Figure 1.

Intrinsic postzygotic barriers play a dynamic role throughout speciation. Incompatibilities can allow for the build up of divergence, while introgression can reverse incompatibilities by replacing incompatible allelic combinations with compatible ones (bottom arrow; lighter colour indicates increasing genetic homogenization, while darker colouring indicates increasing divergence). Three stages of incompatibilities are outlined: polymorphic (involving segregating alleles within species or between incipient species), simple (involving few interacting alleles), and genetically complex (involving many interacting incompatibility alleles with potential for genetic redundancy). The evolutionary processes that connect these stages are listed below the arrows in italics. Early in the speciation process, various evolutionary forces may generate polymorphic incompatibilities (outlined in [17–20]). These in turn may become species-wide, genetically simple incompatibilities if incompatibility alleles fix. Simple incompatibilities can become genetically complex if incompatibility alleles continue to accumulate in a snowball like fashion, in turn creating genetic redundancy [18,21–30]. Although these are drawn as discrete stages, we note that the path of speciation can be substantially more complex (for example, polymorphic incompatibility may be genetically complex). If there is sufficient selection against hybrids, and sufficient production of unfit hybrids, processes such as reinforcement can complete speciation ([31]; note the dotted line between ‘hybridization & introgression’ and ‘reinforcement’). We highlight that divergence is reversible at almost any point along this continuum (although it becomes increasingly more difficult as incompatibilities become more complex). Also, all of these processes can occur relatively rapidly, and may not reflect divergence times between incipient species (e.g. incompatibilities may remain polymorphic for long periods of time if they are maintained by local selection, or reinforcement may happen relatively rapidly if there is strong selection against the production of unfit hybrids).