Global challenges Study Guide

Selective breeding, also known as artificial selection, is a process where humans choose organisms with desirable traits and breed them together to produce offspring with an increased chance of inheriting those same traits. This practice has profoundly impacted the development of food plants and domesticated animals over thousands of years. For food plants, selective breeding has led to varieties that produce higher yields, are more resistant to pests and diseases, have better nutritional value, or can grow in challenging environments. For example, modern wheat varieties produce much more grain per plant than their wild ancestors, and many fruits are larger and sweeter. In domesticated animals, selective breeding has resulted in livestock that produce more meat, milk, or eggs, grow faster, or have more docile temperaments, making them easier to manage. Examples include dairy cows bred for high milk production, chickens bred for rapid growth or prolific egg-laying, and sheep bred for wool quality. While selective breeding has been instrumental in increasing food production and improving quality, it also has potential drawbacks such as reduced genetic diversity within populations, making them more vulnerable to new diseases or environmental changes, and sometimes leading to health problems in animals due to extreme selection for certain traits.

The human body possesses a robust defence system to protect itself from harmful microorganisms, known as pathogens. This defence can be broadly categorised into non-specific and specific immunity. Non-specific defence systems, also called innate immunity, are the body's first line of defence and act generally against all pathogens, regardless of their specific type. These systems are always present and ready to defend, providing immediate protection. Key components include physical barriers like the skin, which forms a tough, waterproof layer preventing entry. Chemical barriers are also crucial, such as stomach acid, which destroys most pathogens swallowed with food and drink, and tears, which contain enzymes that break down bacterial cell walls. The mucous membranes lining respiratory and digestive tracts trap pathogens, which are then expelled, for example, by coughing or sneezing. Furthermore, some white blood cells, specifically phagocytes, provide an internal non-specific defence by engulfing and digesting pathogens. This rapid and general response is vital for preventing the initial establishment of infection and gives the more specific adaptive immune system time to mount a targeted attack if needed.