The lack of premating barriers suggests that, if individuals of these species frequently encounter one another in the areas where they coexist, post-mating barriers may play a role in the maintenance of reproductive isolation between them.
Even though there is a recent increase in studies on speciation with gene-flow (e.g. [ 25]), it is often assumed that sympatric species have evolved reproductive barriers that allow them to coexist [ 2, 26].
Further studies on Eurasian shads should focus on multiple molecular markers and on particular ecological features, in order to evaluate the nature of putative reproductive barriers between these species in different river systems where they coexist.
The scheme has low technological barrier, minimal impact on the conventional driving behaviour, and can coexist with the traffic light control.
Some studies show that OECs readily coexist with reactive astrocytes and are able to pass through this barrier when injected at the time of injury and after a delay, following scar formation [15], [17], [21], [40], [59], [60].
If, as suggested earlier, the genetic differentiation of these cryptic species is related to past physical barriers to gene flow, the absence of genetic introgression suggests active reproductive isolation where they coexist sympatrically.
Therefore, the barrier between FACTS and HVDC may no longer be of application nowadays, when AC and DC systems coexist along the entire power system.
Ring species, in which two different forms coexist in one region while being connected by a long chain of interbreeding populations encircling a geographic barrier, provide clear demonstrations of the evolution of one species into two.
The differing cultures might coexist, but there are few obvious signs that they intermingle, in part because of territorialism, in part because of language barriers.
