B15.4 Natural selection and Speciation


Natural selection

Although members of a species have many features in common they are not identical to each other.

Variation occurs as a result of mutations that create new forms of genes, known as alleles, which can be inherited by the next generation.

Members of a species reproduce at a rate that creates more offspring than the environment can support, and some die because of factors such as predationfood shortage or disease.

Well-adapted individuals are those which have inherited characteristics such as the ability to avoid predators, compete strongly for food, or resist disease. They survive and reproduce, passing on the genes that make them successful. They are said to have a selective advantage.

This process is called natural selection, and it ensures that most members of a species are well adapted to their environment.

Natural selection allows species to respond to the changes in the environment.

Industrial melanism

Industrial melanism in light and dark peppered moths. (http://www.bbc.co.uk/staticarchive/3111395608844f7a4615c271c5d8f59ef6e3a954.gif)

Light peppered moths camouflage themselves against light lichens on trees.

During the nineteenth century pollution killed off some of the lichens and soot deposits caused the bark to appear darker. Light coloured moths were no longer camouflaged and were eaten by birds. The dark moths had a better camouflage.

As a result, dark moths had a greater chance of reproducing and passing on the alleles that made them dark. This led to a gradual increase in the proportion of dark moths until light moths became very rare…


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