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By the end of these notes, you should be able to:
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
Explain how environmental factors can act as stabilising, disruptive and directional forces of natural selection
Explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations
Outline how bacteria become resistant to antibiotics as an example of natural selection
Use the Hardy–Weinberg principle to calculate allele and genotype frequencies in populations and state the conditions when this principle can be applied
Describe the principles of selective breeding (artificial selection)
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
Overproduction of offspring: All living things have the ability to produce more babies than are needed to replace their parents. For example, a single pair of rabbits could theoretically produce hundreds of offspring in their lifetime.
Population stability: Despite this ability to overproduce, most populations stay roughly the same size over time. This is because not all offspring survive to adulthood.
Limited resources: The reason populations don't grow forever is that resources like food, water, shelter, and space are limited. There simply isn't enough for everyone.
Competition: When resources are limited, organisms must compete with each other to survive. This competition is often called the struggle for existence.
Genetic variation: Within any population, individuals are not identical. They have different characteristics (called phenotypes) because they have different versions of genes (called alleles). For example, some rabbits might have brown fur while others have white fur.
Heritable traits: These different characteristics can be passed from parents to offspring through genes. A rabbit with brown fur can pass the allele for brown fur to its babies.
Selection pressures: Environmental factors that affect an organism's chance of survival are called selection pressures. These could be predators, disease, temperature, food availability, or competition from other organisms.
Survival of the fittest: Organisms with characteristics that help them survive in their environment are described as being better adapted. They have higher fitness, which means they are more likely to survive and reproduce.
Passing on advantageous alleles: When better-adapted individuals survive and reproduce, they pass their helpful alleles to their offspring. Over many generations, these advantageous alleles become more common in the population, while less helpful alleles become rarer.
Evolution: This gradual change in the frequency of alleles in a population over time is called evolution. Natural selection is the main mechanism that drives evolution.
Imagine a population of rabbits living in a grassy area:
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