Phylogenetic niche conservatism and the evolutionary basis of ecological speciation

<jats:title>ABSTRACT</jats:title><jats:p>Phylogenetic niche conservatism (<jats:styled-content style="fixed-case">PNC</jats:styled-content>) typically refers to the tendency of closely related species to be more similar to each other in terms of niche than they are to more distant relatives. This has been implicated as a potential driving force in speciation and other species‐richness patterns, such as latitudinal gradients. However, <jats:styled-content style="fixed-case">PNC</jats:styled-content> has not been very well defined in most previous studies. Is it a pattern or a process? What are the underlying endogenous (e.g. genetic) and exogenous (e.g. ecological) factors that cause niches to be conserved? What degree of similarity is necessary to qualify as <jats:styled-content style="fixed-case">PNC</jats:styled-content>? Is it possible for the evolutionary processes causing niches to be conserved to also result in niche divergence in different habitats? Here, we revisit these questions, codifying a theoretical and operational definition of <jats:styled-content style="fixed-case">PNC</jats:styled-content> as a mechanistic evolutionary process resulting from several factors. We frame this both from a macroevolutionary and population‐genetic perspective. We discuss how different axes of physical (e.g. geographic) and environmental (e.g. climatic) heterogeneity interact with the fundamental process of <jats:styled-content style="fixed-case">PNC</jats:styled-content> to produce different outcomes of ecological speciation. We also review tests for <jats:styled-content style="fixed-case">PNC</jats:styled-content>, and suggest ways that these could be improved or better utilized in future studies. Ultimately, <jats:styled-content style="fixed-case">PNC</jats:styled-content> as a process has a well‐defined mechanistic basis in organisms, and future studies investigating ecological speciation would be well served to consider this, and frame hypothesis testing in terms of the processes and expected patterns described herein. The process of <jats:styled-content style="fixed-case">PNC</jats:styled-content> may lead to patterns where niches are conserved (more similar than expected), constrained (divergent within a limited subset of available niches), or divergent (less similar than expected), based on degree of phylogenetic relatedness between species.</jats:p>