Diversity as a product of interspecial interactions
  We demonstrate diversification rather than optimisation for highly interacting organisms in a well mixed biological system by means of a simple model and reference to experiment, and find the cause to be the complex network of interactions formed, allowing species less well adapted to an environment to flourish by co-interaction over the `best' species. This diversification can be considered as the construction of many co-evolutionary niches by the network of interactions between species. Evidence for this comes from work with the bacteria Escherichia coli, which may coexist with their own mutants under certain conditions. Diversification only occurs above a certain threshold interaction strength, below which competitive exclusion occurs. 
Daniel Lawson, Henrik Jeldtoft Jensen and Kunihiko Kaneko