I'll preface this as usual by stating that I am an atheist with a strong acceptance of the scientific method, and this is in no way an attempt to "disprove" the theory of evolution.  I am also a layman in the field, however, and am hoping somebody with a little more knowledge can provide me with some elightenment.

It has been said repeatedly that the two fundamental concepts of evolutionary theory are random mutation and natural selection.  As I understand the concepts, this means that (1) random genetic mutations periodically occur in some (if not all) organisms and (2) if a mutation is beneficial to an organism's survival (or somehow makes it more attractive to the opposite sex, if you're talking about higher order organisms), that organism will be more likely to reproduce and pass its genes on to the next generation.  Thus, small changes accumulate over time and eventually lead to new species.

Hopefully I have stated that correctly.

Here comes the apparent paradox, however.  When asked why it's impossible to view evolution as it occurs, the answer is inevitably that it occurs on a very long time scale and that the individual mutations are too small to notice until they have had time to accumulate through many generations.  If the individual mutations are so small, however, how can any of them actually contribute to the survivability or selection of the organism?  I mean, it's easy to think about a gross mutation like an extra finger or new skin coloration affecting my chances of mating and passing along my genes, but how would a mutation too subtle to be detected affect whether I am able to find a mate or not?

Please help me understand what I am missing here.