Phylogeny: Classifying Succulents

Before we dive right in, I want to just let you know that phylogeny is the study of evolutionary relationships between different organisms. Whether you believe in evolution or not, we can all agree that succulents are all related to one another, and the lines are becoming more blurred when hybrids come into play. Today, we will take a look at the relationships between Echeveria and Sedum succulents! This will be Part 1 of a two week series on Succulent Genetics and Cross-Breeding.

Basic Taxonomy (Old School)

Taxonomy is one classification system to put organisms into groups that are part of larger groups that are part of larger groups...Life is the biggest group that contains all the other groups, then domains contain many kingdoms, kingdoms contain many phylums, phylums contain many classes, classes contain many orders, etc. If you go to here, you will be able to see how succulents are classified by these taxonomic categories. Be aware that this can get more complicated with subfamilies and different terms for plants than for animals.

Classification	Echeveria gibbiflora	Humans
Kingdom	Plantae	Animalia
Phylum/Clade	Angiosperms	Chordata
Class/Clade	Eudicots	Mammalia
Order	Saxifragales	Primates
Family	Crassulaceae	Hominidae
Genus	Echeveria	Homo
Species	gibbiflora	sapiens

What's a binomial name?

You will often see organisms given a simple binomial name in italics, which is the Genus (capitalized) followed by the species (lower-case). Echeveria gibbiflora is a binomial name and the genus is often shortened to its initial. You may see instead: E. gibbiflora. Many people don't italicize binomial names, but in research articles and scientific journals, they do.

What about hybrids or cross-bred succulents?

Echeveria "Perle von Nurnberg" is a hybrid of Echeveria gibbiflora of the variety "metallica" and Echeveria elegans. This is denoted by an x, so that you may see the Perle von Nurnberg referred to as E. gibbiflora x E. elegans. With so many hybrids up and about and hybrids of hybrids, you can see how things can get complicated very fast.

Problems of Taxonomy and the Basics of Phylogeny

Before modern DNA-sequencing and other techniques, people classified organisms based more on traits and functions of life. This led to some organisms being placed in entirely different categories from their close relations. Now with modern technology, we've found that we need to move around some organisms on the family tree so that they are put somewhere that makes sense. For example, green algae was considered a protist (Kingdom: protista) that could be either uni or multicellular. Now, it is considered a plant (Kingdom: plantae), and this was within just a few years! 

Phylogeny focuses on the evolutionary development of organisms, so it is more compatible with DNA information. A common way to display a phylogeny is through a phylogenetic tree or cladogram. Some people use these words interchangeably, but basically it is like a family tree where every intersection is a common ancestor. Here's a simple example: You can see that flies and moths have a common ancestor and that bees and flies have a common ancestor. You can also see that butteries and flies are more related to each other than butterflies and bees, and that beetles are more related to ants than they are to flies.

Sedum vs. Echeveria

Succulents in the past were classified by different people in different parts of the world at different times. It is no surprise that sometimes they did not agree. These classifications were based on leaf shape, pattern, and flower, among other physical traits. Now, with DNA testing, scientists have put together a phylogeny (like the diagram shown above) to show how Echeveria and Sedum succulents are all related.

Who's in the (Crassulaceae) family?

Members of the Crassulaceae family are usually pentamerous (have patterns of 5 leaves) and possess radial symmetry (actinomorphic). Within the Crassulaceae family, there are several clades or groups. The Acre clade is a very large group that contains 526 species subdivided between 8 different genera (plural of genus). Within the Acre clade, the two largest genera are Sedum and Echeveria.

What is a Sedum?

Sedum is the largest genus of the of the Crassulaceae family and is part of the Acre clade.  It is also a very ambiguous and relatively undefined genus since its conception in 1753 by Linnaeus, the father of taxonomy. In 1930, Berger recognized 22 different sections or subgenera of the Sedum genus, but now we consider 10 of these to be genera themselves or part of other groups. These include Gaptopetalum, Hasseanthus, Perriorosedum, Populisedum, Prometheum, Pseudorhodiola, Rhodiola, Sedella, and Telephium. Most of the classifications come from papers published in 1930, 1936, 1995, and 2007 by various different scientists who often did not agree.

Above: Jelly beans sedum (aka Pork and Beans or Sedum rubrotinctum)

What is an Echeveria?

Echeveria is the second largest genus in the Acre clade of the Crassulaceae family and it contains about 145 species. Over the years, species have been moved to and from the Echeveria genus to other genera. Some succulents are very closely related, so there is now what is called the "Echeveria group." This group contains Pacyhphytum (think moonstones),  Pachysedum, Thompsonella, and Graptopetalum. Some groups were once considered separate genera like Oliveranthus and Urbinia, but are now considered Echeveria. 

See the phylogenetic results for yourself here

I can't post the phylogenetic trees from the article because of copyright issues, sorry, but here are the highlights in case you can't access the article:

There exists a common ancestor for Sedum nussbaumerianum (coppertone stonecrop), Pachyphytum oviferum (moonstones), Echeveria colorata, Graptopetalum payaguayense (ghost plant), and Echeveria gibbiflora.

The ghost plant is more related to the Echeveria gibbiflora than to any of the others mentioned. The Echeveria colorata is more related to the moonstones than to the coppertone stonecrop, though all three are more related to each other than to the E. gibbiflora. Cool, huh?

I hope that you've learned a little bit more about the family trees of your beloved succulents and where all the names come from. How all of this relates to succulent cross-breeding will be addressed in the next segment of Succulent Science. See you then and hit that subscribe button!

Liked this? Check out other succulent genetic stuff below/coming soon. In order:
Phylogeny: YOU ARE HERE
Hybrids: Cross-breeding (Mendelian genetics, how-to, and flower basics)
Variegation Genetics: (recessive trait, endosymbiotic theory)
Hybridization: Ploidy (compatible ploidy and hybrids)