‘Good fences make good neighbours’: Concepts and records of range dynamics in ground squirrels and geographical barriers in the Pleistocene of the Circum-Black Sea area

Ground squirrels were an important member of the Pleistocene steppe-tundra mammal community. They evolved ecological specialisations and exhibit behaviours that make them particularly informative subjects to study palaeoenvironmental constraints affecting species distribution and speciation. Interspecific competition and isolating geographical barriers are considered as the principal factors that define species range boundaries. The present paper provides a first comprehensive compilation of the living and extinct Spermophilus species in Europe. These data suggest ‘patchwork quilt’ model for the expansion and spatial distribution of ground squirrel species. Here we consider mainly small-sized Spermophilus species because large-sized (e.g., S. superciliosus) ground squirrels consist another ‘patchwork quilt’, which overlap the first one. This overlapping of the species ranges is possible because of the size difference that lowers interspecific competition (Hutchinson's rule). We consider two main types of range boundaries. One type includes roughly ‘sub-parallel’ boundaries that oscillate in concert with climatic and vegetational changes (a case of climatically controlled competitive exclusion). The other type consists of roughly ‘sub-meridional’ boundaries corresponding to geographical barriers (e.g., water barriers, mountain ridges); these boundaries are rather stable. Examples of ‘sub-parallel range modifications include: oscillations of boundaries between S. pygmaeus and S. suslicus; the immigration of S. citellus into the Pre-Carpathian area; the branching of S. suslicus from S. pygmaeus; the regional appearance of the Late Pleistocene species S. severskensis and S. citelloides. Examples of ‘sub-meridional events’ are: the crossing of the Danube by S. citellus; the appearance of an isolated population of S. pygmaeus on right bank of the Dnieper during the Late Pleistocene to Middle Holocene; a crossing of the Dnieper river by S. pygmaeus, which resulted in the appearance of S. odessanus; the intrusion of eastern populations of S. pygmaeus into the Trans-Volga areas

Correlation of rheology and light scattering in isotactic polypropylene during early stages of crystallization

ABSTRACT: Structure development during crystallization of isotactic polypropylene (iPP) at low undercooling is studied by small-angle light scattering (SALS) and light transmission. The structure development is related to network formation (gelation) as previously measured on the same sample in dynamic mechanical experiments. Close to the rheologically measured gel point, small-angle light scattering (SALS) patterns in both Hv and Vv modes show circular symmetry; density fluctuations (maximum in Vv invariant) as determined by SALS go through a maximum. Orientation fluctuations develop much more slowly and appear at much later stages of the crystallization process. Growing clusters are characterized by very low anisotropy and low internal crystallinity. The characteristic length scale associated with the cluster at early stages is about 1 ím. At later times, evolution of 2-fold and 4-fold symmetry patterns suggests the development of the anisotropic superstructures. The growth of crystalline clusters leads to a strong increase in turbidity as monitored by light transmission in the Vv mode