an isolated population with unique selective pressures resulting in evolutionary divergence from the mainland population
I would suggest a different understanding: Much of what's happened in critical theory (especially the parts more visible to "outsiders") is deeply embedded in "mainland" contexts, including, most importantly for critical theory, being embedded in the expansion of higher education in the USA after the GI Bill and the long tradition of "pragmatic" thinking in mainstream US thought that may find its roots in Protestantism and flower in the mythic "American" "everyman".