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An example of the potentially significant role that PNC plays in structuring the distribution of biodiversity relates to discussions of the latitudinal species diversity gradient [reviewed by Mittelbach et al. (2007)]. One longstanding hypothesis has been that this pattern is underlain by a simple historical cause, namely a longer time for diversification in the tropics in many lineages than outside of the tropics. If many extant lineages originated and began their diversification under tropical climatic conditions, and if movements of these lineages out into temperate climates occurred only more recently, this alone would go a long way toward explaining the gradient [e.g., Farrell et al. (1992), Latham and Ricklefs (1993), Wiens and Donoghue (2004), Jablonski et al. (2006)]. Rangel et al. (2007) put this verbal argument to the test in a simulation focused on bird biodiversity in South America, showing that realistic patterns can be obtained under a variety of circumstances.

A key ingredient of this argument, which led John Wiens and I to call it the “tropical niche conservatism” hypothesis (Wiens and Donoghue, 2004), is that not all tropical lineages confronted with the retraction of tropical climates during the Tertiary managed to adapt to colder climates. Instead, many of these lineages simply tracked tropical habitats, and therefore became increasingly geographically restricted. If every tropical lineage had been able to readily adjust to cold temperatures and extreme seasonality, then the latitudinal diversity gradient would be far less steep than the one we observe today. This is the important sense in which PNC has explanatory power beyond the time-for-speciation factor.

NICHE CONSERVATISM AND LOCAL COMMUNITY ASSEMBLY

That there should exist a general relationship between phylogenetic relatedness and ecological interactions that are crucial to community assembly, has been evident from Darwin (1859) onward. As G. Evelyn Hutchinson put it (Hutchinson, 1965): “It is evident that at any level in the structure of the biological community there is a set of complicated relations between species, which probably tend to become less important as the species become less closely allied. These relations are of the kind which ensure niche separation.” With the rapid expansion of phylogenetic knowledge [e.g., see Cracraft and Donoghue (2004)], it has now become possible to study this rigorously.

A series of recent analyses imply that PNC influences community composition both by the filtering of the regional species pool based on abiotic niche parameters and through competition and other biotic interactions. The signals of these processes may be reflected in the distribution of species across the phylogeny of the regional species pool [quantified using a variety of phylogenetic diversity measures; Faith (1992a), Webb (2000),



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