My advisor has recently got me listening to Whad’ya Know. My first reaction: It’s like Wait Wait…Don’t Tell Me! Only not as funny, not as interesting, and not as good. I’ve been downloading the podcasts for the past couple of weeks, and I’m not sure whether I’ll keep subscribing in iTunes.
I’m only bringing this up because last week’s episode contained a very egregious example of someone knowing just enough biology to get themselves in trouble. The sad part was that the person should have known better. Why? She teaches biology at the university level.
What happened? At the midpoint of the first hour of the show, Michael Feldman conducted the quiz. The in-house participant was Deb, a grasslands ecologist. Deb was asked about the diet of early Americans, which turned out to be seaweed. Deb then pointed out that seaweed is not a plant. Huh? Then she said it’s algae, which is a protist. Really? Deb doth protist too much, methinks.
It’s really sad to watch Deb struggle to display her out-dated understanding of eukaryotic taxonomy. It’s especially disconcerting because she’s extremely proud of her knowledge that eukaryotes (a term she does not use) are divided into four kingdoms — animals, plants, fungi, and protists. And she requires her students to know this. That is, she requires her students to gain an incorrect understanding of eukaryotic evolution.
First off, seaweed is a plant. It’s just not a land plant. The green algae, of which seaweed is a member (one of the few things Deb got right), are aquatic plants. They have no vascular system for transporting water and nutrients because they can absorb those from the surrounding water. The water also provides them with support. Algae also have no roots, no seeds, and no fruits. As plants began to colonize land, they evolved adaptations to the terrestrial environments they encountered. First, came a waxy cuticle to prevent water-loss. Then, they evolved vasculature (xylem and phloem) to transport water and nutrients to and from the roots and leaves. After that came pollen and seeds. The pollen allows individuals to fertilize each other without needing water to transport the male gametes. The seeds provide protection for the developing embryo of the plant. Flowering plants, the largest extant radiation of plants, also evolved flowers and fruits to aid in reproduction. See here for more on the evolution of land plants.
But what about that outdated misnomer, protist? If, as Deb claims, anything eukaryote that’s not an animal, fungus, or land plant, then nearly every eukaryote is a protist. That’s because the majority of eukaryotes don’t fall into one of those three small taxa.
Land plants make up a small portion of the plant group (shown in green in the top-left of the figure). Many seaweeds are green algae, a close relative of land plants (close in the scope of this tree). Animals are also a single lineage on this tree, highlighted in a red box in the bottom-right. And fungi, the most diverse of the three non-protist taxa, are made up of a few of the lineages closely related to animals in the Opisthokont clade. For more on this topic, see this post on the eukaryotic phylogeny and this article.
Everything else that I didn’t just describe would be classified as a protist according to Deb. That’s all the Excavates, Rhizaria, Chromalveolates, red algae, and green algae. Protists also make up a large portion of the Unikonts, the group containing animals and fungi. You’re probably asking, the whats? Exactly. Much of those organisms are single-celled amoeba-like critters. They’re not worthy of any sophisticated, evolutionarily-accurate taxonomy, and they get thrown in the meaning-less grab-bag known as “protists”.
What’s wrong with that taxonomic strategy? First, it does not reflect the evolutionary relationships of the organisms. Protist is a paraphyletic taxon — essentially, a group of organisms that excludes some organisms within that evolutionary group (like including reptiles to the exclusion of birds). Also, understanding the true evolutionary relationships is essential for studying the biology of these organisms. For example, the “protists” that are closely related to animals help us understand how animals evolved from single-celled organisms. Anyone knowledgeable in eukaryote systematics does not use the term “protist”, except to point out that it is incorrect.
I would hope that Deb, a woman in charge of teaching college students about evolutionary biology, would realize that protist is a meaningless classification. No contemporary introductory biology course worth anything uses that classification scheme. The taxon protist only still lingers because of its historical use. Let’s hope that it does not linger much longer.