Oscillating selection can produce stasis despite strong directional pressure

Evidence

The Grants showed that year-to-year fluctuations in rainfall led to dramatic shifts in the types and amounts of seeds available to the birds, and this resulted in selection favoring deep beaks in some years but thin beaks in others. Although natural selection was directional and strong for most years of this period, the pattern of selection oscillated, so that the net effect was stasis. Despite multiple bouts of rapid change, birds at the end of the sample period had roughly the same bill shapes as those at the beginning. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 701

Categorization

• XREF: Directly relevant to the book’s core puzzle: why weapons don’t endlessly escalate despite strong sexual selection; connects to Grant’s finch work — connects to: Oscillating directional selection can produce net evolutionary stasis; Sexual selection stays directionally constant while physical natural selection oscillates — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 185; Natural selection stabilizes at local optima, as armor-plate shifts in sticklebacks show — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 180 (+4 more)

Related

• Oscillating directional selection can produce net evolutionary stasis (similarity 0.96)
• Sexual selection stays directionally constant while physical natural selection oscillates (similarity 0.90)
• Sexual selection is uniquely directional because the social environment itself evolves with the weapons (similarity 0.87)
• Social competition environments evolve in tandem with weapons, unlike fluctuating physical environments (similarity 0.86)
• Sexual selection can be far stronger than natural selection and drives weapon extremes (similarity 0.85)
• Weapon size in animal species is under stabilizing selection, not open-ended directional selection (similarity 0.84)
• Competition for reproduction drives evolution of nature’s biggest weapons (similarity 0.83)
• Natural selection stabilizes at local optima, as armor-plate shifts in sticklebacks show (similarity 0.83)
• Species with huge weapons often show weak current selection on them. (similarity 0.83)
• Balancing selection normally curbs weapon size until some lineages escape into runaway arms races (similarity 0.83)
• Coquetry’s power lies in suspended, non-committal oscillation between yes and no (similarity 0.82)
• Variation in animal weapon size is explainable by species ecology and behavior. (similarity 0.81)
• Balanced selection keeps most animal weapons modest rather than extreme. (similarity 0.81)
• Natural selection usually favors modest weapon size because cost offsets benefit (similarity 0.81)
• Each weapon-size increase resets the competitive baseline, ratcheting populations upward (similarity 0.81)

Broader Connections

• Sexual selection stays directionally constant while physical natural selection oscillates #complicates (0.70) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 185
• Sexual selection is uniquely directional because the social environment itself evolves with the weapons #complicates (0.65)
• Natural selection stabilizes at local optima, as armor-plate shifts in sticklebacks show #complicates (0.60) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 180
• Social competition environments evolve in tandem with weapons, unlike fluctuating physical environments #complicates (0.60) — 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 (0.60) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 850