As a rule, convergent evolution occurs when two groups of largely unrelated organisms are exposed to very similar environments and develop similar adaptations to survive. According to Princeton University, the evolution of the wing among bats and birds is convergent, as their last common ancestor was wingless.
Convergent evolution can be observed among groups separated geographically, as is the case with North American cactus species and the African Euphorbia. Another example can be found with sharks, icthyosaurs and dolphins. In each case, according to Princeton University, the superficial similarities in body plan are largely the result of common environmental pressures acting on the species to drive it toward an optimum solution.