Progressive evolution is development of organisms with more elaborate and specialised structures from those having less elaborate features e.g. amphibians from reptiles. Regressive or degenerative evolution is development of simpler forms from more complex ones. Such evolution has occurred in case of vestigial organs, parasitic forms, and in reduction of over specialised structures such as wings in flightless birds. Parallel evolution is formation of similar trains in related groups of organisms independently due to similar requirement e.g. running of two toed deer and one toed Horse. Evolution of wings in insects and birds serve as example of convergent evolution