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.

Approaches to Macroevolution: 2. Sorting of Variation, Some Overarching Issues, and General Conclusions

“…183–184; “species drift” of Levinton et al 1986 , p. 178 and Gould 2002 , p. 736) can change the amount and nature of variation available for selection at multiple levels. At this level, the random processes involve the wide variety of events encountered on geologic timescales; hence Turner’s ( 2015 , p. 87) statement that “contingency is to species selection as drift is to selection” (see also Eble 1999 ; Chevin 2016 ; and the contingency discussion in Jablonski 2017 ). In fact, the small number of species contained in most clades at any one time suggests that species drift will often be a more significant factor at that level than at the level of organisms within populations (Gould 2002 , p. 736, 893; but see Simpson and Müller 2012 , who argue that the overall scarcity of sustained trends in the fossil record suggests that species drift is a minor factor).…”

“…Despite their different underlying dynamics, all such trends would be fit by OU models with a temporal shift in the “optimum,” or a starting point far from the “optimum” (Hansen 1997 ). (McShea’s 1994 , 2000 “driven trend” includes directional speciation but excludes differential speciation and extinction; see also Turner 2015 and Hopkins 2016 .) Passive trends can arise by diffusion from a fixed boundary (Stanley 1973 , 1979 ; Fig.…”

Evolvability and Macroevolution: Overview and Synthesis

“…Caution is needed, however: the relatively small number of species present at a given time for most genera and many families suggests that apparent differences in evolvability could arise by chance, an effect that has been termed species drift (Levinton et al, 1986:178;Gould, 2002:736;Stanley, 1979 [as "phylogenetic drift"]; Turner, 2015;Chevin, 2016;Jablonski, 2017b). Such scaling effects may be important in comparative analyses, although the general scarcity of sustained macroevolutionary trends, as opposed to the increases or decreases in ranges of trait values that can notoriously mimic trends (Gould, 2002;Jablonski, 2017b), has been taken as evidence against the pervasiveness of species drift (Simpson & Müller, 2012).…”

Towards a Dynamic Interaction Network of Life to unify and expand the evolutionary theory

“…Consequently, some evolutionary concepts may become more important than they currently are to explain evolution. For example, contingency, which means the dependence of an evolutionary chain of events upon an event that itself is contingent, in the sense that it can’t be understood as a selective response to environmental changes [ 18 , 208 , 209 ], is often associated with extraordinary events, like mass decimation. Contingency could come to be seen as a less extraordinary mode of evolution in the history of life, since the ordinary course of evolution might include many cases of contingent events, that is, associations of entities in a transient collective, including any scaffolds—associations that are not necessarily selective responses or the outcomes of processes modeled in population genetics.…”