Based on context, I think an evolutionary extremist is a person who holds that the theory of evolution is a reasonable explanation to explain the diversity of life on the planet. I envision an evolutionary extremist as one of those ideologues that examines the world around them comes up with some plausible ideas, sees what works, revises their ideas, and repeats as necessary. At the end of the day, this person has a slightly more lucid understanding of themselves and the world with live in. The converse of this (based on the links that use the term) is a christian fundamentalist that believes the bible as explained to them by their pastor is the inerrant word of
So, I understand much about the theory of evolution, I also understand much of biblical creation. One works in my life on a day-to-day basis and one doesn't. Since the one that works contradicts the narrow viewpoint some people have regarding the bible, I am the extremist. See, confidence in a theory with explanative and predictive power over the ultra-specific interpretation in a collection of a subset of ancient stories with no explanative/predictive power makes one an extremist.
I know extremist is a word with significant negative connotations, but I think in this case I'll take on that mantle.
Oh and for all you keeping score at home only 57,826 more websites need to state "Hitler was an atheistic darwinian evolutionary extremist." before it will indeed become true. Oops. 57,825....way to go!