A process in a population through which individuals resistant to some destructive factor become gradually more numerous, while non-resistant individuals are eliminated.
This process had been observed already in the 19th century, for instance in a moth living on the bark of birches in England, adapting to the blackening of the colour of the bark due to industrial pollution and as a result of selection pressure exerted by predator birds.
The danger to produce similar side-effects by the use of insecticides or antibiotics, was signaled as early as 1953, but not heeded. Adaptive shifts have now produced the appearance of resistant populations of many insects and pathogens. The lesson has however not yet been learned, as shown for example by the dubious use of genetic engineering to create selective resistance to chemical killers in cultivated plants with the hope to selectively kill weeds, which supposedly could not acquire such resistance.