This divergence is at a smaller, intraspecific level, which may also imply a persistent dispersal barrier near the northern tip of the peninsula [18], limiting dispersal between east and west for most stygofauna.
This is surprising because phytoplankton drift passively with tides and currents, so that they appear to have enormous potential for long-range and persistent dispersal along with high levels of gene flow between populations.
This view was challenged by "extensionists" such as Joseph Dalton Hooker (1844), who argued that long-distance dispersal across persistent barriers was unlikely, and instead continents must have been connected in the past by tracts of dry land and now-submerged landbridges over which organisms dispersed.
Vegetation dynamics are now determined by this species, and the persistent seed bank and continuing dispersal input have little obvious future in vegetation development.
Hence, two opposite evolutionary hypotheses can let us expect either that isolation mechanisms have resulted in the subdivision of A. cajennense into genetically distinct populations, or that persistent gene flow, maintained by dispersal on hosts, has created a genetic gradient of progressive differentiation throughout the tick distribution range.
Simulations suggest the evolutionary rate signatures are a combination of surface transport mechanisms, which are occurring with irregular periodicity but over longer distances across YNP, and on a relatively slow but persistent hydrothermal "conveyer" that would drive dispersal of chemotrophic communities through a YNP subsurface biosphere.
Here, using the results of Pringle and colleagues [ 12, 13] and a simple numerical model, we quantify Ne for populations whose dispersal is subject to persistent directional flow and find a complementary mechanism for the reduction of Ne.
In particular, we find that the intermediate dispersal principle, namely that globally persistent metapopulations correspond to dispersal rates that are neither too low nor too high, turns out to be very robust even in this explicit context.
This difference in understory community composition could be caused by the persistent environmental differences we observed or by dispersal limitation, as evidenced by greater species richness in post-agricultural sites with greater historical landscape connectivity.
Dispersal must be considered in the persistent discussion of Ne/N ratios (e.g., [ 11]) if there is to be a better empirical understanding of how variation is maintained in natural populations, and for management and conservation questions [ 44].
This study aims to understand this fundamental issue in the evolutionary ecology of populations, the apparently contradicting evidence for large scale dispersal mediating vast (re colonisations concurrently with persistent, fine scale genetic discontinuities in older refugial regions.
