In simple terms
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Natural and artificial selection
Cambridge 9700 Paper 4 — Natural and artificial selection (17.2). A-Level Notes diagram-backed lesson with premium structure and live visuals.
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Variation: Individuals within a population show genetic variation (due to mutation, meiosis, random fertilisation) in their phenotypes. Some variations are heritable.
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Overproduction: Organisms typically produce more offspring than can survive to reproductive age.
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Struggle for Existence: Resources are limited, leading to competition for food, mates, space, and avoidance of predators or disease.
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Differential Survival and Reproduction: Individuals with advantageous heritable traits are better adapted to the environment. They are more likely to survive the struggle, reproduce successfully, and pass on those advantageous alleles.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 17.2.1
Explain that natural selection occurs because populations have the capacity to produce many offspring that compete for resources; in the 'struggle for existence', individuals that are best adapted are most likely to survive to reproduce and pass on their alleles to the next generation
- 17.2.2
Explain how environmental factors can act as stabilising, disruptive and directional forces of natural selection
- 17.2.3
Explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations
- 17.2.4
Outline how bacteria become resistant to antibiotics as an example of natural selection
- 17.2.5
Use the Hardy-Weinberg principle to calculate allele and genotype frequencies in populations and state the conditions when this principle can be applied (the two equations for the Hardy-Weinberg principle will be provided, as shown in the Mathematical requirements)
- 17.2.6
Describe the principles of selective breeding (artificial selection)
- 17.2.7
Outline the following examples of selective breeding: • the introduction of disease resistance to varieties of wheat and rice • inbreeding and hybridisation to produce vigorous, uniform varieties of maize • improving the milk yield of dairy cattle
Explore the concept
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Full topic notes
Formal explanation with the rigour you need for the exam.
Natural Selection: The Survival of the Fittest (or Fitter)
Natural selection is the mechanism proposed by Charles Darwin and Alfred Russel Wallace for how evolution occurs. It's a non-random process where individuals better adapted to their environment are more likely to survive and reproduce, passing on their advantageous alleles to the next generation. This leads to a gradual change in allele frequencies within a population over time. Here's how it works:
Variation: Individuals within a population show genetic variation (due to mutation, meiosis, random fertilisation) in their phenotypes. Some variations are heritable.
Overproduction: Organisms typically produce more offspring than can survive to reproductive age.
Struggle for Existence: Resources are limited, leading to competition for food, mates, space, and avoidance of predators or disease.
Differential Survival and Reproduction: Individuals with advantageous heritable traits are better adapted to the environment. They are more likely to survive the struggle, reproduce successfully, and pass on those advantageous alleles.
Inheritance: The offspring inherit the advantageous alleles, increasing their frequency in the gene pool of the next generation.
Adaptation: Over many generations, the proportion of individuals with these advantageous traits increases, leading to a population that is better adapted to its environment.
Types of Natural Selection
Natural selection can act on populations in different ways, depending on the selection pressures. This leads to three main modes of selection:
A classic example of directional selection is the evolution of antibiotic resistance in bacteria. When antibiotics are used, susceptible bacteria are killed, but resistant mutants survive and reproduce, quickly dominating the population. The antibiotic acts as the selection pressure.
Stabilising Selection: This is the most common type. It favours intermediate phenotypes and selects against extreme variations. For example, human birth weight. Babies that are too small are less likely to survive, and babies that are too large can cause complications during birth. The result is a population where most individuals have a birth weight in a narrow, intermediate range.
Directional Selection: This occurs when environmental conditions change, favouring individuals with phenotypes at one extreme of the existing range. A classic example is antibiotic resistance in bacteria. The presence of an antibiotic selects for resistant bacteria, shifting the population's average phenotype towards higher resistance.
Disruptive (or Diversifying) Selection: This type of selection favours individuals at both extremes of the phenotypic range, while selecting against intermediate phenotypes. This is less common but can lead to the formation of two distinct subpopulations. For example, in a habitat with two distinct food sources (e.g., large and small seeds), birds with very large or very small beaks might be favoured over birds with medium-sized beaks.
Artificial Selection: Humans Taking the Reins
Artificial selection, also known as selective breeding, is a process where humans intentionally choose organisms with desirable traits to breed, in order to enhance those traits in subsequent generations. Unlike natural selection, the 'selection pressure' is human preference rather than environmental fitness.
