Where large non-beneficial changes occur in the "important" DNA, the recipient of those changes will probably: be aborted (as happens with most pregnancies - the fertilized egg-cell does not implant), be still-born, be horribly disadvantaged physically or mentally - or both, die young, be aborted (hopefully), be born sterile and thus not propagate that gene. Where small non-beneficial changes happen in the important DNA, we may simply have a "deficient" person who would not be successful or we get what we call "genetic diseases" and if we were animals, we would "breed the gene out". Being human we have not done that and suffer from many "genetic diseases" in consequence*.
Where beneficial changes occur (large or small) in the "important" DNA, the recipients tend to live well and have many offspring** which ensures that this "helpful" genetic material is passed on into society.
Ultimately, if a large enough change in DNA (good or bad) spreads to enough of a breeding group to swamp out alternate DNA possibilities, speciation will occur.
TheHermit
*Examples are Haemophilia, Down's Syndrome, Achondroplasia, Marfan's Syndrome,Sickle Cell Anemia, Thalassemias, Albinism, Phenylketonuria, Tay Sachs Disease, Muscular Dystrophy, Huntington's Disease,Cystic Fibrosis, Tourette's Syndrome, Turner's Syndrome. A more interesting possibility is that we can already detect the vulnerability to heart disease, vascular disease and many forms of cancer and will probably be able to "splice in" healthy DNA to remove these tendencies within 20 to 30 years if people are not so daft as to attempt to prevent this research.
**This process is being inverted by modern society where healthy sucessful people have fewer or no children. Which leads me to believe that in self defence "breeding programs" will be implemented. I would suggest that society consider a program along the following lines:
A voluntary (state-funded) program of registration prior to breeding, where your DNA is sampled and if correctable "faults" are detected, you have genetic material (sperm or eggs) "filed" for later use and then you are sterilized. At any time in the future, you and a partner (also a member of this program) should be able to request (state funded) implantation of material after the faults are corrected, thereby removing these diseases from the DNA pool. If no genetic faults are detected, you receive a "clean" card.
On pregnancy, participants in this program would agree to have (again state funded) a full DNA check for Downs, Ancephelitus and other similar detectable genetic problems, and to terminate the pregnancy in the event of a "problem" being confirmed.
In exchange for these agreements on the part of prospective parents, the state would agree to pay for all reproductive and genetically related health costs on the part of the offspring in the event of a problem occurring.
People declining to participate in this program would not be permitted access to state funded medical care in the event of a genetically based medical problem, and the right for the state or their offspring to sue them to recover medical care costs would not be limited.
This entire program would be paid for by the huge savings in support costs for the children who would not be born with genetically caused medical problems and the improvements in the health of populations as the gene pool recovered from the effects of modern medicine before Malthus takes his post-mortem revenge on us all.
If this program were successful - could the memetic engineers among us work out a way to sell it to people who fear both medicine and science? - it would have the effect of dramatically reducing defective genes and their tragic consequences from the general population within just a few generations.