How Does Evolution Work by Natural Selection?
Learn how evolution by natural selection works for AQA GCSE Biology: inherited variation, survival, reproduction and antibiotic resistance explained clearly.
Evolution by natural selection happens when inherited characteristics that help organisms survive and reproduce become more common in a population over generations. The population gradually changes; an individual organism does not evolve during its lifetime.
How does it happen?
Individuals of the same species show variation: they do not all have identical characteristics. Some variation is genetic, meaning it comes from differences in inherited genes. Mutations are changes in genetic material that can produce new variants, although most mutations have no effect on an organism’s visible characteristics.
Organisms compete for resources such as food, space or mates. If an inherited characteristic makes an individual better suited to its environment, that individual is more likely to survive and breed. Its offspring may inherit the helpful characteristic. Over many generations, the characteristic can become more common. This process is natural selection, and the resulting change in a population’s inherited characteristics is evolution.
Whether a characteristic is helpful depends on the environment. A variant that improves survival under one set of conditions may not help under another. Natural selection does not work towards a goal: variants become more common when the organisms carrying them are more likely to survive and reproduce.
An example: antibiotic resistance
A mutation may give a bacterium resistance to an antibiotic. When the antibiotic is used, bacteria without resistance are killed, while resistant bacteria are more likely to survive and reproduce. The proportion of resistant bacteria in the population increases. Bacteria reproduce rapidly, so this change can happen quickly.
The key exam mistake is to say that the antibiotic makes bacteria mutate because they need to survive. Mutations produce variation; the antibiotic selects resistant bacteria that can survive it. In an exam answer, link the helpful inherited characteristic to survival, reproduction and its increase over generations.
This explanation covers AQA GCSE Biology separate science at both Foundation and Higher tier.
