Balanced selection keeps most animal weapons modest rather than extreme.

Evidence

Huge animal weapons look too awkward to be favored by selection, and in the vast majority of cases, this appearance isn’t misleading. Big weapons are ungainly, and most individuals fare poorly with them. For most weapons in most species, selection favors modest size and minimal cost—enough tooth to bite or grab prey, for example, but not enough to slow you down or impair your ability to maneuver. What this actually means is that selection on weapons is balanced: bigger weapons may be better for stabbing or biting, but they are also more expensive to produce and more difficult to carry. The result is a tenuous balance, much like a game of tug-of-war, which accounts for the multitude of unimpressive weapons adorning the vast majority of animal species. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 21 (#corroborates, via linked claim)

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 (#corroborates, via linked claim)

Yet, every so often this balance is tipped. Here and there, sprinkled across the tree of life, are animal lineages where modesty of proportion was cast aside. The evolution of weapons in these species surged forward unencumbered. Gone were the shackles of balancing selection, and all that remained was selection favoring bigger and bigger weapon sizes. Here, individuals with the most grotesque or extreme armaments beat opponents with smaller weapons. In so doing, they secured opportunities to breed. Their progeny, as impressive in their weapons as their parents, quickly replaced earlier forms and advanced the population another notch in weapon size. As soon as the next new innovation arose—an even larger or more complex variant of the existing design—the process repeated itself. Again and again, individuals with the newest and biggest weapons won, replacing those with earlier and smaller forms, and this process pushed the population still further on the path to extreme. In short, the weapons of these species got caught up in an arms race. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 22 (#expands, via linked claim)

Categorization

• XREF: connects to: Some elaborate animal weapons are astonishingly cheap to produce and carry; Natural selection usually favors modest weapon size because cost offsets benefit — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 21; Balancing selection normally curbs weapon size until some lineages escape into runaway arms races — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 22 (+6 more)

Related

• Weapon size in animal species is under stabilizing selection, not open-ended directional selection (similarity 0.95)
• Natural selection usually favors modest weapon size because cost offsets benefit (similarity 0.94)
• Extreme weapon size faces stabilizing, not open-ended, selection in real populations (similarity 0.94)
• Species with huge weapons often show weak current selection on them. (similarity 0.93)
• Balancing selection normally curbs weapon size until some lineages escape into runaway arms races (similarity 0.92)
• Bigger weapons usually fail because their costs outweigh their benefits (similarity 0.91)
• When balancing selection fails, bigger weapons win every round and escalate the race. (similarity 0.91)
• Reproductive success, not survival, drives the largest animal weapons (similarity 0.90)
• Sexual selection drives weapons to extremes because payoffs for winners are enormous. (similarity 0.90)
• Sexual selection can be far stronger than natural selection and drives weapon extremes (similarity 0.89)
• Weapon costliness keeps signals honest by excluding weaker males (similarity 0.88)
• Each weapon-size increase resets the competitive baseline, ratcheting populations upward (similarity 0.87)
• Arms races inevitably stall as weapon costs neutralize reproductive benefits (similarity 0.87)
• Dueling plus competition and defensibility is the general rule for extreme weapons (similarity 0.87)
• Markets and battlefields select among rifle designs like natural selection acts on weapons (similarity 0.87)

Broader Connections

• Natural selection usually favors modest weapon size because cost offsets benefit #corroborates (0.90) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 21
• Weapon size in animal species is under stabilizing selection, not open-ended directional selection #corroborates (0.80) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 850
• Balancing selection normally curbs weapon size until some lineages escape into runaway arms races #expands (0.75) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 22
• Animal weapons evolved to extremes far beyond practical size #contradicts (0.70) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 2
• Sexual selection can be far stronger than natural selection and drives weapon extremes #qualifies (0.60) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 177
• Armor evolution tracks a benefit-cost trade-off identical to that shaping animal weaponry #expands (0.55) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 62
• Reproductive success, not survival, drives the largest animal weapons #contradicts (0.50)
• Releasing soldiers from other tasks lets weapons evolve to extremes #contradicts (0.50) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 128