14.1.6 Origin if differences within breeds (2024)

Within breeds genetic drift, migration, selection and mutation are also relevant actors. In addition to these evolutionary forces the way the breed is created is essential for the genetic variation within the breed established today.

E.g. in dogs (standardized) breeds were created in crossing often only a few animals from a limited number of breeds. Their offspring was selected according to a strict breeding standard. A dog breed is often based on a limited number of founder animals and this is the cause that often a limited genetic variation within dog breeds is established.

E.g. in pigs and cattle, breeds were developed from landrace breeds by culling the animals that did not fit into the breeding standard (not the right colour or the wrong conformation) and by promoting the use of males that highly fitted the breeding standard.

Genetic drift can be avoided when breeds are kept in large numbers.

Migration of animals has often a positive effect on the size of the genetic variation within a breed. In practical terms: when studbook regulations allow that animals from outside the breeds can be used (under certain regulations) in that breed, the genetic variation will be enlarged. Therefore it is highly recommended to work with ”open” studbooks instead of closing the breed for animals from outside.

Selection of animals as parents for the next generation might also have a rather high negative impact on the genetic variation within a breed when the selection is very intense. Then only a few parents are selected that determine the genetic variation in the next generation.

Mutation is at short term of minor importance for the genetic variation within breeds. The mutation rate is estimated to be that low, that in a short period of time the number of animals within the breeds is too small to have the opportunity that a mutation takes place.