Phylogenetic niche conservatism: What are the underlying evolutionary and ecological causes?

Contents: Summary 681 I. Introduction 681 II. What is phylogenetic niche conservatism? 682 III. Pattern or process? 683 IV. What causes a pattern of PNC? 683 V. Tests for PNC 687 VI. Conclusion: is a concept of PNC scientifically useful? 961 Acknowledgements 692 References 692 Summary: Phylogenetic niche conservatism (PNC) is the tendency of lineages to retain their niche-related traits through speciation events. A recent surge in the availability of well-sampled molecular phylogenies has stimulated phylogenetic approaches to understanding ecological processes at large geographical scales and through macroevolutionary time. We stress that PNC is a pattern, not a process, and is found only in some traits and some lineages. At the simplest level, a pattern of PNC is an inevitable consequence of evolution - descent with modification and divergence of lineages - but several intrinsic causes, including physicochemical, developmental and genetic constraints, can lead directly to a marked pattern of PNC. A pattern of PNC can also be caused indirectly, as a by-product of other causes, such as extinction, dispersal limitation, competition and predation. Recognition of patterns of PNC can contribute to understanding macroevolutionary processes: for example, release from constraint in traits has been hypothesized to trigger adaptive radiations such as that of the angiosperms. Given the multiple causes of patterns of PNC, tests should address explicit questions about hypothesized processes. We conclude that PNC is a scientifically useful concept with applications to the practice of ecological research.