The theory of evolution by natural selection is now widely accepted.

The idea

Evolution by natural selection explains how species change over many generations. Individuals in a population vary because of differences in their genes. Competition for limited resources means some variants survive and reproduce more successfully than others. These survivors pass their advantageous alleles to offspring, so those characteristics become more common in the population over time. Darwin proposed this mechanism in the nineteenth century, but acceptance grew only as supporting evidence accumulated. Today the theory is widely accepted because many independent lines of evidence agree: the fossil record, the similarities between organisms, and observations of rapid evolution such as antibiotic resistance in bacteria. A concrete example is the peppered moth, whose colour distribution shifted as pollution changed tree bark colour.

What earns the marks

• Variation exists within a population because individuals have different alleles.
• Competition and environmental pressures mean some variants are more likely to survive and reproduce.
• Successful variants pass their advantageous alleles to offspring, so allele frequencies change over generations.
• The theory is widely accepted because multiple independent sources of evidence support it, not because of a single observation.
• Acceptance increased over time as new evidence, such as genetics and antibiotic resistance, confirmed predictions made by the theory.

Common mistakes and how to put them right

• Thinking individual organisms evolve during their lifetime.

  Put it right: Populations evolve across generations as allele frequencies change.

• Believing that organisms choose to change or that need alone causes change.

  Put it right: Variation arises randomly, and selection acts on existing variation.

• Assuming the theory is accepted because Darwin was famous.

  Put it right: Acceptance depends on evidence that supports the mechanism.

Exam tips

• Use the phrase ‘survive and reproduce’ rather than just ‘survive’, because reproduction passes alleles on.
• Link each point back to alleles or genes to show the genetic basis of natural selection.
• When asked why a theory is accepted, refer to multiple types of evidence rather than one example.