Actually, it isn't. Evolution is just change in allele frequencies over time. That can be caused by selection, which can take a number of forms including natural, sexual and artificial.
This renders the term 'evolution' meaningless since it easily accomodates populations accumulating deleterious mutations.
This would allow evolutionists to claim that populations that are in catastrophic error-catastrophe are 'evolving' when they are actually in genetic meltdown.
Coincidental?