Selective breeding has fed the world, but it also creates several problems.
Reduced gene pool (loss of genetic variation)
The biggest issue is that selectively bred populations have a much smaller gene pool than wild ones. Only a handful of "best" individuals are used as parents, so most of the species' genetic variation gets thrown away.
The consequences:
- All individuals carry similar alleles. If a new disease arrives that can attack one individual, it can probably attack all of them. The whole population can be wiped out by a single outbreak. (This is the famous concern about the Cavendish banana, where billions of plants are clones of one variety.)
- Adaptation to a changing environment is harder. Without genetic variation, there are no alternative alleles for natural selection to work with if conditions change.
- Loss of traits that might be useful later. Many wild plants have disease-resistance alleles that crop varieties lack. Once those wild relatives are extinct, we cannot get the resistance back.
Inbreeding depression
When closely related individuals (siblings, cousins) are repeatedly bred together to maintain a particular trait, they share most of their alleles. They are likely to share harmful recessive alleles too. When two carriers of the same recessive disease meet, their offspring are likely to be homozygous recessive and show the disease.
This is called inbreeding depression and is a serious problem in many pedigree dog breeds:
- Pugs and bulldogs have breathing problems because their flat faces compress their airways (a feature they have been bred for)
- King Charles Spaniels often have a brain condition (syringomyelia) where the skull is too small for the brain
- German Shepherds are prone to hip dysplasia
- Many breeds have shortened lifespans compared to mixed-breed dogs
Loss of natural behaviour and survival skills
Animals selectively bred for thousands of years may have lost the ability to survive in the wild:
- Domesticated turkeys cannot reproduce by natural mating; they have to be artificially inseminated because the males are too heavy
- Modern broiler chickens cannot fly and have weak hearts (their bodies grow faster than their organs can support)
- Many vegetable varieties cannot reproduce without human help (e.g. seedless bananas have to be propagated by cuttings)
Ethical concerns
- Many breeds bred for extreme physical features suffer chronic pain or breathing problems (some flat-faced breeds of dogs and cats)
- Intensive selection for fast growth in farm animals raises welfare concerns
- Loss of traditional and rare breeds reduces genetic diversity and cultural heritage
Mitigating the problems
- Breeders outcross occasionally to introduce fresh alleles
- Rare breed conservation programmes maintain populations of traditional breeds that hold useful genetic variation
- Seed banks (like the Svalbard Global Seed Vault) preserve traditional crop varieties for the future
- Some kennel clubs are tightening breeding standards to discourage extreme physical features