Steep clines have been observed with assortative mating and strong postzygotic barriers [ 42].
Given the dispersal abilities of field crickets (Harrison 1979), strong selection and tight linkage across the region are required to maintain the observed steep clines.
Over time, the initially steep clines established by secondary contact erode (Endler 1977; Barton and Gale 1993), flattening individual genotype and trait distributions and expanding the spatial extent of introgression.
If there is neutral mixing, steep gradients in allele frequencies will decay over time resulting in wide, shallow clines; conversely, if there is a strong barrier to gene flow because of selection against hybrids and/or assortative mating, steep clines may be maintained [ 2, 10, 39].
Thus, the general picture emerging is a complex response of eelgrass variables to the interacting gradients of water quality, salinity and potential top-down control (see below) with steep clines in both chemical and biological variables particularly at the entrance of the Baltic Sea.
Our results support a key prediction from Haldane's Rule that mtDNA will show a steeper cline across a hybrid zone than nuclear markers.
Specifically, we predict that if Haldane's Rule is operating in this system, we will see a narrower (i.e., steeper) cline in the transition of mtDNA haplotypes compared to nuclear markers.
Under steeper clines, the maximum values of θ W were slightly higher, 1.5 vs 1.4, and its change over generations was slightly delayed.
Our results showing steeper clines in mtDNA compared to nuclear markers add to a gathering corpus of research showing discordant rates of DNA transmission across hybrid zones among different marker types [ 11- 18, 22], a pattern that has often been attributed to Haldane's Rule [ 21].
This circular island is about 30 km in diameter and has a steep climatic cline in altitude, from broad-leaved evergreen forest in lowland areas 0-7000 mixedixed broad-leaved evergreen and coniferous forest (700-1200 m), and coniferous forest (1200-1500 m) to bamboo thicket in central areas (1500-1936 m).
