tree of eukaryotes (Keeling et al., 2005; Leander, 2008). The occurrence and adaptive significance of convergent evolution in microbial eukaryotes, by contrast, is poorly understood, but it is clear from several examples that convergent traits can evolve over vast phylogenetic distances (Leander, 2008). Convergence in very distantly related lineages is particularly compelling because the influence of homologous developmental programs (i.e., intrinsic conditions) in constraining subsequent evolution should be minimal if not absent altogether (Leander, 2008). Therefore, improved understanding of convergent evolution in distantly related microbes will provide a much broader framework for evaluating the forces of natural selection and the potential role of constructive neutrality during the evolution of ultrastructural systems and complex molecular processes.
Eukaryotic cells are built from a few core systems that have become tremendously diverse over the course of evolutionary history. Some systems are remarkably conserved, in particular fundamental molecular processes such as information flow or core metabolism, but even in these systems substantial modifications accumulated in some lineages. In other cases, conserved ancestral building blocks (such as the proteinaceous cytoskeleton involved in locomotion and feeding) are widely shared, but have been used in different ways with diverse outcomes. The origins of other components are less clear and likely more recent, but also show a great deal of morphological variation (examples include photoreception systems or surface armor). Taken together, the diversity of cellular and molecular systems in microbial eukaryotes is simply staggering, and some emerging patterns indicate that convergent evolution played a major role in shaping the overall organization of eukaryotic cells at all levels (Arndt and Reznick, 2008; Leander, 2008).
Below, several features will be described, for which an excessive complexity is a common denominator. This is counterintuitive in single-celled organisms, especially when selective advantages for these complex structures and/or mechanisms remain elusive. We argue that the theory of constructive neutral evolution (Stoltzfus, 1999), which invokes nonselective factors such as excess capacities, can best account for their emergence.
The comparable combinations of ultrastructural features in euglenozoans and alveolates have been appreciated for decades (Taylor, 1987; Bouck and Ngo, 1996). For instance, the cells of benthic predatory species of euglenids and dinoflagellates are streamlined and dorsoventrally flattened and possess batteries of extrusive organelles, or extrusomes, that are similar in morphology and behavior (Fig. 4.2). The mucocysts of euglenids