In this case, there would be considerable opportunity for selection-the genotypes of parents producing many children would increase in frequency at the expense of those having few or none. Assume, on the other hand, that the mean number of children per family is low, but some families have no children at all or very few, whereas others have many. In this case, there would not be fertility selection whether couples all had very few or all had very many children. Assume that all people of reproductive age marry and that all have exactly the same number of children. The intensity of fertility selection depends not on the mean number of children per family, but on the variance in the number of children per family.

It might seem at first that selection due to differential fertility has been considerably reduced in industrial countries as a consequence of the reduction in the average number of children per family that has taken place.

It was the appearance of culture as a superorganic form of adaptation that made mankind the most successful animal species. From their obscure beginnings in Africa, humans have become the most widespread and abundant species of mammal on earth. The exploration of outer space has started without waiting for mutations providing humans with the ability to breathe with low oxygen pressures or to function in the absence of gravity astronauts carry their own oxygen and specially equipped pressure suits. People travel the rivers and the seas without gills or fins. Humans did not wait for genetic mutants promoting wing development they have conquered the air in a somewhat more efficient and versatile way by building flying machines. The discovery of fire and the use of shelter and clothing allowed humans to spread from the warm tropical and subtropical regions of the Old World to the whole Earth, except for the frozen wastes of Antarctica, without the anatomical development of fur or hair. To extend its geographical habitat, or to survive in a changing environment, a population of organisms must become adapted, through slow accumulation of genetic variants sorted out by natural selection, to the new climatic conditions, different sources of food, different competitors, and so on. In the future, therapeutic cloning will bring enhanced possibilities for organ transplantation, nerve cells and tissue healing, and other health benefits. Genomes can be cloned individuals cannot. The proposal to enhance the human genetic endowment by genetic cloning of eminent individuals is not warranted. Germ-line gene therapy could halt this increase, but at present, it is not technically feasible. Health care and the increasing feasibility of genetic therapy will, although slowly, augment the future incidence of hereditary ailments.

More than 2,000 human diseases and abnormalities have a genetic causation. Nevertheless, natural selection persists in modern humans, both as differential mortality and as differential fertility, although its intensity may decrease in the future. For the last few millennia, humans have been adapting the environments to their genes more often than their genes to the environments. In turn, cultural inheritance leads to cultural evolution, the prevailing mode of human adaptation. Cultural inheritance makes possible for humans what no other organism can accomplish: the cumulative transmission of experience from generation to generation. There are, in mankind, two kinds of heredity: biological and cultural.

Clone x cross reference