Decreasing genetic diversity of animals

Animal breeding programmes, designed to produce animals with hardy high-yield characteristics after careful selection, can be counter-productive. The more successful a variety is, the more likely farmers are to choose the hybrid in question. An entire society may come to depend on a small range of highly selected varieties. When a new or mutant form of a disease appears to which the favoured hybrid has no inbred resistance, an entire breeding stock may be destroyed.

Both male and female animals which are finely bred or which have particularly commendable points from a breeding point of view, may be overused for breeding purposes, giving rise to inferior progeny or infertility. Females of certain species may be mated consecutively when it may be better for them to have a resting period between pregnancies. Males may be overused for financial reasons, or for their prowess or virility, or because of their fine breeding or special qualities. Males may be used too often with the same herd or the same individual animals. They may be overused for artificial insemination purposes, where demand for the services of sires of high merit is extensive, and where one ejaculation may be extended to 100 or more services, almost on the level of mass production.

Inbreeding of domestic animals is a common practice to improve stock and to preserve qualities found in one or two exceptionally fine animals. However, the effect of inbreeding also serves to increase the proportion of defective, weak, slow-growing animals, and unless the method is used very selectively, especially with species that are normally cross-fertilizing and outbred, these characteristics will predominate. Reasons for excessive inbreeding include lack of due care, general inexperience, overzealousness, and financial rewards.