A comparison of the effects of random and selective mass extinctions on erosion of evolutionary history in communities of digital organisms.

The effect of mass extinctions on phylogenetic diversity and branching history of clades remains poorly understood in paleobiology. We examined the phylogenies of communities of digital organisms undergoing open-ended evolution as we subjected them to instantaneous "pulse" extinctions, choosing survivors at random, and to prolonged "press" extinctions involving a period of low resource availability. We measured age of the phylogenetic root and tree stemminess, and evaluated how branching history of the phylogenetic trees was affected by the extinction treatments. We found that strong random (pulse) and strong selective extinction (press) both left clear long-term signatures in root age distribution and tree stemminess, and eroded deep branching history to a greater degree than did weak extinction and control treatments. The widely-used Pybus-Harvey gamma statistic showed a clear short-term response to extinction and recovery, but differences between treatments diminished over time and did not show a long-term signature. The characteristics of post-extinction phylogenies were often affected as much by the recovery interval as by the extinction episode itself.

Selective press extinctions, but not random pulse extinctions, cause delayed ecological recovery in communities of digital organisms.

A key issue concerning recovery from mass extinctions is how extinction and diversification mechanisms affect the recovery process. We evolved communities of digital organisms, subjecting them to instantaneous "pulse" extinctions, choosing survivors at random, or to prolonged "pulse" extinctions involving a period of low resource availability. Functional activity at low trophic levels recovered faster than at higher levels, with the most extensive delays seen at the top level. Postpress communities generally did not fully recover functional activity in the allotted time, which equaled that of their original diversification. We measured recovery of phenotypic diversity, observing considerable variation in outcomes. Communities subjected to pulse extinctions recovered functional activity and phenotypic diversity substantially faster than when subjected to press extinctions. Follow-up experiments tested whether organisms with shorter generation times and low functional activity contributed to delayed recovery after press extinctions. The results indicate that adaptation during the press episode degraded the organisms' ability to re-evolve preextinction functionality. There are interesting parallels with patterns from the paleontological record. We suggest that some delayed recoveries from mass extinction may reflect the need to both re-evolve biological functions and reconstruct ecological interactions lost during the extinction. Adaptation to conditions during an extended disturbance may hinder subsequent recovery.