STRESS, EXTINCTIONS AND EVOLUTIONARY CHANGE: FROM LIVING ORGANISMS TO FOSSILS

1. Natural populations are exposed to environmental stress of varying intensities. This provides a reference point for extrapolations from the living biota to fossils and vice versa. 2. Evolutionary change is likely when there are resources in excess of maintenance and survival needs. It is largely precluded at species borders by the metabolic costs of stress; from this follows climatic tracking by species. 3. A relatively small increase in abiotic stress could underlie extinctions of stress-sensitive endemic species and the spread of stress-resistant generalist and widespread species. Widespread fossil species appear resistant to extinction under the stress level of normal background extinctions. 4. Synergistic interactions among generalized stresses should increase the likelihood of extinctions, especially for stresses with energetic consequences. 5. Some marine organisms survived the K-T mass extinction event because of stress-evasion mechanisms such as stress-resistant life-cycle stages with low metabolic rates. 6. In moderately stressed and narrowly fluctuating environments, sufficient genetic variability and metabolic energy should be available to permit adaptation. In these environments phyletic gradualism is expected. 7. In highly stressed and widely fluctuating environments, a punctuated evolutionary pattern is expected whereby stasis occurs most of the time. 8. Evolutionary patterns therefore can vary depending on the details of the interaction between stress, environmental fluctuations, energy availability and genetic variability. 9. Little evolutionary change is expected when the availability of energy is severely restricted. Examples include cave animals in stable but stressed environments and ‘living fossils’ in widely fluctuating but stressed environments. 10. Since the primary effect of abiotic stress may be at the level of energy carriers, a reductionist approach permits generalisations in considering extinctions and conditions under which diversification is likely.