Artificial selection reduces genetic diversity because humans allow only organisms with preferred traits to reproduce. This narrows the gene pool, removes alternative alleles and makes the selected population more genetically uniform.
This is HL-only content in IB ESS subtopic 3.1, Biodiversity and evolution. Artificial selection occurs when humans choose breeding individuals based on desirable phenotypes, such as high milk production, rapid growth or disease resistance.
| Stage | Effect on genetic diversity |
|---|---|
| Humans select a small number of parents | Only a limited sample of the population's alleles is passed on. |
| Selected individuals reproduce repeatedly | Desirable alleles become more frequent, while unselected alleles decline or disappear. |
| Closely related individuals are crossed | Inbreeding increases and the population becomes more homozygous. |
| Selection continues over generations | Allelic richness and heterozygosity decrease, producing genetic uniformity. |
For example, selective breeding of Holstein dairy cattle has strongly favoured high milk yield. Extensive use of semen from a relatively small number of elite bulls has reduced the effective breeding population, increasing relatedness and the risk that harmful recessive alleles will be expressed.
Lower genetic diversity can reduce resilience because fewer genetic variants are available to help a population survive a new disease, pest or environmental change. A genetically uniform crop or livestock population may therefore be highly productive under current conditions but vulnerable when conditions change.
A common misconception is that artificial selection always eliminates every unwanted allele immediately. In reality, allele frequencies change across generations, and some recessive alleles may remain hidden in heterozygous individuals. Managed outcrossing, gene banks and the conservation of traditional breeds can limit diversity loss.
From a societal perspective, producers may prioritize yield and profit, while conservationists and small farmers may prioritize long-term resilience and food security.
Exam technique: For an IB ESS explain question, give a causal chain: restricted breeding population → fewer alleles passed on → increased homozygosity → reduced genetic diversity and resilience. Support the mechanism with a named example rather than merely stating that selective breeding is harmful.