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in this group harbor photosynthetic symbionts that are intermittently replenished by feeding.

Both euglenozoans and alveolates have a reputation for “doing things their own way,” which is to say that they have developed seemingly unique ways to build important cellular structures or carry out molecular tasks critical for their survival. Why such hotspots for the evolution of novel solutions to problems should exist in the tree of life is not entirely clear. However, the deeper we look into these groups, the more often it is found that they are also evolving strikingly similar mechanisms for achieving these essential biological functions. Significantly, however, there is a great weight of phylogenetic data that show these lineages are not closely related: of the 5 eukaryotic supergroups hypothesized to explain all eukaryotic diversity, alveolates and euglenozoans fall into 2 different supergroups, chromalveolates and excavates, respectively (Fig. 4.1). The support for these supergroups as a whole remains contentious (Keeling et al., 2005; Leander, 2008; Hampl et al., 2009; Keeling, 2009), but there is strong support from phylogenomics and many individual phylogenies and rare genomic characters for a specific relationship between alveolates and stramenopiles on one hand, and euglenozoans and heteroloboseans on the other hand (Hampl et al., 2009). Moreover, no analysis of eukaryotic phylogeny has ever suggested they are closely related to one another. Still more significantly, the majority of the characteristics we discuss below are not universal to all members of either alveolates or euglenozoans, but rather appear to have evolved within a subgroup of each lineage. Altogether, the distribution of these characteristics can really only adequately be explained by convergent evolution. Below, we will examine some of these examples of convergence and what the cooccurrence of convergent traits may tell us about how they evolved.

CONVERGENT EVOLUTION

Recognizing the independent origins of similar traits in distantly related lineages—convergent evolution—allows us to better understand how different environmental and intrinsic conditions have shaped the characteristics of organisms over time; each specific example of convergence reflects a fundamental biological problem and its possible solutions. The causes of convergent evolution are varied and can involve camouflage, mimicry, biomechanical optimization, molecular constraints, developmental canalization, and character-state reversals. Examples of convergent evolution range from the biochemical level to the behavioral level and are best characterized within animals and land plants (Conway Morris and Gould, 1998; Zakon, 2002; Emery and Clayton, 2004; Arndt and Reznick, 2008), which collectively represent only a small portion of the full



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