Tuesday, November 3, 2009

More to Evolution than Natural Selection?

Is there really more to evolution than natural selection? There’s no denying that natural selection, acting at the organismic or individual level, probably explains most phylogenetic change. I have to do a little more studying on neutral theory starting with Sewall Wright, then Kimura’s neutral theory of genetic change, and genetic drift to really give a fair treatment of these theories so I won’t address them yet. I’ll leave that for another day except to say that I’m a little suspicious of mechanisms that are claimed not to significantly influence phenotypic change (on which selection obviously acts), yet are touted as important refinements to evolutionary theory.

I thought it might be more interesting initially to talk about a couple of theories or concepts of change that may have played part in evolutionary history, but might really be outside of traditional Darwinism. The first is symbiotic theory advanced by Lynn Margulis (more recently in her book, Symbiotic Planet) and others. It may not be known widely in the general public that perhaps one of the most significant events in evolution (at its barest definition, that of change, or more synonymous with natural history) was the incorporation of bacteria into early eukaryotic cells. Mitochondria are apparently vestiges of a symbiotic association of ancient eukaryotic cells with bacteria. It would be difficult to explain such an event as a result of genetic variation and selection. It may have been the result of a long association of organisms in a common ecosystem, but without this contingent happening multicellularity may have not evolved. For the bacteria once incorporated into cells as mitochondria allowed early eukaryotes to take advantage of an environment that was becoming increasingly oxygenated (around two billion years ago). Another significant symbiotic association was that of cyanobacteria and eukaryotes that gave rise to algae; the result was the chloroplast and a way of generating energy from sunlight.

Other perhaps significant agents of change are the concepts of emergence and self-organization. Stuart Kaufmann (in his book Origin of Order and earlier writings) proposed that stabilization of organic systems that fluctuate outside of equilibrium may and probably have produced new organizational states that result in the emergence of new properties that provide the new construct (organism) with a selective advantage. A simplistic example may be the rise of multicellular organisms. Before multicellularity, the biological world was microscopic. Early on this probably was the results of natural selection: the consequence of genetic variation that led to the ability of cells to associate intimately and become tightly integrated colonies of cells that are successful. But once such an organizational change took place, larger organisms could evolve that would have the selective advantage of being bigger and being able to exploit new resources at this higher level of biological organization. I’d like to conclude with the caveat that Ernst Mayr gave in the introduction of Margulis’ Symbiotic Planet, namely that whatever the source of change that was produced by symbiosis (and we could add here emergence), well weathered natural selection would afterward take over and be the primary source of diversification after emergent characteristics or symbiotic associations had arisen in these lineages.

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