Fire-Proneness as a Prerequisite for the Evolution of Fire-Adapted Traits

Fire as a major evolutionary force has been disputed because it is considered to lack supporting evidence. If a trait has evolved in response to selection by fire then the environment of the plant must have been fire-prone before the appearance of that trait. Using outcomes of trait assignments applied to molecular phylogenies for fire-stimulated flowering, seed-release, and germination, in this Opinion article we show that fire-proneness precedes, or rarely coincides with, the evolution of these fire-adapted traits. In addition, fire remains central to understanding germination promoted by smoke among species occurring in non-fire-prone environments because of the historical association of their clade with fire. Fire-mimicking selection and associated exaptations have no place in understanding the evolution of fire-adapted traits because we find no support for any reversal in the fire–trait sequence through time. Ancestral trait reconstruction using accurately dated molecular phylogenies is revolutionizing our understanding of fire-directed evolution among plants. Ancestral fire-prone lineages may also be identified on molecular phylogenies using fossil charcoal and reconstruction techniques. Ascertaining whether or not the onset of exposure to fire preceded the advent of putatively fire-adapted traits enables the identification of unique adaptations to fire. Fire-mimicking (multi-agent) selection and associated exaptations are alternative explanations of apparent fire-adapted traits that require selection via drought or non-unique components of fire to precede selection by fire. Smoke-stimulated germination among plants in non-fire-prone habitats may not be an anomaly if it can be shown that they possess a dormant gene mechanism inherited from a fire-prone past.