Table 1.

Proposed models of diversification leading to heterogeneity in populations

Proposed mechanism	Example(s)	Reference(s)
Alternative niches or resources in the new environment	Specialization on one of several carbon sources or one of several locations (e.g., air-water interface) in the environment	171, 377
Mutation-selection balance	Can occur when a high mutation rate and the slow fixation of mutations lead to several coexisting genotypes in a population	74, 374
Negative frequency-dependent selection	The competitive fitness of a type increases as it becomes rarer so does not eliminate competitor	178
Trade-offs	With a SPaNC trade-off, a type with higher stress resistance but a lower growth rate coexists with a variant with less resistance but a better growth rate; trade-offs can also lead to coexistence of rate yield variants	126, 127
Trade-offs plus frequency-dependent selection	Theoretical, no example available	76
Trade-offs plus mutation rate differences	Theoretical, no example available	19
Convergent evolution of equally fit types	Different genotypes result from the same regulatory change and provide the same benefit in the selection environment	125, 353
Niche creation	Cross-feeding between a producer of a fermentation product and an evolved user organism; cannibalism	107, 280, 284