When taxa have an ecological relationship so that they affect each other’s evolution. If the relationship is mutually beneficial they may become reliant on each other, or in a host-parasite relationship the evolution of a slight competitive advantage to one leads to natural selection in the other so that virulence and resistance broadly keep pace with each other. When these gradual step-wise processes continue in regions isolated from each other a native host may be wiped out by an alien parasite, or it may spread uncontrollably..

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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