Coevolution happens when two or more species influence each other's evolution. It is most often invoked to explain coadaptations between species. We examine parasites and hosts as an example of antagonistic coevolution, which can lead to evolutionary 'arms races'. Possible examples of escalating coevolution in the fossil record are examined.

Key Questions:

• What is coevolution?
• Are parasites and their hosts coevolving?
• How does natural selection effect virulence?
• What is an evolutionary 'arms race' ?
• What happens if a species falls behind in an arms race?
• Summary