I guess the main point of evolution is to capitalise on variants that give us an edge over natural predators. At the moment, the only natural predator that faces us is the environment (and Otto's asteroid), so my prediction is that darker skin will win out at the expense of lighter skin to cope with increased UV radiation (we will also become generally more coffee-coloured as the world becomes more mixed anyway, but that isn't evolution (is it ?)). Maybe a hotter world means we will need to develop a greater surface area to sweat it all out, and we could get covered in fleshy folds to achieve this - We will also need to develop different breathing mechanisms to cope with the pollution, so maybe we'll absorb air through the filter of our skin rather than breathing in in future too..
If water levels continue to rise, and the earth becomes uninhabitable, I suppose we could go full circle and turn back into fish again ?