A level Biology exam revision resources written by A level Examiners


Natural selection is the process that explains how one species can change into another species. It explains how one type of organism can change into another, or into two others.

The theory of evolution by natural selection was proposed by Charles Darwin and is now a key part of modern biology. Evolution by natural selection draws on four factors:

  1. Within a species there is variation in characteristics.
  2. Many of these variations can be inherited.
  3. Organisms usually overproduce (superfecundity). Far more offspring are produced than survive to become adults and breed themselves
  4. Populations of organisms usually remain fairly constant in size.

For any new variation (that has arisen as a result of gene mutation) to be passed on to the next generation it must confer a survival advantage to the organisms that have the variation. Darwin referred to this as the 'survival of the fittest' that is the organism that best fits it's environment will be more likely to survive to reproductive age, and pass on it's genes to the next generation.

The peppered moth is a common moth in woods and is normally found in a white colouration. A melanic (blackend) form of the moth also exists.


In Britain the white form was better camouflaged on the bark of birch trees. The melanic form was more visible, and therefore was more highly predated by birds. White moths predominated as they survived to reproductive age, and passed on their genes to their offspring.


However, when the Industrial revolution occurred, the amount of factories in major cities increased dramatically, and huge numbers of trees became soot blackened around major cities. This caused the environment of the moths to change. The melanic form became the form which was more camouflaged.


When this occurred the melanic form became the one which was 'most fit' to the environment, and therefore became more likely to survive to reproductive age, and pass on its genes. The Melanised form therefore became the more abundant form of moth.

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