Historical Contingency and the Explanation of Evolutionary Trends

Abstract

One “big question” of macroevolutionary theory is the degree to which evolutionary history is contingent. A second “big question” is whether particular large-scale evolutionary trends, such as size increase or complexity increase, are passive or driven. Showing that a trend is passive or driven is a way of explaining it. These two “big questions” are related in both a superficial and a deep way. Superficially, defending historical contingency and showing that major trends are passive are two complementary ways of downplaying the importance of natural selection in evolutionary history. A passive trend is one that’s not explained by selection. In order to appreciate the deeper connection between the two issues, it is necessary to distinguish different senses of contingency (especially sensitivity to initial conditions vs. unbiased sorting). It’s plausible that passive trends are generally due to unbiased sorting processes. Serendipitously, thinking of contingency as unbiased sorting also helps to clarify its relationship to species selection, which some think of as biased sorting. Macroevolutionary theory thus turns out to have considerable unity.