Last week’s Phylogeny Friday introduced the three domains of life: bacteria, archaea, and eukaryotes. The bacteria and archaea are commonly referred to as prokaryotes, although that creates a paraphyletic taxon. Today, we will focus on the eukaryotes (organisms with nuclei and organelles). This taxon contains plants, animals, fungi, and a bunch of other lineage with which you are probably not familiar. Recently, a group of Canadian researchers reviewed our current understanding of the evolutionary relationships of the eukaryotes, creating the following phylogeny.
The eukaryotes can be divided into five supergroups (they left off the Damn Yankees), but the relationship of these groups is unresolved. The green supergroup in the upper left contains the photosynthetic lineages (both the green and red ones), including the land plants. In the bottom right are the unikonts, a taxon that contains the fungi (ascomycetes, basidiomycetes, zygomycetes, chytrids, and microsporidia which were recently classified as fungi) and animals (in the red box). The other three supergroups — excavates, rhizaria, and chromalveolates — are made up entirely of organisms commonly known as protists.
What does this tree tell us about eukaryotes? Animals and land plants, the two taxa people are most familiar with, are represented by single lineages — twigs on the tips of the tree. The fungi make up a substantially larger part of the tree, but they are also extremely diverse relative to animals and land plants (in terms of ecology and physiology). The rest of the lineages of the tree (including the other unikonts and non-land plants) are lumped into the paraphyletic taxon protists. Considering the diversity of this group, and the fact that these organisms make up the majority of the tree, we could say that protist is a synonym for eukaryote.
Next week we’ll take a closer look at one of the many lineages of the eukaryotes: the animals. Why? It could be because animals are the most interesting, it could be our bias (we are animals and we’re interested in learning about ourselves), or it could be because I know more about this lineage than any of the other ones.
Keeling, PJ, G Burger, DG Durnford, BF Lang, RW Lee, RE Pearlman, AJ Roger, MW Gray. 2005. The tree of eukaryotes. Trends Ecol Evol 20: 670-676. doi: 10.1016/j.tree.2005.09.005