Same constraint, same solution.
Each environment selects a small set of viable forms. Anyone who arrives is shaped into one of them. The list under each environment is what got shaped, and how.
Deep ocean (cold, dark, high-pressure)
Almost no light. Sparse, unpredictable food. Crushing pressure. The body must locate food with non-visual cues, conserve energy, and not collapse.
Anglerfish · firefly squid · lanternfish · ostracods (independently)
Gulper eel · viperfish · dragonfish (multiple)
Greenland shark · orange roughy · giant tube worms
Many cave-fish parallel; some hadal isopods
Hot desert (water-scarce, high UV, hot/cold cycles)
Lose water and you die. Get too hot in the day, too cold at night. Food and shade are spotty. The body must hoard water, dump heat, and time activity.
American cacti · African euphorbias (independent of cacti)
Kangaroo rats · jerboas · gerbils · fennec foxes
Multiple desert mammals; many reptiles
Saharan silver ants (cuticular hairs) · pale desert reptiles
Polar / extreme cold
Lose heat slower than you generate it, or die. Liquid water is rare. Food is sparse for half the year. The body must insulate, antifreeze, and hibernate.
Antarctic icefish (Notothenioidei) · Arctic cod (independently)
Penguins · whales · seals · arctic mammals (multiple)
Polar bear · seal · penguin · musk ox
Arctic fox · ermine · ptarmigan · snowshoe hare
Dark cave systems
No light. Stable temperature. Sparse food. Selection switches off the visual system and amplifies non-visual senses.
Mexican cavefish · olm · cave crayfish · cave isopods (many independent origins)
Cave crickets · whip spiders · cave fish barbels
Olm · most cave-adapted fish
Oilbird · cave swiftlets · some bats
High altitude
Less oxygen per breath. More UV. Cold. The body must extract more O₂ per breath and tolerate hypoxia.
Andean condor · bar-headed goose · Tibetan humans · Ethiopian highlanders (independent)
Llama · vicuña · yak · highland deer mice
Snow leopard · Himalayan tahr · Andean rodents
Parasitic life
The host gives you everything. So selection strips away anything you don't need: legs, eyes, gut, brain. The shape collapses toward the simplest form that can find a host and reproduce.
Tapeworms · barnacle parasites of crabs (Sacculina) · root parasitic plants
Tapeworms · flukes · lamprey · leech
Helminths broadly; Sacculina; brood parasitic birds
Toxoplasma · cordyceps fungi · parasitoid wasps · hairworms
Fast aerial pursuit
Catch a moving target in three-dimensional air. The body must produce thrust, lift, and instant control corrections, all at the same time, against drag.
Falcons · swifts · dragonflies · pterosaurs
Birds of prey · dragonflies (compound-eye equivalent)
Dragonflies · falcons · bats hunting moths