Rising predator pressure quickly enlarged lake stickleback armor

Evidence

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 (#exemplifies, via linked claim)

Today, sticklebacks in many lakes lack defensive weapons. Dolph Schluter and his students at the University of British Columbia found that lake habitats have far fewer predators than marine habitats, and this appears to relax the pattern of natural selection for larger plates and longer spines. With fewer predators, lake fish benefit less from large weapons than marine fish. Armor also costs more in lakes than it does in the ocean. Low freshwater concentrations of the ions necessary for bone growth mean that fish pay a higher price for mineralizing bony plates in lakes. Unarmed sticklebacks are larger as juveniles and begin breeding sooner than their armed counterparts. In freshwater, it appears, the costs of long spines and large plates are steeper than the benefits they provide. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 56 (#expands, via linked claim)

Categorization

• XREF: connects to: Lake sticklebacks repeatedly gained then lost their defensive weapons; Natural selection stabilizes at local optima, as armor-plate shifts in sticklebacks show — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 180; Weapons shrink when predator pressure falls and armor gets costly. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 56 (+1 more)

Related

• Natural selection stabilizes at local optima, as armor-plate shifts in sticklebacks show (similarity 0.93)
• Weapons shrink when predator pressure falls and armor gets costly. (similarity 0.92)
• Stickleback fossils document armor gain and loss across one hundred thousand years. (similarity 0.87)
• Oscillating directional selection can produce net evolutionary stasis (similarity 0.80)
• Ambush predators evolve large weapons because they strike rather than chase prey (similarity 0.80)
• Each advance in weapon metallurgy forced a corresponding advance in protective armor (similarity 0.79)
• Armor evolution tracks a benefit-cost trade-off identical to that shaping animal weaponry (similarity 0.78)
• Fortification-artillery escalation mirrors predator-prey arms races, not rival contests (similarity 0.78)

Broader Connections

• Natural selection stabilizes at local optima, as armor-plate shifts in sticklebacks show #exemplifies (0.75) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 180
• Weapons shrink when predator pressure falls and armor gets costly. #expands (0.65) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 56
• Fortification-artillery escalation mirrors predator-prey arms races, not rival contests #expands (0.35)