SelectionCambridge IGCSE Biology: Revision notes
Section 1
What is natural selection and how does it work?
Natural selection is the process by which populations of organisms become better adapted to their environment over many generations. It relies on four key principles:
- Genetic variation exists within a population – individuals have different alleles for the same genes
- Many offspring are produced – organisms reproduce in large numbers, creating competition for limited resources
- Struggle for survival occurs – not all offspring survive because there are finite resources (food, space, light)
- Better-adapted individuals have a greater chance of reproduction – those with advantageous traits are more likely to survive and reach reproductive maturity
- Beneficial alleles are passed to the next generation – successful reproducers pass their alleles to offspring, increasing their frequency in the population
Over many generations, advantageous alleles become more common and disadvantageous alleles become rarer, causing the population to become better suited to its environment.
Examiners want you to explain the full chain of causation: variation → competition → differential survival → differential reproduction → change in allele frequency. Don't just state that organisms with advantageous traits survive; explain why competition creates this scenario.
Think of natural selection as a filtering process: the environment 'filters out' poorly adapted individuals, leaving the well-adapted ones to pass on their genes. Each generation is filtered again, gradually refining the population.
Section 2
How does antibiotic resistance develop in bacteria?
The development of antibiotic-resistant bacteria is a textbook example of natural selection in action:
Initial situation: A population of bacteria exists with genetic variation – most are susceptible to antibiotics, but a few individuals have alleles that confer resistance.
Selection pressure: When antibiotics are introduced, they kill the susceptible bacteria (struggle for survival). However, resistant bacteria survive and reproduce without competition from other bacteria.
Result: The population rapidly becomes dominated by antibiotic-resistant bacteria as the resistant alleles increase in frequency with each generation.
Why this matters: Overuse and misuse of antibiotics accelerates this process by creating repeated strong selection pressures. This is why completing antibiotic courses and avoiding unnecessary antibiotic use is crucial.
This example demonstrates all key aspects of natural selection: variation, competition, differential survival based on adaptation, and inheritance of beneficial traits.
Year 1: A population of 1000 bacteria includes 10 with resistance alleles. Antibiotic is applied; 990 susceptible bacteria die, 10 resistant bacteria survive and divide rapidly. Year 2: Population now contains 500 bacteria, 450 with resistance (90%). The resistant allele has increased from 1% to 90% in just one generation through natural selection.
Students often say 'antibiotics create resistance' – this is incorrect. Antibiotics do not cause mutations; they select for resistance that already exists in the population. Resistance alleles pre-existed, but became common due to selection pressure.
Section 3
What is selective breeding and how is it carried out?
Selective breeding (artificial selection) is the process by which humans deliberately select individuals with desirable features and breed them together to produce offspring with those desired traits.
The selective breeding process over multiple generations:
- Identify desirable traits in the current population (e.g. high crop yield, disease resistance, fast growth)
- Select individuals showing these traits and prevent other individuals from breeding
- Cross the selected individuals to produce offspring
- Examine offspring and identify those showing the strongest desirable features
- Repeat the process through multiple generations, selecting and crossing the best individuals each time
- Result after many generations: The population shows a marked increase in the frequency of desired alleles, and the desirable traits are much more pronounced
Key difference from natural selection: Humans make the selection decisions based on usefulness rather than survival advantage. Selection pressure is artificial (human choice) rather than environmental.
When describing selective breeding, always mention that the process must be repeated over many generations to see significant changes. Examiners expect you to explain that each generation is selected and crossed, not just that desirable individuals are identified once.
Section 4
How is selective breeding used to improve crops and livestock?
Selective breeding has been used for thousands of years to improve both crop plants and domesticated animals. Here are key examples:
Crop plants: Farmers select plants with:
- High yield (more grain, fruit, or seeds per plant)
- Disease resistance (ability to resist fungal, bacterial, or viral infections)
- Drought tolerance (able to survive with less water)
- Better nutritional content
- Suitability to local climate conditions
These plants are then bred together; offspring showing the best combination of traits are selected and bred again in the next season.
Domesticated animals: Farmers select for:
- Rapid growth rate (reach market weight quickly)
- High productivity (eggs, milk, meat yield)
- Good health and disease resistance
- Desirable physical features (meat quality, wool quality)
- Temperament suitable for farming
Selected animals are mated; offspring showing desired traits are kept for breeding, others are removed from the breeding population.
Results: Over many generations (often 10–20+ cycles), populations can change dramatically. Modern wheat produces far more grain than wild ancestors; modern dairy cows produce vastly more milk than cattle of 100 years ago. However, this process is slow compared to natural selection because humans can only breed animals seasonally and each generation takes years to mature.
Modern wheat: Wild wheat ancestors had small grains and low yield. Over many generations, farmers selected plants with the largest seed heads and highest grain number. Today's wheat produces 30+ times more grain per plant. This required consistent selection across 50+ human generations (thousands of years).
Selective breeding is like using a filter that gets progressively finer: each generation removes organisms without desired traits, concentrating the 'good' alleles. Repeat this enough times, and you've refined the population entirely.
Section 5
What are the key differences between natural and artificial selection?
Natural selection and artificial (selective) breeding are both mechanisms that change allele frequencies over time, but they differ in important ways:
| Feature | Natural Selection | Artificial Selection |
|---|---|---|
| Selection agent | Environment (predators, food availability, climate, disease) | Humans |
| Selecting for | Traits that improve survival and reproduction | Traits humans find useful or desirable |
| Speed | Slow (depends on generation time and selection pressure) | Slow but can be faster if strong human preference exists |
| Source of variation | Genetic variation already present in population | Genetic variation already present in population |
| Mechanism | Better-adapted individuals survive and reproduce more | Humans prevent non-selected individuals from breeding |
| Outcome | Population becomes better adapted to environment | Population becomes more suited to human purposes |
| Long-term effects | Increases fitness for survival in the wild | May reduce fitness for wild survival (e.g. farm animals often cannot survive in wild) |
| Example | Antibiotic resistance; peppered moths in industrial areas | Modern wheat varieties; dairy cattle; dog breeds |
Both processes increase the frequency of selected alleles across generations, but natural selection is 'non-random survival based on fitness', while artificial selection is 'non-random breeding based on human choice'.
Examiners often ask 'compare natural and artificial selection' – always use a structured approach. State what is the same (both change allele frequency), then clearly explain the key differences (what creates selection pressure and what is being selected for).
Students sometimes confuse the two by saying natural selection is 'faster' or 'more powerful'. Both are slow processes requiring many generations. The key difference is the selection agent (environment vs humans) and the criteria for selection (survival fitness vs human preference).
Must Know
- Natural selection is the process by which populations become better adapted to their environment because organisms with advantageous alleles are more likely to survive and reproduce, passing these alleles to offspring
- Natural selection requires genetic variation, overproduction of offspring, struggle for survival, and differential reproduction of better-adapted individuals
- Antibiotic resistance in bacteria is a key example of natural selection: resistant alleles become common when antibiotics kill susceptible bacteria, and resistance spreads rapidly through repeated selection
- Selective breeding is when humans deliberately select individuals with desirable traits and breed them together; this process must be repeated over many generations to significantly increase the frequency of desired alleles
- Selective breeding is used to improve crop plants (yield, disease resistance) and domesticated animals (growth rate, productivity, milk/meat quality)
- Natural selection is driven by environmental pressures and selects for traits that improve survival; artificial selection is driven by human choice and selects for traits humans want, which may not aid wild survival
That's the notes covered.
Carry on to the next subtopic.