Selective Breeding

Selective breeding (also called artificial selection) is when humans choose which individuals reproduce in order to produce offspring with desired characteristics.

The 4-step cycle:

1. IDENTIFY individuals in the population that show the DESIRED TRAIT — e.g. highest milk yield, biggest fruit, fastest growth, brightest petals.

2. BREED ONLY THE CHOSEN INDIVIDUALS together. Stop the others from contributing. In farm animals this often uses artificial insemination. In plants, controlled cross-pollination (remove anthers, transfer pollen by hand).

3. SELECT THE OFFSPRING that show the desired trait most strongly. Discard / cull / not breed from the others.

4. REPEAT over many generations. This is essential — substantial change only happens after many generations of repeated selection.

Why it works at the genetic level:

• The desired trait is controlled by particular ALLELES in the gene pool.
• By choosing only individuals that show the trait, you select individuals that carry the favourable alleles.
• These individuals pass the favourable alleles to their offspring.
• Over generations, the FREQUENCY of the favourable alleles INCREASES.
• Eventually most individuals are HOMOZYGOUS for the favourable alleles → almost all show the desired trait.

Time scale. Substantial change takes many generations:

• Crops (wheat, rice): decades. Modern semi-dwarf high-yield wheat took 20+ years.
• Chickens: years. Broiler growth rate has tripled since the 1950s.
• Cattle: decades. Holstein-Friesian milk yields have more than doubled.
• Dogs: thousands of years. Over 400 breeds from one wolf ancestor.

Selective breeding has been done for ~10 000 years (since the dawn of agriculture). Most modern crops and livestock are products of intensive selective breeding.

Cattle:

• Dairy cattle (e.g. Holstein-Friesian): selected for HIGH MILK YIELD. Modern dairy cows produce 30+ litres per day; wild cattle produce ~2 litres.
• Beef cattle (e.g. Aberdeen Angus, Limousin): selected for large muscle mass and fast growth.

Chickens:

• Broiler chickens: selected for FAST GROWTH and large breast muscles. Reach 2 kg in 6 weeks (16 weeks in 1950s). Welfare concerns: leg disorders, heart failure.
• Battery layers: selected for HIGH EGG YIELD — 300+ eggs per year (wild chickens lay ~12).

Wheat:

• Short stem (semi-dwarf): reduces lodging (falling over) — more energy goes to grain.
• High grain yield: larger grain heads.
• Disease resistance: resistance to rust fungi, mildew, etc.
• Norman Borlaug's semi-dwarf wheat (1960s Green Revolution) is credited with saving an estimated 1 billion lives.

Other crops:

• Maize: dramatic increase in cob size and grains per cob from teosinte ancestor.
• Rice: IR8 'miracle rice' transformed Asian agriculture.
• Brassicas: ONE wild plant (Brassica oleracea) → cabbage, broccoli, cauliflower, kale, Brussels sprouts. All from selective breeding of different parts of the same species.

Domestic animals:

• Dogs: all 400+ breeds derive from grey wolves. Selected for working roles (herding, hunting, guarding) and appearance.
• Sheep: wool quality (merino), milk yield, meat carcass size.
• Horses: speed (thoroughbred racehorses), strength (Shire horses), size (Shetland ponies).

Ornamental plants:

• Roses: colour, scent, double flowers, repeat flowering.
• Tulips: flower colour and shape (famous 17th-century Dutch tulip mania bred from a handful of wild bulbs).

Modern methods. Today selective breeding often uses GENOMIC SELECTION — DNA testing to identify which individuals carry favourable alleles BEFORE breeding decisions are made. This accelerates the process compared to phenotype-only selection.

Darwin called selective breeding 'artificial selection' because he saw it as evidence for his theory of natural selection — both processes work by the same mechanism.

