Ecological Divergence and the Origins of Intrinsic Postmating Isolation with Gene Flow

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.

Model	Process driving divergence	Migration and gene flow	Discrete patches (populations)	Method	Notes
[16]	Divergent selection	Yes	Clinal divergence rather than discrete patches	Analytical	Divergent 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 selection	Yes	Clinal divergence rather than discrete patches	Analytical	Divergent 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)	No	Yes	Simulation	Drift fixed new mutations in small populations; two loci considered.
[28]	Unspecified	No	Yes	Analytical	The classic paper on the accumulation of DMIs. No assumptions are made about evolutionary causes of substitutions driving DMIs.
[29, 30]	Uniform selection, not explicitly divergent	Yes	Yes	Analytical	Migration 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 drift	No	Yes	Analytical and simulation	Population 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 alleles	Yes	Yes, stepping stone	Analytical	Focused on the effect of intrinsic incompatibilities on the barrier to neutral gene flow.
[33]	Unspecified	No	Yes	Analytical	Adds stochasticity of molecular evolution to the results of Orr [28].
[34]	Uniform selection	Yes	Yes, between neighboring patches	Simulation	Speciation in subdivided populations occurs most rapidly when there is some migration (which spreads incompatible alleles); many (250) loci are considered.
[35]	Uniform selection	Yes	Yes	Simulation	Strong selection counters the inhibitory effects of gene flow, permitting the evolution of intrinsic postmating isolation.
[36]	Uniform selection	Yes	Yes,	Analytical	Migration occurs through a single, spatially structured population and affects the accumulation of DMIs.
[37]	Uniform selection	Yes	Yes	Analytical	Loci 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 drift	Yes	Yes	Analytical	Incompatible 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 selection	Yes	No, clinal with IBD	Simulation	IBD and outbreeding depression were sufficient to drive speciation, in the absence of ecological differences.
[41]	Uniform selection	Yes	Yes	Simulation	Focused 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 study	Divergent selection	Yes	Yes	Analytical and simulation	Focus 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.