【F1】
Genetic variation derives from several sources and provides only the raw materials of evolution; random variation can only have a disrupting effect on genetic equilibrium unless deleterious combinations of genes are eliminated and advantageous ones are preserved.
The process by which this occurs is called natural selection.【F2】
Although Darwin is credited with the first full statement of the theory of evolution through natural selection, the origins of the concept are deeply rooted in European thought and may be traced to the early 17 th century.
Indeed, the practices of artificial selection of domesticated plants and animals, which so influenced Darwin, can be traced at least as far as the Romans and perhaps even to Neolithic(New Stone Age)times. The theory of natural selection can be stated as follows: All living things vary and reproduce themselves many times, yet the number of a given group tends to remain constant. Therefore there is a competition for survival, and only those most adapted to external conditions survive. In essence, the idea of natural selection is statistical; those members of a population who are most evolutionarily fit are those who will leave the greatest number of offspring.【F3】
These will not be the only members of the population to leave offspring, but it is probable, in a statistical sense, that they will leave more living descendants in the long run than the less well-adapted members of the population.
Fitness therefore refers to a population"s ability to cope successfully with a particular environment at a particular time; it is tied to time and place in an absolute way.【F4】
The factors that help determine the course and direction of evolution are many; predation and disease, migration and conflict, behaviour and temperament, competition for breeding space and mates, and competition for living space and food.
【F5】
Purely physical factors in the environment are no less important; stability or instability of the climate; solar radiation; natural disaster; pollution of the soil, water, and air—all will have their effect and take their toll on living groups.
Evolution is not, therefore, something that has occurred and been completed. Any population is constantly evolving, assimilating changes and variations in its gene pool in response to stimuli from a large number of sources—some that are recognized and some that are not.