Sexual selection is uniquely directional because the social environment itself evolves with the weapons

Evidence

This is not how sexual selection works. In battles for access to reproduction, males compete with rival males. The environment that matters for performance is a social one—other males with whom a male does battle—rather than temperature or sea level or other physical features of the landscape. And this social environment evolves in tandem with the weapons. As antlers or horns become bigger, so, too, does the standard against which a male must contend. Like a sliding scale, each increase in weapon size resets the baseline of the population, selecting, in turn, for yet another increase. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 182 (#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 (#contradicts, via linked claim)

Because social environments evolve in tandem with increases in weapon size, sexual selection can push populations along a path of unending directional change. Selection in this social context is not likely to oscillate. Measure selection in a beetle population now, measure it in a decade, measure it one thousand years in the future, and you’re likely to find the same thing: males with the biggest horns win. The particular weapon size performing best will change (one-half-inch horns give way to three-fourths-inch horns, and then to one-inch horns, and so on), but the direction of selection remains constant. All else being equal, this form of selection is going to generate a lot more change than selection oscillating back and forth or holding fast. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 185 (#expands, via linked claim)

Categorization

• XREF: connects to: Operational sex ratio skew measures the intensity of sexual selection — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 688; Oscillating directional selection can produce net evolutionary stasis; Sexual selection drives weapons to extremes because payoffs for winners are enormous. (+7 more)

Related

• Sexual selection stays directionally constant while physical natural selection oscillates (similarity 0.95)
• Social competition environments evolve in tandem with weapons, unlike fluctuating physical environments (similarity 0.94)
• Sexual selection can be far stronger than natural selection and drives weapon extremes (similarity 0.92)
• Sexual selection drives weapons to extremes because payoffs for winners are enormous. (similarity 0.88)
• Oscillating directional selection can produce net evolutionary stasis (similarity 0.88)
• Oscillating selection can produce stasis despite strong directional pressure (similarity 0.87)
• Weapon size in animal species is under stabilizing selection, not open-ended directional selection (similarity 0.86)
• Competition for reproduction drives evolution of nature’s biggest weapons (similarity 0.85)
• Extreme weapon size faces stabilizing, not open-ended, selection in real populations (similarity 0.85)
• Each weapon-size increase resets the competitive baseline, ratcheting populations upward (similarity 0.85)
• Reproductive success, not survival, drives the largest animal weapons (similarity 0.84)
• Natural selection usually favors modest weapon size because cost offsets benefit (similarity 0.84)
• Balanced selection keeps most animal weapons modest rather than extreme. (similarity 0.83)
• Balancing selection normally curbs weapon size until some lineages escape into runaway arms races (similarity 0.83)
• Extreme skew in mating success drives arms-race escalation in weaponry (similarity 0.83)

Broader Connections

• Social competition environments evolve in tandem with weapons, unlike fluctuating physical environments #corroborates (0.85) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 182
• Weapon size in animal species is under stabilizing selection, not open-ended directional selection #contradicts (0.80) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 850
• Sexual selection stays directionally constant while physical natural selection oscillates #expands (0.75) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 185