I think it's been a while since I've seen or heard anyone mention that the world is overpopulated. It is actually the opposite. There is a sharp decline in birth rates in a lot of countries, and many are worried about it.
I think the term survival of the fittest shouldn't be taken literally. I like to interpret it as those fit to survive in a given time. In prehistoric times, it is those that can hunt and gather, and make healthy children. As time passed, physicality became less and less important. Intelligence slowly became the necessary factor. With the advancements in technology, I wouldn't be surprised if it changed to creativity and ingenuity.