Second, separate introductions from native populations to similar latitudes in the introduced range can result in parallel clines in the absence of selection.
Clines of increasing body size with latitude have been known in the Old World since the 1950s (Prevosti 1955; Misra and Reeve 1964; Pegueroles et al. 1995), and parallel clines in female wing size had evolved in the New World within two decades of the introduction (Huey et al. 2000; Gilchrist et al. 2004).
Here we investigate whether flies on all three continents have evolved parallel clines in desiccation and starvation tolerance, such that flies in low-latitude regions (hot, dry) might have increased stress resistance.
Secondary contact along such a front would initially produce parallel clines similar in width (i.e., concordant) and position (i.e., coincident).
Parallel clines on different continents or along independent temperature gradients can thus offer an invaluable opportunity to study thermal coadaptation since the role of temperature in driving those clines is quite compelling.
Recently, McNab (2010) further emphasized the role of food availability by combining different geographical clines in body size under a common 'resource rule'.
The pattern of isolation by distance, with no clines in diversity, is a signature of a genetic equilibrium situation, with no trace of a past colonization process.
Here the cline of the difference in potential fitness in two morphs is replaced by the cline in morph frequency as long as the difference in potential fitness is counteracted by NFDS, as follows.
Many clines evidenced in marine population genetics studies actually result from selection acting at multiple loci, as revealed by the finding of concordant clines in contact zones between hybridizing taxa, that is hybrid zones (Duggins et al. 1995; Bierne et al. 2003; Sotka et al. 2004; Murray and Hare 2006; Zbawicka et al. 2014).
Within this network of upland rivers, watershed position was accompanied by a strong cline in genetic diversity.
