Well...neither. * Birth defects have (basically) two causes. #1 - unfortunate gene combination inherited from parents #2 - genetic mutation (i.e. the person's genes are, in some significant way, different from the genes of either of the parents) * In the case of genetic mutation - if the mutation is advantageous - that is: if gives the person who has it has an advantage in reaching maturity and reproducing - then it propagates and we have the possible beginnings of an instance of biological evolution. If it is advantageous enough that it (over several generations) becomes part of the genome of a significant portion of the population, then we have an actual instance of biological evolution (that is: the genome of the whole population changing significantly as a result of environmental pressures favoring that particular "new" gene). * If, on the other hand, the mutation is deleterious to reaching maturity and/or reproduction, then the "birth defect" is not evolution at all and the mutation would - under what we might call "primitive conditions" - not propagate significantly in the population. * Our society is sufficiently advanced (we have made the environment in which we live sufficiently "friendly") that birth *defects* - genomes that are deleterious to survival and reproduction - can actually survive and propagate. In more primitive situations this was typically not the case, and birth defects would naturally fail to propagate to a significant portion of the population and (therefore) naturally fail to result in biological evolution. * There are exceptions to this, but generally speaking and - importantly - statistically speaking. this is true. * So - for example - if a genetic mutation that someone has today helps them to be more successful at producing children, those children (and/or their descendants) are more likely to possess the same genetic mutation, and if it remains (over several generations) advantageous in reproduction it will eventually become a part of a significant portion of the population. * Now consider that each of us has (on average) between 50 and 150 genetic mutations when comparing our DNA to that of our parents. All of us are mutants. * If only 1 person in a particular generation (out of 5 billion people or whatever) has only one beneficial genetic mutation, and if that person survives to produce offspring, and they also, then - eventually - that particular mutation will become part of the human genome at large. * Now imagine we are talking about a million years, and millions of generations of humans. Even if only one person out of each generation has a mutation that is beneficial to survival and reproduction of the average human, then in a million years we're talking about millions of "new" beneficial genetic mutations entering the human population in a significant way. That "accumulation" of genetic changes in a population - once the accumulated changes are sufficiently different from the "original" population - eventually results in the evolution of a new species. It might take a million years - or 10 million years - for it to happen, but unless we change our environment so much that it is advantageous to only one (existing, H sapiens) genome, we should expect it to happen. * Not in our lifetimes. Not in our great-great-grandchildren's lifetimes. But eventually.