This is what happens when two species, or two body parts in different species, evolve to look similar.


  • many of the Australian mammals look like counterparts in other parts of the world, even though, unlike these, they are all marsupials;
  • before the mammals existed, giant birds took the top of the food chain in some places
  • before mammals and birds outcompeted them (a process that is still continuing), many types of reptiles existed that look much like certain modern birds or mammals
  • eyes with very similar designs have provably developed independently in very different types of animals, but the set of designs in actual use is limited
  • the sabretooth tiger developed and died out in North America at least 4 times (each time from smaller species of felines)
  • vertebrates have a tendency to learn how to fly, and the results are limited by functional constraints: Pteranodon looks much like a pelican in overall shape, and its wings are much like a bat's

Apparently, function dictates form, and environmental stability dictates function.