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examBoard: Cambridge
examType: IGCSE
lessonTitle: Selective Breeding in Agriculture
Humans have been changing plants and animals for thousands of years! Long before we understood genetics, farmers were picking the best crops and animals to breed. This process, called selective breeding, has transformed wild species into the food crops and farm animals we rely on today.
Key Definitions:
Modern maize (corn) was developed from a wild grass called teosinte. Early farmers in Mexico began selecting and breeding teosinte plants with larger kernels around 9,000 years ago. The difference is amazing — teosinte kernels are tiny compared to modern corn cobs!
Selective breeding is all about choosing parent organisms with desirable traits and breeding them together. Over many generations, this increases the frequency of these desirable traits in the population.
1. Identify the desired traits (e.g., higher yield, disease resistance)
2. Select parent organisms that show these traits
3. Breed these selected parents
4. Select offspring that best show the desired traits
5. Repeat the process over multiple generations
Crossbreeding: Mating two different varieties or breeds
Inbreeding: Mating closely related individuals to maintain pure lines
Hybridisation: Crossing two genetically different individuals to produce hybrids with hybrid vigour
Crop plants have been transformed through selective breeding to increase yields, improve nutritional content and develop resistance to pests and diseases.
Modern wheat varieties produce up to 3 times more grain than varieties from 100 years ago. Breeders select for more grains per plant, larger grains and stronger stems.
Varieties of potatoes have been bred to resist potato blight, the disease that caused the Irish Potato Famine. This reduces the need for chemical pesticides.
Apples have been selectively bred for sweetness, crispness and storage life. Modern varieties like Gala and Pink Lady were developed through careful breeding programmes.
One of the most significant applications of selective breeding was during the Green Revolution (1950s-1960s). Scientists developed high-yielding varieties of wheat, rice and maize that dramatically increased food production in developing countries.
Norman Borlaug, often called the "Father of the Green Revolution," developed semi-dwarf, high-yield, disease-resistant wheat varieties. His work in Mexico, India and Pakistan is credited with saving over a billion people from starvation. He was awarded the Nobel Peace Prize in 1970 for his contributions to world food security.
Farm animals have been selectively bred for thousands of years to improve meat production, milk yield, egg laying and other useful traits.
Modern dairy cows like Holstein-Friesians can produce over 30 litres of milk per day - about ten times what their wild ancestors could produce. Breeders select for higher milk production, better fat and protein content and good udder health.
Through intensive selective breeding, modern broiler chickens reach market weight in just 6-7 weeks, compared to 16 weeks in the 1950s. They've been bred to convert feed to meat extremely efficiently.
Today's selective breeding programmes use advanced techniques to speed up the process:
While selective breeding has been crucial for increasing food production, it raises several important considerations:
One major concern with selective breeding is the loss of genetic diversity. When we focus on just a few desirable traits, we may inadvertently reduce the overall genetic variation in a population. This can make crops and livestock more vulnerable to new diseases or environmental changes.
In the 1950s, nearly all commercial bananas were of a single variety called Gros Michel. When Panama disease struck, it devastated plantations worldwide because all plants were genetically similar and equally vulnerable. This forced the industry to switch to the Cavendish variety, which is now facing similar threats.
Some selectively bred animals suffer health problems as a result of their modified traits. For example, broiler chickens grow so rapidly that they can develop leg problems and some dairy cows suffer from increased rates of mastitis due to their high milk production.
Modern breeding programmes increasingly focus on sustainability traits such as:
Selective breeding continues to evolve with new technologies and approaches:
Modern genomic tools allow breeders to identify exactly which genes control which traits, making selection more precise and efficient. This helps develop new varieties faster and with more predictable results.
Breeders are increasingly focusing on developing crops that can withstand extreme weather, changing rainfall patterns and new pest pressures resulting from climate change.
New Rice for Africa (NERICA) varieties were developed by crossing Asian rice (high yield) with African rice (pest resistance and drought tolerance). These varieties have helped increase food security in parts of Africa, producing up to 50% more rice without requiring expensive fertilisers.
Selective breeding has been fundamental to increasing agricultural yields throughout human history. By carefully choosing which plants and animals reproduce based on desirable traits, farmers and scientists have dramatically improved productivity, disease resistance and quality of our food.
While selective breeding offers many benefits for food security, it must be balanced with considerations of genetic diversity, animal welfare and environmental sustainability. Modern breeding programmes increasingly incorporate these concerns alongside traditional goals of improving yields.
As we face challenges like climate change and growing global food demand, selective breeding will continue to play a vital role in developing resilient, productive agricultural systems for the future.
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