Types of Trends in Complexity
If evolution possessed an active trend toward complexity, then we would expect to see an increase over time in the most common value (the mode) of complexity among organisms, as shown to the right. Indeed, some computer models have suggested that the generation of complex organisms is an inescapable feature of evolution. This is sometimes referred to as evolutionary self-organization. Self-organization is the spontaneous internal organization of a system. This process is accompanied by an increase in systemic complexity, resulting in an emergent property that is distinctly different from any of the constituent parts.
However, the idea of increasing production of complexity in evolution can also be explained through a passive process. As shown on the right, this involves an increase in variance but the mode does not change. The trend towards higher complexity over time exists, but also involves increasingly smaller portions of biological life.
In this hypothesis, any appearance of evolution acting with an intrinsic direction towards increasingly complex organisms is a result of people concentrating on the small number of large, complex organisms that inhabit the right-hand tail of the complexity distribution and ignoring simpler and much more common organisms. This passive model predicts that the majority of species are microscopic prokaryotes, which is supported by estimates of 106 to 109 extant prokaryotes compared to diversity estimates of 106 to 3ยท106 for eukaryotes. Consequently, in this view, microscopic life dominates Earth, and large organisms only appear more diverse due to sampling bias.
Read more about this topic: Evolution Of Biological Complexity
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