A central challenge in evolutionary biology concerns the mechanisms by which complex adaptations arise. Such adaptations depend on the fixation of multiple, highly specific mutations, where intermediate stages of evolution seemingly provide little or no benefit. It is generally assumed that establishment of complex adaptations is very slow in nature, as they demand special population genetic circumstances. However, blueprints of complex adaptations in molecular systems are pervasive and generally have multiple independent origins, indicating that they can readily evolve. Here we discuss the potential mechanisms whereby such complex adaptations evolve with the aim to derive testable predictions. We first summarize the limits of non-adaptive scenarios, and argue that complex molecular traits can readily evolve through series of adaptive steps in dynamically changing environments.

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