|MadSci Network: Evolution|
The course of future human evolution is open to multiple scientific controversies involving not only the scientific facts but also ethics. But human evolution may be treated as another science issue. In fact, even though humankind has achieved a high degree of technological development, evolution goes on. There are always evolutionary pressures acting on the genetic background of every linving organism. The fact that humans possess the means to control many diseases does not make us immune to selection and hence to evolution. The change that did occur is that we incorporated into our environment all the cultural achievements for the past several thousand years. On that complex environment of natural and cultural constraints our genetic background is developing. Selection is different in every environment and our milieu is simply, a more complex one. In any case, due to the large size of the human population and its long generation time, evolutionary change is very slow, but it is undoubtedly happening. This led me to the common belief that undesirable traits are always eliminated by natural selection. Sometimes it does but in many other cases these 'undesirable' traits are hidden to selection and then kept in a population genes. That is the case of the mutations that manifest themselves only when present in homozygosis in an individual, but when individuals are heterozygotes there is no deleterious effect on the individual. If there is no difference at all between such an individual and the 'normal' one, selection cannot eliminate it, and the mutated gene can be trasmitted to the next generation. This is the reason of still so many potentially lethal genes in human and animal populations.
For selection to act it is necessary a priori to have gene variation in a population. This means that several variants of a gene with different fitness for the organism exist. From these usually slight differences selection picks the fittest for a given environment. When genetic variation diminishes, for instance when a population gets isolated or that only a portion of the genetic variants in the population are present then it is when we can talk about 'genetic decline.' But we cannot talk of genetic decline referred to the supposed accumulation of lethal genes in the human common gene pool. These mutations are so rare in the population that the probability of an individual getting two or three lethal genes from their parents is in practice infinitesimal. Moreover, even genes with deleterious effects represent an variation for selection to act. We do not know if some variant genes could be advantageous to the individual in another environment. Take the case of sickle-cell anemia, caused by a mutation in one of the hemoglobin proteins. In homozygosis it is deleterious and often life-threatening, but in heterozygosis it makes the red blood cells more resistant to malaria. The revolution in molecular biology leads me to mention our ability to consider using techniques of DNA manipulation to eliminate lethal genes from the population. Apart from the ethical questions posed by such a manipulation of the human germ line, it is difficult to imagine the enormity of the task involved in such a procedure. We carry about 100000 genes and we do not know more than several dozens of potentially lethal mutations. DNA manipulation should be reserved for somatic procedures, like the gene-therapy experiments being carried out to correct genetic defects as another tool for medicine. The elimination of undesirable traits by DNA manipulation would be highly impractical (every gene in every individual should be screened for mutations in order to eliminate it) and probably uneconomical. Furthermore it would reduce the genetic diversity of the human gene pool, and since evolution is blind how are we sure that all these genes should have been eliminated?