Biological evolution is defined as the process by which new species or populations of living things develop from preexisting forms through successive generations; it explains the appearance of new species and varieties through the action of various biological mechanisms such as natural selection, genetic mutation or drift, and hybridization. Sudden or intensely visible accelerations in evolution are certainly examples of dramatic environmental change [sometimes the former are caused by the latter], with wide-ranging effects on human society. The materials under this theme represent just a few of the stories waiting to be told in such a rapidly changeable world.