Same underlying mechanism:

1. There is HERITABLE VARIATION in the population (different alleles).
2. Not all individuals reproduce equally — some have more offspring than others.
3. Those that reproduce more pass their alleles to the next generation.
4. Over generations, the frequencies of those alleles INCREASE in the population.

This is true for BOTH natural and artificial selection.

Key differences:

Why the speed difference? Natural selection rarely kills 90% of individuals each generation — that would risk extinction. Selective breeding can be ruthless: only the top 5% of individuals (or 1%) might be allowed to breed. This very high selection intensity drives rapid change.

Why the fitness difference? Natural selection favours all traits that aid survival + reproduction in nature. Selective breeding selects only the trait humans want — even at the cost of welfare or wild survival. A broiler chicken with massive breast muscles cannot fly; a Holstein cow needs human assistance to give birth; a pug cannot breathe properly.

Important link to Topic 3 (Inheritance). Selective breeding works ONLY because of VARIATION caused by sexual reproduction (meiosis + fertilisation) and mutation. Without variation, there is nothing to select from.

Selective breeding has produced enormous gains in agricultural productivity, but it has serious long-term disadvantages.

1. Reduced genetic diversity.

• Repeatedly choosing the same desirable individuals reduces the variety of alleles in the population (a smaller GENE POOL).
• Many modern crops and livestock are extremely uniform.
• A smaller gene pool means fewer options for future breeding — useful alleles may be lost forever.

2. Inbreeding depression.

• In a small breeding population, close relatives often have to mate.
• INBREEDING increases the chance that offspring are HOMOZYGOUS for harmful RECESSIVE alleles.
• These alleles are then EXPRESSED → genetic disorders appear.

Famous examples:

• Pedigree DOGS:
  • Pugs, bulldogs → flat faces → breathing problems (brachycephalic syndrome), overheating.
  • Cavalier King Charles spaniels → skull too small for brain → syringomyelia (severe pain).
  • German shepherds → hip dysplasia (lameness).
  • Dalmatians → deafness, urinary stones.
• Dairy CATTLE:
  • Holsteins prone to mastitis and lameness.
  • BLAD (bovine leucocyte adhesion deficiency) — recessive disorder spread by overuse of one bull.
• Pedigree CATS:
  • Persians → flat faces and tear-duct problems.
  • Munchkins → dwarfism with joint disorders.

3. Vulnerability to disease.

• A genetically uniform population is vulnerable to ONE disease wiping out the whole crop or herd.
• Cavendish banana — almost all commercial bananas are clones of a single variety. Highly susceptible to PANAMA DISEASE (a soil fungus). Wiped out the previous Gros Michel variety in the 1950s; the same may happen to Cavendish.
• Irish potato famine (1845-1849) — Ireland depended on one potato variety, devastated by late blight. Around 1 million people died, 1 million emigrated.
• MODERN WHEAT — concern about Ug99 stem rust attacking high-yield varieties globally.

4. Loss of traditional landraces.

• For thousands of years, farmers grew thousands of locally-adapted crop varieties (landraces) and livestock breeds.
• Modern uniform varieties have replaced most of these worldwide.
• Useful alleles (e.g. drought tolerance, disease resistance, regional adaptation) carried by traditional varieties may now be lost.
• 'Gene banks' (e.g. Svalbard Global Seed Vault) try to preserve these as insurance.

5. Slow process.

• Selective breeding takes many generations.
• One reason GM has become popular — it can introduce a useful trait in a single generation.

6. Ethical concerns about animal welfare.

• Extreme selective breeding can produce serious welfare problems:
  • Broiler chickens unable to support their weight.
  • Dairy cows with painful udders.
  • Pedigree dogs unable to give birth naturally (e.g. bulldogs almost always need caesarean).
• Many breed societies and vets are now working to reverse these trends — outcrossing, breed standard changes, health screening.

Mitigations. Modern breeding programmes try to preserve genetic diversity by maintaining gene banks, periodically outcrossing, and using genomic selection rather than only phenotype.