Contingency (evolutionary biology)

In evolutionary biology, contingency describes how the outcome of evolution can depend on the particular history of a lineage, since evolution is a historical process in which later outcomes are shaped by the events and interactions that preceded them. Stephen Jay Gould developed the idea at length in his 1989 book Wonderful Life, framing it as a thought experiment: if the history of life were rewound and replayed many times, would the same outcomes appear again, or would small historical differences send it down drastically different paths. Laboratory and field studies inspired by the idea have found that evolutionary outcomes are often repeatable, especially among similar founding populations over observable experimental timescales, and that convergent evolution is common in nature, particularly among closely related species that share genes and developmental patterns, suggesting that both contingency and convergence shape the diversity of life; examples include the Escherichia coli long term evolution experiment, in which only one lineage evolved the ability to use citrate as a food source, woodpeckers and aye-ayes which solve similar ecological problems with different anatomical solutions reflecting their different evolutionary histories, and isolated regions such as New Zealand whose distinctive wildlife reflects a contingent history.