Sometimes people think of evolution as a binary thing - you reproduce or you don't. But a few generations on, some people leave huge numbers of descendants and some don't. There isn't a huge variance in the number of children people have, but that gets multiplied for grandchildren, and again for great-grandchildren...so the scale is not 0 to 1 (or 2 or 3), but 0 to <big number>. I wouldn't make any sweeping statements a priori about how economic inequality affects that, but if some people have all their resources vacuumed up and some don't, it's going to affect their lives on average somehow.