vol 170 supplement the american naturalist august 2007 Correlates of Diversification in the Plant Clade Dipsacales Geographic Movement and Evolutionary Innovations Brian R Moore and Michael J Donoghue Department of Ecology and Evolutionary Biology Yale University New Haven Connecticut 06520 abstract We explore patterns of diversification in the plant clades Adoxaceae and Valerianaceae within Dipsacales evaluating correlations between biogeographic change i e movements into new areas morphological change e g the origin of putative key innovations associated with vegetative and reproductive characters and shifts in rates of diversification Our findings indicate that rates of diversification in these plants tend to be less tightly correlated with the evolution of morphological innovations but instead exhibit a pronounced correlation with movement into new geographic areas particularly the dispersal of lineages into new mountainous regions The interdependence among apparent novelties arising from their nested phylogenetic distribution and the correlation between morphological and biogeographic change suggests a complex history of diversification in Dipsacales Overall these findings highlight the importance of incorporating biogeographic history in studies of diversification rates and in the study of geographic gradients in species richness Furthermore these results argue against a simple deterministic relationship between dispersal and diversification like other factors that may influence the probability of speciation and or extinction the impact of dispersal on diversification rates depends on being in the right place at the right time Keywords biogeographic movement dispersal diversification rates key innovation speciation extinction Differences in species richness among geographic regions ultimately reflect differences in the durations that these areas have been occupied by the organisms of interest i e historical biogeography and or differences in rates of diversification i e speciation minus extinction of the groups in those areas Our focus here is on shifts in diversification rate as a factor underlying geographic patterns in species diversity Both the origin of intrinsic traits Corresponding author e mail brian moore yale edu Order of authorship is arbitrary Am Nat 2007 Vol 170 pp S28 S55 2007 by The University of Chicago 0003 0147 2007 1700S2 42201 15 00 All rights reserved including morphological behavioral and physiological novelties and extrinsic events including episodes of biogeographic or climatic change have been hypothesized to promote shifts in diversification rate by altering the probability of speciation and or extinction A comprehensive understanding of the causes of diversification therefore requires incorporation of both elements Surprisingly the relative impact of both biotic and abiotic factors on diversification rates has seldom been evaluated in an explicitly phylogenetic context In plants the search for intrinsic correlates of diversification has tended to focus on a few putative key innovations relating to floral morphology growth form and fruit type dispersal mechanism The role of extrinsic factors in plant diversification has received less attention however an important class of abiotic factors pertains to the impact of biogeography on diversification Dispersal to new geographic areas could conceivably increase the probability of diversification in several ways For example the dispersing species may diversify in response to ecological opportunities afforded by the new area if for instance it is well suited to exploit resources in the new area that are underexploited by residents of that community Alternatively the new area may provide a measure of ecological release for the dispersing species if that area lacks antagonistic elements competitors predators etc that inhibit diversification e g via competitive exclusion Conversely abiotic aspects of the new area may promote diversification either by increasing the opportunity for allopatric speciation e g owing to greater topographic complexity Carlquist 1974 von Hagen and Kadereit 2003 Xiang et al 2004 Hughes and Eastwood 2006 or by decreasing the incidence of extinction e g owing to greater climatic stability reduced environmental harshness Cracraft 1985 Qian and Ricklefs 1999 The relationship between geographic dispersal morphological change and shifts in diversification rate can be conceived of as a cube in which the cells delimit the set of possible interactions fig 1 Of special interest are outcomes in which shifts in diversification are correlated with geographic movement which we refer to as dispersifica Correlates of Diversification in Dipsacales S29 patterns of species richness Overall we hope to illustrate how such studies are likely to benefit from the adoption of an explicitly historical perspective Two Experiments in Dipsacales Adoxaceae and Valerianaceae Figure 1 Graphical model of the relationship between morphological and biogeographic variables and diversification rate The axes of this cube designate change or stasis in morphology biogeographic distribution and rates of diversification such that its cells collectively circumscribe the set of possible correlations between each of the three variables Our focus is on scenarios restricted to the upper tier of the cube which entails significant shifts in diversification rate Within this tier shifts in diversification rate may occur independent of changes in the other two variables cell 1 which may be correlated with some unexplored factor rate shifts may be correlated with the evolution of a morphological trait cell 2 which is consistent with a key innovation scenario rate shifts may be correlated with the dispersal to a new biogeographic area cell 3 which we refer to as a dispersification scenario or rate shifts may be correlated with both morphological and biogeographic change cell 4 Our goal is to explore the extent to which inferred diversification rate shifts populate the cells of this cube tion character evolution key innovation or changes in both variables Our goal is to explore how the cells of this cube are occupied Note that this exercise falls squarely in the realm of data exploration To the extent that we are testing hypotheses they are fairly prosaic the null hypotheses under evaluation are that rates of diversification are stochastically constant and that any shifts in diversification rate are not significantly correlated with
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