Revealing the factors that promote speciation Timothy G Barraclough1 Alfried P Vogler1 2 and Paul H Harvey3 1 Department of Biology NERC Centre for Population Biology Imperial College at Silwood Park Ascot Berkshire SL5 7PY UK Department of Entomology The Natural History Museum Cromwell Road London SW7 5BD UK 3 Department of Zoology University of Oxford South Parks Road Oxford OX1 3PS UK 2 What biological attributes of organisms promote speciation and ultimately species diversity This question has a long history of interest with proposed diversity promoters including attributes such as sexual selection ecological specialism and dispersability However such ideas are di cult to test because the time scale of processes involved is too great for direct human observation and experimentation An increasingly powerful solution is to investigate diversity patterns among extant groups to infer the nature of processes operating during the evolution of those groups This approach relies on the use of robust phylogenetically based null models to overcome some of the problems inherent in observational inference We illustrate this area by i discussing recent advances in identifying correlates of diversity among higher taxa and ii proposing new methods for analysing patterns in species level phylogenies drawing examples from a wide range of organisms Keywords species diversity species richness evolution diversi cation phylogeny sister group Darwin 1871 West Eberhard 1983 Similarly species range size has been postulated as an attribute with a strong e ect on extinction probability Jackson 1974 Jablonski 1987 and small body size has been proposed as an attribute permitting high equilibrium species numbers within lineages Hutchinson MacArthur 1959 Morse et al 1985 To test ideas of this kind we need to look for replicate evidence for an association between the trait of interest and species richness A recent approach is to compare sister groups that di er in their expression of the trait in question There are four reasons why sister group comparison is currently the best approach for identifying evolutionary correlates of species richness Mitter et al 1988 Zeh et al 1989 Barraclough et al 1998 1 INTRODUCTION Life is made up of species and so understanding the evolution of species richness is fundamental to our understanding of the natural world Traditionally however this area is plagued by the di culties inherent in studying a phenomenon operating over time scales many orders of magnitude greater than our own lifespans Panchen 1992 Recent accumulation of phylogenetic information through the use of molecular techniques has provided novel possibilities for statistical tests of hypotheses concerning the evolution of species richness in extant taxa Mooers Heard 1997 Nee et al 1996a Purvis 1996 Sanderson Donoghue 1996 We illustrate this approach by concentrating on two issues First can biological attributes of lineages in uence the tendency for those lineages to accumulate species richness Second can we use species level phylogenies to identify modes of speciation and patterns of subsequent phenotypic change 1 Replication By including several sister group comparisons in our tests we gain replicated statistical evidence for any trend we observe This replication increases our ability to make general conclusions about an e ect reducing the possibility of detecting accidental historical associations between traits and species richness Cracraft 1990 2 Rates of diversi cation Our interest is in testing whether lineages with a particular attribute accumulate more species than those without it However di erent taxa have di erent ages and so we might expect older taxa to have more species than younger taxa if they are growing simply because they have had time to accumulate more species Sister taxa are by de nition the same age and so by comparing the number of species between sister groups we obtain direct estimates of relative net rates 3 Non independence of taxa Several taxa may share a high species accumulation rate and the same value of a biological attribute simply because they inherited both 2 BIOLOGICAL FACTORS PROMOTING SPECIES RICHNESS Lineages vary in the number of species they contain Possible causes for this variation include di erences in the environment experienced by those lineages Cracraft 1985 Ricklefs Schluter 1993 Rosenzweig 1995 Kerr Packer 1997 or mere chance variation in probabilities of speciation and extinction Raup et al 1973 Raup 1985 An additional possibility is that lineages may vary with respect to biological attributes that in uence the net rate of cladogenesis either through an e ect on speciation rate or extinction rate or the equilibrium number of species a lineage can realize For example many authors have suggested that strong sexual selection by female choice may promote speciation and ultimately species richness Phil Trans R Soc Lond B 1998 353 241 249 241 1998 The Royal Society 242 T G Barraclough and others Factors promoting speciation from a common ancestor In this case these taxa do not represent independent data points supporting an evolutionary correlation between species diversity and the biological variable as both traits have evolved only once Sister group comparisons overcome this problem because di erences between sister taxa have evolved subsequent to their divergence and so necessarily represent independent evolutionary events M ller Birkhead 1992 4 Confounding variables and noise Taxa di er in many attributes apart from the trait of primary interest X and diversi cation rate Some of these attributes may in uence diversi cation rate causing error variation or noise in the data that may obscure any pattern arising from an e ect of trait X on diversi cation rates In addition trait X may have no direct in uence on diversi cation rates but we may observe a correlation through the in uence of unaccounted for confounding variables The shared common ancestry of sister taxa means they will tend to be similar to each other in many respects By comparing sister taxa we are comparing like with like thereby controlling for much potential noise and confounding variation which might otherwise a ict our analysis Read Nee 1995 Harvey et al 1995 Nee et al 1996b Two additional features of sister group comparison should be mentioned with respect to drawing conclusions First these tests detect a correlation and so ultimately it is not possible to determine the direction of causation
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