Diversification Rates Increase With Population Size and Resource Concentration in an Unstructured Habitat

M. H. H. Stevens^*^,1, M. Sanchez^*^,, J. Lee^* and S. E. Finkel

^* Department of Botany, Miami University, Oxford, Ohio 45056-3653, University of Puerto Rico, Mayaguez, Puerto Rico, 00681 and Molecular and Computational Biology Program, Department of Biological Sciences, University of Southern California, Los Angeles, California 90089-2910

^¹ Corresponding author: Department of Botany, Miami University, Oxford, OH 45056.
E-mail: hstevens{at}muohio.edu

Understanding the mechanisms controlling the generation andmaintenance of biodiversity provides some of the planet's greatestand most pressing challenges. Variation in resource concentration,which varies widely at multiple scales, may cause biodiversityto increase, decrease, or exhibit a unimodal response and underlyingmechanisms remain obscure. We established experimental culturesof long-term stationary phase (LTSP) Escherichia coli to testwhether per capita heterozygosity varies with resource concentration,and, if so, whether population sizes associated with differentresource concentrations contributed to these patterns. Our resultsprovide the clearest example to date of increasing per capitaheterozygosity with increasing resource concentration. Further,our experimental manipulations of population size, independentof resource concentration, provide the first unequivocal evidencethat population size is one of the underlying factors controllingper capita heterozygosity along such resource gradients. Specifically,we show that cultures with higher maximum population sizes,associated with higher resource concentrations, have higherper capita heterozygosity. These experiments provide the firstexperimental evidence for an underappreciated factor controllingbiodiversity along resource gradients—population size.This direct evidence of population size influencing diversificationrates has implications for regional and global scale patternsof biodiversity.