You claim it is their "basis for existence". How would they exist without it? You didn't say so explicitly, but it is a logical consequence of what you did say. If your statements will not bear examination of their logical consequences, what good are they?
If you can't be forthright about what you're actual qualm is, why should I take it seriously?
That being said, I would indeed say that, without evolutionary philosophy, much of modern Communism and Nazism wouldn't have been possible, at least not in the forms they took. Marxism relies upon the assumption that "progressive scientism" will bring about the "inevitable" establishment of a classless society, once the dialectical middleman is gotten through. This is viewed as being more or less the natural order of things, hence the aura of inevitability that shrouded foundational Marxist thought. Anything that hinders the establishment of this end result, at least in revolutionary Communism, is fair game for being eliminated as unfit, regressive, and retrograde. The philosophy that begat all this, and the resultant attempts to eliminate millions of kulaks, capitalists, and other reactionaries, is the notion that the world is constantly evolving, progressing upward, that change is inevitable, and that those who are unfit deserve to be removed from the equation by the "New Men" who are better fitted for the glorious classless society of the future. Hence, yes, there is an underlying evolutionary aspect to Marxist thought that in many ways mirrors the evolutionary theory of the natural world which later inspired Darwin to refine the theory in his On the Origin of Species, and it is doubtful that Marxism would have had this aspect in its dialectic had there not been intellectual injection from the evolutionary philosophy sweeping Europe at the time.
Likewise, it is doubtful that Nazism would have gotten the idea of exterminating "inferior" groups as unfit, or the rest of its overall eugenic programme, had not the idea appeared to be completely natural and wholesome per naturalistic evolution.