (similarity 0.95)
• Natural selection usually favors modest weapon size because cost offsets benefit (similarity 0.94)
• Bigger weapons usually fail because their costs outweigh their benefits (similarity 0.92)
• Species with huge weapons often show weak current selection on them. (similarity 0.92)
• Balancing selection normally curbs weapon size until some lineages escape into runaway arms races (similarity 0.91)
• Reproductive success, not survival, drives the largest animal weapons (similarity 0.90)
• Sexual selection can be far stronger than natural selection and drives weapon extremes (similarity 0.89)
• Competition for reproduction drives evolution of nature’s biggest weapons (similarity 0.89)
• When balancing selection fails, bigger weapons win every round and escalate the race. (similarity 0.89)
• Weapon costliness keeps signals honest by excluding weaker males (similarity 0.89)
• Variation in animal weapon size is explainable by species ecology and behavior. (similarity 0.89)
• Each weapon-size increase resets the competitive baseline, ratcheting populations upward (similarity 0.88)
• Weapons evolve only when reproductive benefits exceed costs (similarity 0.88)
• Sexual selection drives weapons to extremes because payoffs for winners are enormous. (similarity 0.88)

Broader Connections

• Natural selection usually favors modest weapon size because cost offsets benefit #corroborates (0.80) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 21
• Animal weapons evolved to extremes far beyond practical size #contradicts (0.80) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 2
• Economic logic explains weapon evolution across virtually all animal species #contradicts (0.70) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 198
• Armor evolution tracks a benefit-cost trade-off identical to that shaping animal weaponry #expands (0.60) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 62
• Competition for reproduction drives evolution of nature’s biggest weapons #complicates (0.50) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 136
• Weapon size is favored when males defend resources females need #complicates (0.40) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 960