i think it is competition; at least when considering survival......for example, on certain remote islands, bats no longer need to fly because there is less competition on the ground for insects as well as less danger from bigger animals......on other remote islands, where some food is scarce, birds (other than penguins) discovered that eating marine life helps, so they adapted to become swimmers.......

evolution is based on survival......even the mind has been considered as an evolutionary product of survival.....for example, animal A1 watches animal B eat fellow animal A2; therefore, animal A1 learns that animal B is a threat, and somehow through evolotion all animal A's know animal B is a threat