Extinction and viruses.

The micropaleontological record of deep-sea sediments is demonstrably continuous through time and, therefore, it can be used to study in detail the evolutionary process. Several taxa, including Coccolithophorida, planktic Foraminifera, Radiolaria, and diatoms, are abundantly represented. Some species, apparently evolved in the marginal marine environment, appear suddenly in the record while other species grade into different ones at rates that range from 100,000 years to millions of years. The rates are usually different for different morphological characters within the same lineage. Every few million years a mass extinction occurs--the disappearance of a number of taxa apparently caused by an environmental upset of some kind. These events are used by stratigraphers to subdivide geologic time. Interspersed between mass extinctions are the background extinctions, the individual extinctions of well-established species while the sympatric species exhibit no sign of stress. It is hypothesized that background extinctions are caused by host-specific viral action. It is further hypothesized that background extinctions are a fundamental component of the process of evolution.