Cytachrome C and neutral drift

First, just because all versions of cytochrome C do basically the same job, it does not follow that there are no functional differences between them. For example (based on zero knowledge of the details of this particular protein), two versions might function best at two different temperatures. So more than drift might be involved -- but most of the observed differences probably are due to drift.Second, cytochrome C is a highly conserved gene, which means that relatively few nonsynonymous changes in its sequence are neutral, so you should see polymorphic sites pretty infrequently.Third, what makes you think that none of these sites are polymorphic? Looking at the UCSC genome browser, I see a dozen or more nonsynonymous single nucleotide polymorphisms in humans for the coding sequence of CYCS.

Note that of the variant sites listed for the human CYCS coding sequence, not one has been validated, i.e. seen more than once. The detection method used for finding all of these SNPs is not the most reliable, so it is likely that some of these sites are not really variant, and possible that none of them are. Keep in mind that CYCS is a small gene, with only 312 base pairs of sequence. Even if there were no constraints, you'd only expect to find one reasonably high frequency (> 1%) variant in such a small bit of sequence; in coding sequence there's no reason to expect even one.