Evidence for self-organized criticality in evolution

Data of a number of diierent types have been put forward as evidence for self-organized critical behavior in evolution. These include fossil data on extinction rates and taxon lifetimes, evidence of fractal structure in taxonomic trees, and simulation data from experiments on artiicial life. Here we present a new and very simple model of species origination and extinction which, although not critical, reproduces all of these data well. We suggest therefore, that data of these kinds should not be taken as evidence of self-organized criticality. There has in recent years been considerable interest in the idea that evolution might be a self-organized critical (SOC) process 1{3]. Although this suggestion has been made primarily on the basis of modeling studies, it is important also to consider the empirical evidence in its favor. In this paper we discuss a number of diierent forms of evidence which have been presented and ask whether they really show what they claim to. The most direct form of evidence for SOC behavior in evolution comes from experiments on artiicial life. Using the Tierra system, Adami 4] has measured the lifetimes of competing species and nds that their distribution is well approximated by a power law, which is a common signature of systems at or close to criticality. In biological evolution a similar power-law distribution has been pointed out by Sneppen et al. 5] in data on the lifetimes of genera drawn from the fossil record. Burlando 6] has shown that taxonomic trees have a self-similar hierarchical structure which may taken as evidence of SOC behavior (see Ref. 3] for a discussion). For example, Burlando nds that the frequency distribution of the number of species per genus follows a power law over two or three decades. If we make certain assumptions about speciation, we can demonstrate that this is equivalent to the power-law distribution of genus lifetimes, since the longer-lived genera give rise on average to larger numbers of species.