If every member of a population has a new ‘gene’, than it has almost become another species (and so cut that population off the general pool, so that the positive effects of the gene might be off-set by a ‘black swan’:). If you mean ‘a new allele of an existing gene’, then it might exist for thousands of generations without becoming much more wide-spread. Consider albinism. Is it ‘fixated’ in humans?
If every member of a population has a new ‘gene’, than it has almost become another species (and so cut that population off the general pool, so that the positive effects of the gene might be off-set by a ‘black swan’:). If you mean ‘a new allele of an existing gene’, then it might exist for thousands of generations without becoming much more wide-spread. Consider albinism. Is it ‘fixated’ in humans?