Functional complexity in organisms: Parts as proxies

The functional complexity, or the number of functions, of organisms has figured prominently in certain theoretical and empirical work in evolutionary biology. Large-scale trends in functional complexity and correlations between functional complexity and other variables, such as size, have been proposed. However, the notion of number of functions has also been operationally intractable, in that no method has been developed for counting functions in an organism in a systematic and reliable way. Thus, studies have had to rely on the largely unsupported assumption that number of functions can be measured indirectly, by using number of morphological, physiological, and behavioral ``parts'' as a proxy. Here, a model is developed that supports this assumption. Specifically, the model predicts that few parts will have many functions overlapping in them, and therefore the variance in number of functions per part will be low. If so, then number of parts is expected to be well correlated with number of functions, and we can use part counts as proxies for function counts in comparative studies of organisms, even when part counts are low. Also discussed briefly is a strategy for identifying certain kinds of parts in organisms in a systematic way.