No one really disagrees that such strange bedfellows need to be separated and returned to their true relatives, i.e. to be arranged in monophyletic categories - by definition a common ancestor and all its descendants. Much of the taxonomic busywork of the past century has been to ferret out illicit polyphyletic associations and rearrange organisms into a classification based on true relationship. That’s not where the butter battle comes in, however.
The classic Butter Battle Book by Dr. Suess satirized the human propensity for escalating silly disagreements into cataclysmic space-time-continuum-threatening warfare. The story began with an argument between two groups of whimsical creatures, one of which believed passionately that toast should be eaten butter-side up, and the other that it should be eaten butter-side down. Something akin to this feud began in the botanical world during the latter part of the 20th century, and it’s not over yet. It wasn’t about polyphyletic groups, but about a much subtler distinction that results in some traditional taxa being called “paraphyletic.”
Both traditional and phylogenetic systems of classification have the form of a hierarchy of categories. The level of the hierarchy is referred to as ranks. Domains are currently the highest rank of taxon, and they are subdivided into Kingdoms. Progressively smaller subcategories are given lesser rank. The principle ranks are Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species, and in-between ranks (subclass, superorder, etc.) can be inserted as needed.
2. Each branch point on a cladogram results in two sister taxa, which must be given equal rank (genus, family, class, etc.).
This is probably as clear as mud at this point. Let me use a familiar example from the animal world to quickly explain this concept. Reptiles and birds are two of the traditional classes of vertebrates, but modern studies have overwhelmingly confirmed that birds descended from reptiles, in particular from a group of dinosaurs that included velociraptors and Tyrannosaurus. While birds are a monophyletic group, the reptiles are paraphyletic (also referred to as a grade to differentiate it from a clade).
In the greatly simplified diagram to the right, clades A through F are different groups of reptiles. Technically, the sister group of birds is clade F, which includes their closest dinosaur relatives. Therefore, according to the rules, birds must be have the same rank as group F. Birds are still a distinct group, but taxonomically must be considered just a highly specialized subgroup of reptiles. Note also that the traditional classification obscures the fact that group F is much more closely related to birds than it is to group A (turtles and their relatives). Remember that it is the goal of phylogenetic taxonomy to better reveal these relationships in the classification system.
|An acceptable solution in phylogenetic systematics|
is to lump reptiles and birds into a single class, with
birds recognized as a subclass (along with clades A
|The alternate solution recognizes many classes of|
reptiles along with the birds.
Walter Judd, Roger Sanders and Michael Donoghue, in a truly landmark paper in 1994, identified a number of pairs of traditional angiosperm families, of which one was paraphyletic and theo other monophyletic. In most cases, the monophyletic family at the top of the tree was primarily temperate in distribution and consisted primarily of herbaceous plants, while the clades branching along the grade consist mainly of woody trees and shrubs. The pairs include: Apocynaceae (oleander family)/Asclepiadaceae (milkweed family), Araliaceae (Aralia family)/Apiaceae (carrot family), Capparaceae/Brassicaceae (mustard family), among others. To keep this posting under control, I’ll just talk briefly about the first case.
|The traditional Apocynaceae, like this |
Pachypodium, have relatively
conventional flowers with petals united
into a flaring tube that often twists like a
I’ve provided this lengthy explanation as a prelude to getting into the real butter battle, which may take one or two more posts to explore fully. It is first of all a battle between two views of how we divide up an undisputed diagram of relationship into taxonomic entities that are convenient, practical, and informative. Phylogenetic taxonomists, in forbidding paraphyletic taxa, are emphasizing information on phylogenetic branching patterns. Clades are real entities that represent evolutionary history and groups of related organisms. They are predictive of the characteristics to expect within the group. Recognizing birds and reptiles as equivalent groups obscures the fascinating relationship of birds to dinosaurs, and the fact that Tyrannosaurus rex is much more closely related to ostriches than it is to turtles.
|The Asclepiadaceae, represented by this milkweed |
(Asclepias), has the same basic features as the Apocynaceae,
but adds some unique floral features related to specialized
modes of pollination.
There is, moreover, another dimension to the debate brought up by traditionalists that is not so easily dismissed, and which we might sum up as the “ancestor problem.” This arises when one views, theoretically at least, the entire sweep of evolutionary history. The first member (common ancestor) of every genus (or family, etc.) logically had ancestors that were in an older genus. Whenever a new genus-worthy group of species became distinct, it would have rendered its ancestral genus paraphyletic (just as the evolution of birds as a distinct new class of organisms rendered the reptiles paraphyletic). So the evolution of new genera (or families or classes) appears to be impossible according to the rules of phylogenetic taxonomy, or else to force the retroactive fragmentation of the older taxa. I will take up this tricky issue in a future posting (See "Making the Ancestor Problem Go Away," October 18, 2012).