The Evolution of Evolution: Seen through the Eyes of a Slime Mold

All eukaryotic organisms evolved through the aegis of natural selection, but there is a big difference in how this occurs depending on the size of the organism. In unicellular microorganisms, which at one time in early Earth history were probably the only living eukaryotic forms, natural selection plays a relatively minor role, but with size increase, first made possible by the invention of multicellularity, selection plays an increasingly central role in evolutionary change. This has come about because larger forms isolate themselves from their environment and become self-sufficient. However, microorganisms are at the total mercy of changes in their immediate environment. These differences have had some interesting consequences. For instance, extinctions, as in dinosaurs, are common among mega-organisms, and living fossils, such as the horseshoe crab, are rare. This is in sharp contrast to microorganisms, among which we find living fossils to be common.