The phylogeny of protists is an area of exciting, challenging research. The marvelous diversity of protist body forms and nutritional lifestyles seems reason enough for a fascination with these organisms, but questions about how the multi-cellular eukaryotic kingdoms originated from the protists stimulate further interest. Fortunately, the tools of molecular biology, such as rRNA sequencing, are making it possible to explore evolutionary relationships among the protists in ever greater detail and with greater confidence (see Chapters 25 and 26).
We will discuss several protist clades in this chapter, as well as a few other groups of more uncertain phylogenetic status. Some biologists refer to many of these clades as kingdoms; others refer to them as subkingdoms, and still others refer to them as phyla. This choice of words is not of immediate concern to us here, so we'll just call them "groups." We'll describe the following groups: diplomonads, parabasalids, eugleno-zoans, alveolates, stramenopiles, red algae, chlorophytes, and choanoflagellates (Table 28.1; Figure 28.9).
As we shall see, some of these protist clades consist of organisms with very diverse body plans. On the other hand, certain body plans, such as those of amoebas and those of slime molds, have arisen again and again during evolution, in groups only distantly related to one another. We'll begin our tour of protist clades with two of apparently ancient origin.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.