Cultural Diversification and Decimation in the Prehistoric Record

The history of human cultures is frequently marked by a distinctive pattern of evolution that paleobiologists term diversification and decimation. Under this process, fundamentally new socioeconomic systems appear during periods of dramatic cultural diversification, typically through cultural cladogenesis. Significant diversification episodes come about under conditions that favor group economic success under effective or geographic isolation. Typically shortlived, they are often followed by abrupt decimation under more competitive economic conditions. Regional archaeological sequences, viewed from this perspective, suggest that (1) cultural evolutionary trends are strongly conditioned by historical contingency, though general evolutionary processes are continuously active; (2) the emergence of new systems may be contingent on economic opportunities associated with niche reorganization; and (3) severe competition such as that associated with demographic stress will generally favor decimation

Changes in the tension in dsDNA alter the conformation of RecA bound to dsDNA–RecA filaments

The RecA protein is an ATPase that mediates recombination via strand exchange. In strand exchange a single-stranded DNA (ssDNA) bound to RecA binding site I in a RecA/ssDNA filament pairs with one strand of a double-stranded DNA (dsDNA) and forms heteroduplex dsDNA in site I if homology is encountered. Long sequences are exchanged in a dynamic process in which initially unbound dsDNA binds to the leading end of a RecA/ssDNA filament, while heteroduplex dsDNA unbinds from the lagging end via ATP hydrolysis. ATP hydrolysis is required to convert the active RecA conformation, which cannot unbind, to the inactive conformation, which can unbind. If dsDNA extension due to RecA binding increases the dsDNA tension, then RecA unbinding must decrease tension. We show that in the presence of ATP hydrolysis decreases in tension induce decreases in length whereas in the absence of hydrolysis, changes in tension have no systematic effect. These results suggest that decreases in force enhance dissociation by promoting transitions from the active to the inactive RecA conformation. In contrast, increases in tension reduce dissociation. Thus, the changes in tension inherent to strand exchange may couple with ATP hydrolysis to increase the directionality and stringency of strand exchange.Physic