The Pleistocene and post−Pleistocene evolutionary history of many North Atlantic intertidal invertebrate species is well known, but the evolutionary history of the deep North Atlantic fauna is poorly understood, specifically whether colonization of the deep North Atlantic paralleled the patterns observed in shallow water. Contemporary pan−Atlantic species distributions could result from several colonization pathways that connected different regions of the Atlantic at different times ( e.g. Arctic, Antarctic or Panamanian path− ways). To test potential colonization pathways we quantified geographic variation in nu− clear and mitochondrial markers from Atlantic samples of Nucula atacellana, a pan−Atlantic deep−sea protobranch bivalve, using N. profundorum in the eastern central Pacific as an outgroup. We combined existing 16S data from North and South Atlantic populations of N. atacellana with new sequences of 16S, COI, and an intron of calmodulin from those populations, and newly sampled populations near Iceland. Population genetic analyses indicated a subtropical expansion via the Central American Seaway. We found no evidence for Transarctic migration to the Atlantic in N. atacellana , which suggests that colonization pathways may differ significantly between shallow− and deep−water fauna.

Habitat fragmentation is one of serious threats to biodiversity of nature in today's world. The present study of a typical steppe species Iris pumila L. (Iridaceae) has analyzed the impacts of geographical isolation and population size on genetic diversity and population structure in conditions of habitat fragmentation. The key indices of population genetic variability calculated from the ISSR markers data were on average as follows: Shannon diversity index (S) – 0.188; unbiased Nei’s gene diversity (He) – 0.123; and the average measure of Jaccard’s genetic distances between individuals within populations – 58.4%. Although the largest population had significantly higher values of S and He, the small and marginal populations also showed a comparable level of variation. Most of the genetic variation of I. pumila was distributed within the populations. A strong correlation was found between Nei’s genetic distances and geographic distances between the populations. According to the Bayesian analysis, genetic structure of the populations was highly homogeneous; however, the presence of admixed genotypes indicated the possibility of gene flow between the populations at present.