Oscillating directional selection can produce net evolutionary stasis

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 (#corroborates, via linked claim)

Such is the nature of most natural selection. Populations adapt to surrounding conditions until they reach a local optimum. Environments may change and, when they do, selection kicks in once again favoring new trait sizes or colors that work better in the new surroundings. But these shifts also are expected to stabilize. Ocean sticklebacks have sported three long spines and fifty-two armor plates for hundreds of thousands of years. When some of these fish found themselves dumped into freshwater habitats, natural selection in their new environment led to a rapid shift from three spines to one, and from many armor plates to few (fourteen plates). But once these populations reached their new optimum, the change in armor stopped. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 180 (#complicates, via linked claim)

Categorization

• XREF: connects to: Oscillating selection can produce stasis despite strong directional pressure — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 701; Natural selection stabilizes at local optima, as armor-plate shifts in sticklebacks show — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 180; Sexual selection is uniquely directional because the social environment itself evolves with the weapons (+2 more)

Related

• Oscillating selection can produce stasis despite strong directional pressure (similarity 0.96)
• Natural selection stabilizes at local optima, as armor-plate shifts in sticklebacks show (similarity 0.89)
• Sexual selection stays directionally constant while physical natural selection oscillates (similarity 0.89)
• Sexual selection is uniquely directional because the social environment itself evolves with the weapons (similarity 0.88)
• Social competition environments evolve in tandem with weapons, unlike fluctuating physical environments (similarity 0.85)
• Sexual selection can be far stronger than natural selection and drives weapon extremes (similarity 0.82)
• Weapon size in animal species is under stabilizing selection, not open-ended directional selection (similarity 0.81)
• Competition for reproduction drives evolution of nature’s biggest weapons (similarity 0.80)
• Rising predator pressure quickly enlarged lake stickleback armor (similarity 0.80)
• Balancing selection normally curbs weapon size until some lineages escape into runaway arms races (similarity 0.80)
• Coquetry’s power lies in suspended, non-committal oscillation between yes and no (similarity 0.80)
• Variation in animal weapon size is explainable by species ecology and behavior. (similarity 0.80)
• Species with huge weapons often show weak current selection on them. (similarity 0.79)
• Balanced selection keeps most animal weapons modest rather than extreme. (similarity 0.79)
• Natural selection usually favors modest weapon size because cost offsets benefit (similarity 0.79)

Broader Connections

• Oscillating selection can produce stasis despite strong directional pressure #corroborates (0.90) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 701
• 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
• Sexual selection is uniquely directional because the social environment itself evolves with the weapons #qualifies (0.50)
• Social competition environments evolve in tandem with weapons, unlike fluctuating physical environments #qualifies (0.50) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 182
• Weapon size in animal species is under stabilizing selection, not open-ended directional selection #complicates (0.50) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 850