While highly effective for achieving specific goals, artificial selection often comes with drawbacks, particularly a reduction in genetic diversity. When a population is bred for specific traits, many alleles not associated with those traits are lost from the gene pool. This can lead to:
Human Intervention: Humans actively decide which individuals are allowed to breed.
Desired Traits: Selection is based on traits that are beneficial or appealing to humans (e.g., high yield, specific appearance, disease resistance, docility).
Selective Breeding: Individuals showing the desired traits are interbred over successive generations.
Faster Change: Artificial selection can often produce significant changes in a shorter timeframe compared to natural selection, as the selection pressure is intense and directional.
Applications: Widely used in agriculture (e.g., modern wheat varieties bred for high yield, short stalks (dwarfism) to prevent lodging; maize from teosinte), livestock (e.g., Belgian Blue cattle for increased muscle mass), and pet breeding (e.g., various dog breeds from wolves).
Reduced Genetic Diversity: Less variation makes populations more vulnerable to new diseases or environmental changes, as there are fewer alleles that might confer resistance or adaptability.
Inbreeding Depression: Repeated breeding of closely related individuals (to maintain desired traits) increases the chance of homozygous recessive alleles expressing deleterious traits, leading to reduced fitness, fertility, and health problems.
Loss of "Wild" Alleles: Potentially useful alleles found in wild relatives might be lost from cultivated or domesticated species, limiting future breeding options.
Comparing Natural and Artificial Selection
It's crucial to be able to clearly differentiate between these two processes in your exams. Here’s a summary of their key differences and similarities:
Selective Agent: Environment (Natural) vs. Humans (Artificial).
Purpose/Goal: Increased survival and reproductive success/adaptation (Natural) vs. Fulfilment of human desires/economic benefit (Artificial).
Speed: Generally slower, gradual changes (Natural) vs. Often faster, more intense changes (Artificial).
Impact on Genetic Diversity: Tends to maintain a broader range of diversity (Natural, though can reduce in specific directional pressures) vs. Significantly reduces genetic diversity (Artificial).
Outcome: Leads to species adapted to their environment (Natural) vs. Leads to varieties/breeds adapted to human needs (Artificial).
Variation: Both processes rely on pre-existing heritable variation within a population.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Discuss how the selective breeding of dairy cattle for increased milk yield can lead to a reduction in genetic diversity and other potential problems. (6 marks)
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Selection for specific traits: Farmers identify cows with high milk yield and bulls from high-yielding lineages. Only these individuals are chosen for breeding, discarding those with lower yields.
A bacterial population of 2,000,000 cells is found on a hospital surface. A mutation for antibiotic resistance is present in 0.05% of the population. The surface is treated with an antibiotic that kills 99.9% of susceptible bacteria but has no effect on resistant bacteria. Calculate the frequency of the resistance allele in the population after one round of selection and reproduction.
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Step 1: Calculate initial numbers of susceptible and resistant bacteria.
Total population = 2,000,000 Resistant percentage = 0.05%
How it all connects
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Glossary
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Quick check
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Revision flashcards
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What is natural selection?
The process where organisms better adapted to their environment tend to survive and reproduce more successfully, leading to an increase in the frequency of advantageous alleles in a population over generations. The environment is the selective agent.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
Variation: Individuals within a population show genetic variation (due to mutation, meiosis, random fertilisation) in their phenotypes. Some variations are heritable.
- ✓
Overproduction: Organisms typically produce more offspring than can survive to reproductive age.
- ✓
Struggle for Existence: Resources are limited, leading to competition for food, mates, space, and avoidance of predators or disease.
- ✓
Differential Survival and Reproduction: Individuals with advantageous heritable traits are better adapted to the environment. They are more likely to survive the struggle, reproduce successfully, and pass on those advantageous alleles.
- ✓
Inheritance: The offspring inherit the advantageous alleles, increasing their frequency in the gene pool of the next generation.
- ✓
Adaptation: Over many generations, the proportion of individuals with these advantageous traits increases, leading to a population that is better adapted to its environment.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
9700/42 · Q3(c)(i)
One of the changes that occurred during the domestication of wild rice to cultivated rice was the loss of the awns from rice grains. Farmers found that long awns made storing and processing rice grains more difficult. It was also observed that rice plants that have grains with no awns have an increased grain yield. Explain the principles used by farmers to produce rice plant grains with no awns.
9700/42 · Q2(a)
Outline the processes that may affect allele frequencies in wildlife populations.
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