Several studies have now used large-scale comparative approaches to test how reproductive isolation (henceforth abbreviated as “RI”) evolves over time and how it is influenced by the biogeographic and ecological context. For instance, it has been shown that the evolution of prezygotic isolation is enhanced in sympatry (Dobzhansky et al. 1968; Coyne and Orr 1989, 1997; Noor 1997; Yukilevich 2012) and is correlated with the degree and direction of hybridization costs between sympatric species (Yukilevich 2012). It has also been shown that hybrid sterility typically evolves faster than hybrid inviability and that hybrid male sterility often precedes hybrid female sterility (Haldane 1922; Coyne and Orr 1989, 1997; Wu 1992; Price and Bouvier 2002). Finally, RI has been shown to correlate with the degree of ecological divergence (Funk et al. 2006) and the evolutionary rates of RI are known to vary substantially across different taxa (e.g., Sasa et al. 1998; Presgraves 2002; Mendelson 2003; Moyle et al. 2004). The discovery of these patterns has provided a clear way to test theoretical predictions about speciation.
One of the most debated topics concerning rates and patterns of speciation revolves around the question of latitudinal biodiversity gradients found in many marine and terrestrial organisms (e.g., Ricklefs and Schluter 1993; Mittlebach et al. 2007; Schemske et al. 2009). Numerous hypotheses for explaining greater species richness in the tropics have been proposed, including those based on differences in rates of extinction and speciation between tropical and nontropical species (see Mittlebach et al. 2007). Although differences in extinction rates may be important (e.g., Weir and Schluter 2007), the question of whether rates of speciation and evolution of RI differ across latitudes is still largely unknown.
Many have proposed that mechanisms of speciation could ultimately differ with latitude. For instance, it has been argued that tropical environments may promote greater genetic differentiation and allopatric speciation due to mountain ranges being more effective as barriers to dispersal (Janzen 1967; Moritz et al. 2000). At the same time, temperate species are more likely to experience repeated secondary contacts and recurrent gene flow due to cyclical glaciation events, limiting allopatric speciation (Dynesius and Jansson 2000). There is general support for greater genetic differentiation among tropical populations compared to nontropical populations (Martin and Mackay 2004). Others suggest that historically greater species richness in the tropics generates a positive feedback loop, further increasing ecological opportunity for specialization and niche divergence (e.g., Gentry 1989; Schemske 2002). Thus, biotic interactions and coevolutionary arms races are predicted to be more intense in the tropics, possibly accelerating rates of speciation as local populations have to adapt to dynamic biological communities (Dobzhansky 1950; Schemske 2002; Schemske et al. 2009). It has also been proposed that warmer temperatures of the tropics accelerate rates of biological processes, including faster molecular evolution and increased mutation rates (Rohde 1992; Martin and Palumbi 1993; Allen et al. 2006, but see Brohman and Cardillo 2003).
These potential differences between tropical and nontropical latitudes may translate to more rapid accumulation of RI in the tropics. This may include faster evolution of prezygotic isolation such as mating and habitat preferences and/or faster evolution of postzygotic isolation resulting from hybrid ecological inferiority, sterility, or inviability. A recent review of latitudinal biodiversity gradients has called attention to the fact that little has been done to explicitly test whether tropical species are evolving RI faster than their nontropical counterparts (Mittlebach et al. 2007). To date, the best dataset available to test this question is in the genus Drosophila (Coyne and Orr 1989, 1997; see recent extension in Yukilevich 2012).
In the present article, I performed such an analysis on species pairs of Drosophila. Although all informative species pairs were analyzed with respect to this question, the major goal was to ask whether RI evolves faster in the tropics particularly at the early stages of divergence. By restricting attention to this well-studied genus, the goal was to reach specific conclusions about latitudinal patterns of speciation in Drosophila in the hope that future work would test whether these findings also apply to other species groups.