Evolution of Complexity
To study the evolution of complexity in organisms, we need an understanding of complexity that enables us to measure it. In biology today, organismal complexity has two main operational senses: (1) a horizontal sense: the number of different part types at a given hierarchical level (e.g., the number of cell types in a multicellular individual) and (2) a vertical sense: the number of levels of nestedness of parts within wholes (e.g., a eukaryotic multicellular individual is one level of nestedness above a free-living protist). How do horizontal and vertical complexity behave in evolution? For horizontal complexity, an increasing trend is predicted by current theory, that is, by the zero-force evolutionary law (ZFEL), but at most hierarchical levels, evidence is lacking and the existence of a trend is uncertain. For vertical complexity, there is unambiguous evidence for a trend in the maximum, a rise in the maximum hierarchical level achieved by organisms over the history of life. However, the underlying mechanism of change and the forces driving the trend are unknown. Interestingly, there is some evidence that the rise in vertical complexity, the addition of new levels, is - when it occurs - accompanied by systematic losses in horizontal complexity at lower levels.
