Figure 4

Schematic representation of character divergence and fixation in several speciation scenarios. Character divergence varies quantitatively during lineage evolution, with later stages being characterized by marked differentiation (character fixation) at multiple levels, reciprocal monophyly of most gene genealogies, and reproductive isolation [21]. In situations of neutrality or when species are not subjected to novel selective regimes (a, and left lineage of b)-- character fixation is stochastic, due to neutral processes as random genetic drift or because characters are not acquired and fixed in the same order under balancing selection-- a situation known as mutation-order speciation [94]. In cases of speciation along new selective pressures (right lineage of b, c and d), characters subjected to selection are expected to become fixed at early stages of divergence [57, 58]. This is especially evident in sympatry or parapatry (c) when disruptive selection causes fitness-dependent character fixation [57, 95]. In most scenarios the origin of reproductive incompatibilities will be the result of the epistatic effect of new mutations, under divergent or balancing selection [96]. Reproductive isolation is expected to proceed more rapidly if there is divergent selection, both in allopatry (b, c; [e.g. [97]]), and in sympatry or parapatry (d; [reviewed by [57]]), or in those situations where a character mediating species recognition becomes fixed [77, 78]. However, recent evidence indicates that reproductive incompatibilities do not necessarily need to have an adaptive origin, and that they can be the cause of lineage splitting in a neutral or nearly neutral scenario. For example, when the accumulation of new mutations, gene movement, or gene duplication lead to hybrid dysfunction [55]. A textbook-example could be the recurrent cases of polyploidy speciation in plants [98].