Despite their intuitive appeal, reciprocal transplant experiments are laborious in the best systems and impossible in most, so alternative approaches for estimating ecogeographic isolation are needed. A potential solution is the application of ecological niche models. This approach uses GIS technology to build a predictive map of suitable environmental conditions from spatial data on abiotic variables and species occurrences ( Peterson and Vieglais 2001; Kozak and Wiens 2006; Kozak et al. 2008; Nakazato et al. 2008; Warren et al. 2008). Niche similarity can be calculated using recently developed methods for comparing ecological niche models ( Warren et al. 2008); however, to assess how differences in niche translate into ecogeographic isolation, it is useful to project niche models onto the geographic range over which the two taxa occur. Niche similarity and overlap in geographically projected niche models will commonly yield different results. For example, two species of plants may be identical in most niche parameters such as precipitation, temperature, seasonality, etc., but grow on different soil types. A niche similarity index would show these two species as highly similar, but if the two soils to which they are adapted occur in disjunct areas, the geographically projected niche model would predict that this small niche difference translates into substantial ecogeographic isolation.

In the past decade, a number of papers have suggested that it is useful to distinguish ecological from nonecological mechanisms to elucidate the role of natural selection in speciation, for example, Schluter (2000, 2001, 2009), Rundle and Nosil (2005), and Nosil et al. (2009). What sets these efforts apart from previous discussion is the proposal of the term, “ecological speciation,” which is defined variously as The strength of divergent selection experienced by the allopatric populations could affect which forms of isolation arise and at what rate. For example, if habitats in allopatry are very different, divergent selection is likely to be strong, and ecogeographic isolation will likely arise first as presented above. However, if selection is only weakly divergent or uniform, ecogeographic isolation may be much slower to arise, and other forms of isolation may outpace its evolution. Inland and coastal forms of M. guttatus experience selection for different growth and flowering times resulting in flowering phenology with little overlap.Coyne and Orr (2004) defined reproductive isolation that is favored by selection (i.e., reinforcement) as “direct,” whereas all other forms of isolation arise as “indirect” outcomes of selection. We prefer to use “reinforcement” instead of “direct” and to use “byproduct” when selection does not favor reproductive isolation per se. Within byproduct, direct isolation arises when the trait conferring reproductive isolation is the target of selection, and indirect isolation arises by pleiotropy or linkage. Although the biological mechanisms described by our system are equivalent to the terminology proposed by Coyne and Orr (2004), we feel that our usage of direct and indirect is more intuitive as it is analogous to the way these terms are used in traditional selection experiments.

Ecological niche modeling and reciprocal transplant data could be combined to take advantage of the strengths of both approaches. If reciprocal transplants were performed across a wide range of climatic conditions, it would be possible to build an ecological niche model using fitness of the transplants in place of presence/absence data. Projecting this “transplant niche model” onto the geographic landscape could then be an excellent tool for measuring the overlap in ecogeographic isolation. Because these models would be based upon the fitness of organisms, they would be a more reliable predictor of adaptation to habitat. Future studies that compare results from the two methods would help determine if niche modeling is an appropriate proxy for transplant studies. Assessing the “Importance” of Reproductive BarriersThe approach of Lowry et al. (2008a) does not eliminate the issue of nonindependence because any correlation in barrier strength for different barriers will still affect the mean. Is it necessary to consider nonindependence when estimating the relative importance of different barriers? When one trait causes multiple forms of isolation, statistical nonindependence does not occur unless the expression of one form of isolation affects the outcome of other forms. Therefore, multiplicative combination of barriers may often be suitable. There may be cases in which the strength of one barrier is moderated by the presence of another. For example, two taxa that reside at different altitudes may be primarily isolated by ecogeography. The timing of reproduction may also differ, but this could be due only to differences in the growing season between the two habitats, and is thus an indirect pleiotropic effect of the genes that contribute to ecogeographic isolation. In these cases, including temporal isolation in the linear sequence of independent barriers may be inappropriate, but it is not known how often this type of nonindependence occurs in nature.