Natural Selection in Action

In this lesson we look at how natural selection works step by step, examine real-world examples, and explore how it can lead to the formation of entirely new species.

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The Mechanism of Natural Selection — Step by Step

It is essential that you can describe the process of natural selection clearly and in the correct order. Here is the full mechanism:

Step 1: Genetic Variation Exists

Within any population of a species, individuals differ from one another. This genetic variation arises from:

• Mutations — random changes in DNA that create new alleles
• Sexual reproduction — meiosis and the random fusion of gametes produce new combinations of alleles

Step 2: A Selection Pressure Acts

A selection pressure is any factor in the environment that affects an organism's ability to survive. Examples include:

• Predation
• Disease
• Competition for food or mates
• Climate or habitat change
• Human activity (e.g., use of antibiotics or pesticides)

Step 3: Differential Survival

Some individuals have characteristics that make them better adapted to the selection pressure. These individuals are more likely to survive — this is what "survival of the fittest" means.