Evolution - thought...

Okay, so my understanding of the theory of evolution is that in a given environment, certain traits are better suited to survival, and therefore organisms with those traits will tend to survive longer and therefore have more opportunites to reproduce, thus resulting in more organisms with those traits as compared to organisms having less advantageous survival traits.
Now, when thinking of humans, we don't have a lot of variation in general traits at birth. So my thinking is that if something really bad happens (global environment changing) that could lead to a mass near-extinction, I don't think we are going to be able to adapt fast enough as a species... But then I had a thought... In any environment changing situation, the surviving organisms will continue to struggle and reproduce. Assuming that the population is now much reduced, there would likely be a higher potential for inbreeding. This may lead to an acceleration of enhancing beneficial traits since the gene pool is biased towards survival traits, but could also lead to more "birth defects" which could result in an accelerated rate-of-change in the general structure and development of new organisms..... just a thought....