Contingency and determinism in evolution: Replaying life’s tape

• Published in: Science (American Association for the Advancement of Science) Vol. 362; no. 6415
• Main Authors: Blount, Zachary D., Lenski, Richard E., Losos, Jonathan B.
• Format: Journal Article
• Language: English
• Published: United States The American Association for the Advancement of Science 09.11.2018
• Subjects: Adaptation; Adaptation, Biological - genetics; Animals; Biological Evolution; Biologists; Biology; Citric acid; Comparative studies; Contingency; Convergence; Determinism; Divergence; E coli; Ecological monitoring; Ecological niches; Empirical analysis; Environmental conditions; Evolution; Evolutionary biology; Experiments; Fitness; Genetic divergence; Genetic drift; Genetics; Gould, Stephen Jay; Humans; Laboratories; Laboratory Experiments; Mutation; Natural selection; Niches; Phenotypes; Planetary environments; Planetary evolution; Population studies; Populations; Reproducibility; Reproductive fitness; Selection, Genetic
• ISSN: 0036-8075; 1095-9203; 1095-9203
• DOI: 10.1126/science.aam5979

The evolutionary biologist Stephen Jay Gould once dreamed about replaying the tape of life in order to identify whether evolution is more subject to deterministic or contingent forces. Greater influence of determinism would mean that outcomes are more repeatable and less subject to variations of history. Contingency, on the other hand, suggests that outcomes are contingent on specific events, making them less repeatable. Blount et al. review the numerous studies that have been done since Gould put forward this question, both experimental and observational, and find that many patterns of adaptation are convergent. Nevertheless, there is still much variation with regard to the mechanisms and forms that converge. Science , this issue p. eaam5979 Historical processes display some degree of “contingency,” meaning their outcomes are sensitive to seemingly inconsequential events that can fundamentally change the future. Contingency is what makes historical outcomes unpredictable. Unlike many other natural phenomena, evolution is a historical process. Evolutionary change is often driven by the deterministic force of natural selection, but natural selection works upon variation that arises unpredictably through time by random mutation, and even beneficial mutations can be lost by chance through genetic drift. Moreover, evolution has taken place within a planetary environment with a particular history of its own. This tension between determinism and contingency makes evolutionary biology a kind of hybrid between science and history. While philosophers of science examine the nuances of contingency, biologists have performed many empirical studies of evolutionary repeatability and contingency. Here, we review the experimental and comparative evidence from these studies. Replicate populations in evolutionary “replay” experiments often show parallel changes, especially in overall performance, although idiosyncratic outcomes show that the particulars of a lineage’s history can affect which of several evolutionary paths is taken. Comparative biologists have found many notable examples of convergent adaptation to similar conditions, but quantification of how frequently such convergence occurs is difficult. On balance, the evidence indicates that evolution tends to be surprisingly repeatable among closely related lineages, but disparate outcomes become more likely as the footprint of history grows deeper. Ongoing research on the structure of adaptive landscapes is providing additional insight into the interplay of fate and chance in the evolutionary process.