Table 2: In chronological order, a summary of some major previous models of Dobzhansky-Muller Incompatibilities (DMIs). Uniform selection refers to scenarios where selection occurs (i.e., some alleles or genotypic combinations are more fit than others), but selection is not divergent between different ecological environments (i.e., no environmental variation). Uniform selection thus fits the “mutation-order speciation” model recently advanced by Schluter [26]. In contrast, divergent selection fits the ecological speciation model. IBD: Isolation-by-distance.

ModelProcess driving divergenceMigration and gene flowDiscrete patches (populations)MethodNotes

[16]Divergent selectionYesClinal divergence rather than discrete patchesAnalyticalDivergent selection is on major locus, modifier locus affects the fitness of alleles at the major locus, but is not under divergent selection itself, barrier to neutral gene flow not examined. Also, focus of the model is how the modifier, once fixed, alters the shape of the cline, rather than on how the modifier fixes in the first place.
[18]Divergent selectionYesClinal divergence rather than discrete patchesAnalyticalDivergent selection is on major locus, modifier locus causes DMI but is not under divergent selection itself, barrier to neutral gene flow not examined.
[27]Stochastic
(drift)
NoYesSimulationDrift fixed new mutations in small populations; two loci considered.
[28]UnspecifiedNoYesAnalyticalThe classic paper on the accumulation of DMIs. No assumptions are made about evolutionary causes of substitutions driving DMIs.
[29, 30]Uniform selection, not explicitly divergentYesYesAnalyticalMigration can allow the spread of a gene combination whose component alleles are not individually favored, but the conditions for this are restricted (low migration, strong advantage for particular genotypic combinations).
[31]Uniform selection or driftNoYesAnalytical and simulationPopulation subdivision has no effect on time to speciation by drift. Under selection, speciation occurs more rapidly between two large populations than between many small ones.
[32]Unspecified; populations initially fixed for different incompatible allelesYesYes, stepping stoneAnalyticalFocused on the effect of intrinsic incompatibilities on the barrier to neutral gene flow.
[33]UnspecifiedNoYesAnalyticalAdds stochasticity of molecular evolution to the results of Orr [28].
[34]Uniform selectionYesYes, between neighboring patchesSimulationSpeciation in subdivided populations occurs most rapidly when there is some migration (which spreads incompatible alleles); many (250) loci are considered.
[35]Uniform selectionYesYesSimulationStrong selection counters the inhibitory effects of gene flow, permitting the evolution of intrinsic postmating isolation.
[36]Uniform selectionYesYes,AnalyticalMigration occurs through a single, spatially structured population and affects the accumulation of DMIs.
[37]Uniform selectionYesYesAnalyticalLoci causing DMIs themselves are under selection (i.e., new alleles at these loci can be favored), but selection is uniform. Barrier to gene flow is considered, including the effect of chromosomal inversions.
(Gavrilets [14]) and previous models by same author (e.g., [38, 39])Divergent selection, uniform selection, or driftYesYesAnalyticalIncompatible alleles advantageous in one environment but neutral in other (thus wider range of scenarios examined in current study), barrier to gene flow examined.
[40]Uniform selectionYesNo, clinal with IBDSimulationIBD and outbreeding depression were sufficient to drive speciation, in the absence of ecological differences.
[41]Uniform selectionYesYesSimulationFocused on the origin of DMIs due to different, incompatible mutations diverging between populations adapting to identical selection regimes. Gene flow strongly impeded strong divergence via such a “mutation-order” process (c.f., Schluter [26])
Current studyDivergent selectionYesYesAnalytical and simulationFocus on the origination of incompatibilities in the face of gene flow. Examined scenarios with and without direct, divergent selection on modifier loci which also contribute to intrinsic isolation. Examined barrier to neutral gene flow caused by evolution of intrinsic isolation.