The process of selective breeding
In selective breeding (also called artificial selection), humans choose organisms with desirable characteristics and breed them together. From the offspring, those showing the desired trait most strongly are selected for the next generation of breeding. This process is repeated over many generations.
Over time, the frequency of the alleles responsible for the desired trait increases in the population. The result is a population of organisms that consistently shows the desired characteristic, such as higher milk yield, larger fruit size, or disease resistance.
Examples of selective breeding
Crop plants: Wheat has been selectively bred for higher grain yield, shorter stems (to resist wind damage), and disease resistance. Maize has been bred from a wild grass (teosinte) with tiny cobs into the large-eared crop we recognise today.
Livestock: Dairy cattle have been bred for higher milk yield over many generations. Chickens have been bred for faster growth and higher egg production. Dogs have been bred from wolves into hundreds of breeds with specific physical and behavioural traits.
Ornamental plants: Roses and other flowers have been bred for colour, scent, petal number and disease resistance. These are purely aesthetic selections rather than agricultural ones.
Advantages and disadvantages
| Advantages | Disadvantages |
|---|---|
| Increased food production (higher yield) | Reduced genetic variation in the population |
| Disease-resistant crop varieties | Inbreeding can cause health problems |
| Animals with desired characteristics | Loss of alleles that might be useful in the future |
| No genetic modification technology needed | Takes many generations to achieve results |
The most important disadvantage for the exam is reduced genetic variation. If the environment changes or a new disease appears, the genetically uniform population may lack the alleles needed to survive, making the entire population vulnerable.
Selective breeding vs natural selection
Both processes involve selection of organisms with advantageous traits and changes in allele frequency over generations. The key difference is the selecting agent: in natural selection, the environment selects, with no human intention; in selective breeding, humans deliberately choose which organisms breed.
Natural selection tends to maintain genetic diversity because different alleles may be favoured in different environments or at different times. Selective breeding typically reduces genetic diversity because humans select for a narrow range of traits and discard individuals without those traits.