6.2.2.1 Natural selection

Natural selection not only occurs in natural populations. Natural selection acts on alleles that contribute to survival and reproductive success, also called fitness. For example, animals that have reduced potential to survive until adult age have a lower fitness (natural selective advantage) than animals that are very fit and healthy and will survive until late age. Also, animals with reduced reproductive capacity will have a lower fitness compared to very fertile animals. In domestic animal species the circumstances in which the animals live generally are quite controlled. Resistance to, for example, food shortage is less essential than it can be in natural populations. Still natural selection also works on domestic animal species. The environment the animals are kept in will require some degree of adaptation. For example, animals that are kept indoors need to be able to withstand the lack of sunlight, whereas animals that are kept outdoors will need to be able to withstand the variation in climate and a potentially larger infection pressure. Not being able to cope with the situation results in reduced fitness. If the selected animals require and receive assistance to get pregnant, for example, this will work against the mechanisms of natural selection.