Natural selection is one of several mechanisms that are responsible for evolution. Three elements need to be present for natural selection to occur. First, traits need to be heritable, meaning that they are inherited from parent to offspring. For example, the colour of a person’s eyes and hair are heritable traits. Second, there needs to be variation in those heritable traits; for example, some people have brown eyes, others blue or green. Third, organisms whose heritable traits are better suited to the environment need to reproduce and leave behind more offspring. It is the variation in those heritable traits that make some organisms better suited to the environment than others. If a trait allows an individual to live for a long time, but doesn’t improve its chances of reproducing, it will not be influenced by natural selection.

For example, think of a population of moths, where some have dark wings, and others have white wings (and wing colour is inherited). If these moths lived in a forest where all of the tree bark was dark in colour, the white coloured moths would be more visible to birds, and birds would eat many of them. Therefore, the dark coloured moths would have more opportunity to reproduce and leave behind more dark coloured offspring. Over time, that particular forest would have a population of moths where dark coloured wings were most common and white wings were very rare.

Where does variation in traits come from? Mutation. Mutation can actually create new traits. Most often, mutations in DNA sequence are either “silent” meaning that they cause no change in the phenotype or they are harmful, meaning that the lead to the death of the organism. But, in some cases, mutations do change the phenotype, and the organism expresses a new trait. This new characteristic will either be advantageous, harmful (but not deadly) or neither.

If you are interested in topics relating to natural selection, see the following articles:
    • Evolution overview
    • Mutations