Selective Breeding

Selective breeding (also called artificial selection) is the process by which humans choose organisms with desirable characteristics and breed them together. Over many generations, this produces offspring with enhanced desired traits. In this lesson we cover the process, examples, advantages and disadvantages.

What Is Selective Breeding?

Selective breeding is when humans — not nature — decide which organisms reproduce. Instead of environmental pressures determining which individuals survive and breed, humans choose the parents based on traits they want to enhance or combine.

Selective breeding has been practised for thousands of years, long before people understood genetics. Early farmers selected the best crop plants and the most productive animals for breeding.

The Process of Selective Breeding

The basic process follows these steps:

Step 1: Identify the Desired Characteristic

Decide which trait you want to enhance. For example:

Higher milk yield in cattle
Larger fruit in tomato plants
Docile temperament in dogs

Step 2: Select Parents

From the existing population, choose the male and female that show the desired characteristic most strongly.

Step 3: Breed the Selected Parents

Allow the chosen parents to mate (or cross-pollinate in plants).

Step 4: Select the Best Offspring

From the offspring, identify those that show the desired characteristic most strongly.

Step 5: Repeat Over Many Generations

Use the best offspring as the parents for the next generation. Continue this process for many generations until the desired trait is consistently expressed at a high level.

Exam tip: When describing selective breeding, always include the phrase "over many generations" — this is essential. One round of breeding is not enough to produce a significant change.

Examples of Selective Breeding

In Agriculture — Crop Plants

Crop	Desired Trait	Outcome
Wheat	Disease resistance, high grain yield	Modern wheat varieties produce far more grain per hectare than wild ancestors
Maize (corn)	Larger cobs, more kernels	Modern corn cobs are ~1,000 times larger than the wild ancestor (teosinte)
Tomatoes	Larger fruit, uniform colour, longer shelf life	Supermarket tomatoes look very different from wild tomatoes
Broccoli, cauliflower, cabbage, kale, Brussels sprouts	Different parts of the same wild mustard plant	All selectively bred from Brassica oleracea (wild cabbage)
Rice	Flood tolerance, high yield	Flood-tolerant rice varieties can survive weeks underwater

In Agriculture — Animals

Animal	Desired Trait	Outcome
Dairy cattle (e.g., Holstein-Friesian)	High milk yield	Modern dairy cows produce ~28 litres of milk per day (vs ~4 litres for wild cattle)
Beef cattle (e.g., Aberdeen Angus)	High muscle mass, good meat quality	Large, muscular animals
Chickens	Egg-laying or meat production	Laying hens produce ~300 eggs per year; broiler chickens grow to slaughter weight in ~6 weeks
Sheep	Wool quality, meat quality	Different breeds optimised for wool or meat

In Companion Animals

Animal	Desired Trait	Outcome
Dogs	Various: speed (Greyhound), herding instinct (Border Collie), small size (Chihuahua), guarding (German Shepherd)	Over 340 recognised breeds, all from the wolf (Canis lupus)
Cats	Coat colour, temperament, body shape	Persian, Siamese, Maine Coon and many other breeds
Horses	Speed (Thoroughbred), strength (Shire), endurance (Arabian)	Highly specialised breeds

Advantages of Selective Breeding

Advantage	Explanation
Increased food production	Higher crop yields and more productive livestock help feed growing populations
Disease resistance	Plants can be bred to resist specific diseases, reducing crop losses and the need for pesticides
Desired characteristics	Animals and plants can be bred for specific human needs (wool, milk, ornamental flowers)
Faster than natural selection	Humans can achieve significant changes in relatively few generations
No genetic modification needed	Uses natural breeding — no laboratory techniques or gene transfer required

Disadvantages of Selective Breeding

1. Reduced Genetic Variation (Reduced Gene Pool)

This is the most important disadvantage and a favourite exam topic.

When only organisms with a specific trait are bred, many other alleles are lost from the population. Over time, the gene pool (the total range of alleles in a population) becomes smaller.

Why this matters:

If a new disease emerges, there may be no individuals with resistance to it, because the allele for resistance was lost during selective breeding.
The population is vulnerable to environmental changes.

2. Inbreeding and Health Problems

Breeding closely related individuals (inbreeding) is often necessary to maintain desired traits. This can lead to:

Increased chance of homozygous recessive genetic disorders
Reduced fitness and vitality
Reduced fertility

Example: Many pedigree dog breeds suffer health problems:

Breed	Health Problem
Cavalier King Charles Spaniel	Heart disease (mitral valve disease)
Bulldog	Breathing difficulties due to flat face (brachycephalic syndrome)
German Shepherd	Hip dysplasia
Dalmatian	Deafness
Pug	Eye problems, overheating, spinal disorders

3. Ethical Concerns

Some animals are bred to have characteristics that cause suffering (e.g., broiler chickens grow so fast their legs cannot support their weight).
Is it right for humans to manipulate other species purely for our benefit?

Exam tip: In exam questions about disadvantages, always explain why reduced genetic variation is a problem — state that it makes the population vulnerable to new diseases because there may be no resistant individuals.