But we are discussing this with a creationist who claims that "all", "most", "many" or some such portion of mutations are harmful.
They should be asked to supply evidence to back those claims.
It may be fair to say that a large percentage of the mutations that change amino acid sequence are deleterious, and that is often where the misunderstandings begin.
If we are born with 50 mutations, and 3% of the genome is translated into protein, this would mean that only 1 or 2 mutations occur in the coding portion of those genes. Each one would have about a 70% chances of changing the amino acid sequence, so about 1 per person. Already, we see that only 2% of mutations (at least for common substitution mutations) really have a chance of being seriously deleterious.
The real deleterious mutations are probably indels and recombination events that happen in coding regions. Indels that change the number of bases in a gene by a number indivisible by three will change more than just one amino acid due to a frame shift, as one example.
If the failure of a large percentage of pregnancies is actually due to mutations it demonstrates the power of selection to weed out those mutations.
My own suspicion is that this isn't the case. We are a somewhat promiscuous species, so there is selective pressure to keep our fecundity a little lower. The mutations that may be responsible for the rate of spontaneous abortions in humans are probably already present in the mother and father, and have been selected for. Again, this is my own speculation and I really don't have anything to back it up with just yet.
Edited by Taq, : No reason given.
Edited by Admin, : Fix spelling: promiscious => promiscuous