AKA "Natural Selection, You Sly Dog!"
Note: extremely incomplete. Mostly intended to be a dumping ground for these curiosities as I stumble upon them.
Amoeba eats paramecia:
Hermit craps line up to swap shells:
Putrid meat is putrid (toxic) because the bacteria release toxins to keep other scavengers (like us) at bay. More here.
Orchids mimic other flowers, lure bees without providing any nectar.
Erodium seed drills itself into the earth:
Parthenogenesis (asexual reproduction among animals). See also.
Mantis shrimp. See The Oatmeal's take.
Immortal jellyfish. "It is unique in that it exhibits a certain form of "immortality": it is the only known case of an animal capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage."
Gladiator spider, spins a net rather than a web:
Golden wheel spider does cartwheels to evade predatory wasps:
Hermit crabs lining up to swap shells:
Naked mole-rats and Damaraland mole-rats — only two known species of eusocial mammals.
From Sarah Perry (Gardens Need Walls):
Islands are surrounded by natural boundaries. Strange varieties of life arise on islands that are not seen anywhere else, precisely because they are cut off from the densely interconnected systems present in the ocean and on large land masses. Islands, small systems surrounded by a natural ocean boundary, give life the opportunity to try amazing stunts that would be impossible on a large landmass. Drifting daisies evolve into trees; drifting iguanas learn to swim; fruit flies evolve into fantastic, showy varieties; crabs grow a meter long and figure out how to open coconuts. Tree kangaroos, ground parrots, and giant tree skinks are niches only available on islands. And when the boundary opens (often because of that great destroyer of natural boundaries, humans), the unique species on islands are often out-competed by species that evolved in the diversity-flattening zones of great land masses. Boundaries drive diversity. [emphasis mine]
Mating and sexual selection
Flamingo mating dance:
Pufferfish "crop circles" (not certain this is sexual selection):
Père David's deer. "During the rut, stags decorate themselves with garlands of vegetation, collected in their antlers."
Surgeons discover woman's embryonic twin lodged deep inside her brain.
Supernumerary body parts. Polycephaly. Janus cats (diprosopus). Conjoined twins.
Genetic chimeras (including some humans). Basically two genetic creatures fused into one — kind of like conjoined twins that are fully-conjoined, swirled together. BBC write-up.