'Contributing to the gene pool' is a very vague phrase to use. I clarified what I meant to make the definition more specific. Under natural selection the 'purpose' (if you will) of living is to reproduce as much as you can and leave behind as many offspring as possible. Now, you can get into genetic theories and rationalize that, every offspring will have slightly different genomes due to genetic recombination and spontaneous mutations, which can have good, bad, or neutral consequences in terms of future biological fitness (the ability to produce offspring). But it seems reasonable to me that given a diverse gene pool (which is facilitated by increased biological fitness), the population as a whole would be more likely to survive some calamity or other.

I don't understand where the tautology is in that.