Understanding Evolution—Naming Matters but Not That Much
January 28, 2016
The arguments go like this: “If Archaeopteryx is classified as a bird, then it isn’t a dinosaur, and you can’t say that birds came from dinosaurs.” Or, “Since Tiktaalik is classified as a fish, it can’t be the missing link between fish and amphibians.” Those of us with training in biology sigh, because this is a horrible misunderstanding of how classification works.
Taxonomy—Classification and Naming Species
One of the first things I teach in any of my paleontology classes is that taxonomy underpins nearly every subdiscipline of paleontology. I then go on to explain that taxonomy is also very subjective and, at times, even seemingly arbitrary.
So then, what is Taxonomy?
Taxonomy is the study of naming or classifying organisms. It is the proper use of scientific names. Taxonomy is why every scientist on Earth, regardless of his or her native tongue, knows that the name Homo sapiens refers to humans.
Thus, taxonomy is important, as it ensures that everyone uses the same name for the same species of organism, despite what word they might use in their local language. In principle anyway.
Taxonomy becomes a challenge where you have distinct species that can interbreed (for example, dogs, wolves, and coyotes). Are they one species or many? Such questions spur discussion at scientific meetings, and biologists work hard to come up with a better definition of the concept of species.
In the case of fossils, we cannot observe behaviors, breeding compatibilities, or distinctions between the forms of males and females of the same species—all of which we know can help us distinguish different species in living organisms. All we have are shapes and sizes of skeletons and occasionally some remains of soft tissues. As paleontologists, we have to decide what differences are sufficient to make two species distinct.
This process is subjective, dependent upon the experience, expertise, and general preference of the paleontologist. One paleontologist may see three different species from two different families, and another may think that they’re all members of the same species with a lot of variability in form. Or that some are male and others are female. Or that some represent juveniles and others adults.
Nevertheless, taxonomy is fundamentally important in paleontology. When paleontologists name new species, they must follow very strict rules to ensure that their new species is valid or acceptable by the rest of the paleontological community. A group of scientists—not just paleontologists but also zoologists—have formed the International Commission on Zoological Nomenclature (ICZN), to ensure that when new species of animals are named it is with good reason. If you’ve ever read a technical paper that names a new species, you’ll notice that it very carefully describes the new species and distinguishes the new species from similar species.
Even with this process, paleontologists may yet argue whether two species are really different or not. For example, the debate about Triceratops and Torosaurus: Some paleontologists think that Triceratops is just a young Torosaurus. Others argue that they are definitely different species.
The fundamental question is does it matter?
The answer is yes!
Even though we might not agree about the details of one species or another, by carefully naming species, we as paleontologists can always be confident that we’re talking about the same organisms by using the scientific names of animals that were named following the rules put forth by the ICZN. In later research, scientists can specify what they mean when they are talking about a particular species. They can specify if they consider Triceratops and Torosaurus the same, or if they believe that they are just closely related species.
Once this is clearly laid out, paleontologists can approach the real questions they want to explore.
Phylogeny—Relationships among Species
Much of paleontology focuses on the evolutionary relationships among fossils and modern species. Fundamentally, we want to know where we came from. This is phylogeny.
In some ways, taxonomy reflects phylogeny. All the mammals are in one group (Class Mammalia) because they’re all related. We put all the birds together (Class Aves). Bigger groups, such as the Class Mammalia, are divided into smaller groups, the Family Equidae (which includes horses, donkeys, and zebras) or Family Canidae, the Dogs (includes wolves, foxes, and our pet dogs). All the members of such taxonomic groups are (presumably) closely related. (This is not always the case, however, and is a worthy topic for later posts.)
The only taxonomic division that has a clear definition is that of species, which as noted above can only be applied to a new animal when certain rules, laid out by the ICZN, are followed. All the other taxonomic groupings, so-called “higher taxa” such as Family, Order, and Class, are considerably more subjective. They exist for convenience to help us keep our thinking on organisms organized, but they have little basis in the reality of the relationships among living things. There’s no defined number of species that need to be grouped to make a Family, nor is there any specified amount of difference needed to place species in different families or classes.
The fossil record provides abundant evidence of where smaller groups (like Family) came from within the larger groups (like Order), but the ancestors of the smaller groups often aren’t terribly similar to the more modern members of the group. For example, horses came from animals that were not particularly horse-like, so how do we classify them? The vast majority of paleontological research shows that birds came from a group of dinosaurs that weren’t birds at all, though they do have many features in common. How do we classify these?
Classifying such organisms into higher categories is difficult. But the classification (the taxonomy), aside from species, does not affect the actual relationships (the phylogeny), because taxonomy and phylogeny are not the same thing. For example, it doesn’t matter if Archaeopteryx is classified as a bird or a dinosaur. What matters is that Archaeopteryx as a species is clearly defined and distinct from other dinosaurs and other birds. Then we can start to understand how it is that birds and dinosaurs are hardly different in some ways but utterly different in others.
Taxonomy is important insofar as the species must be clearly defined. The ICZN does a good job of making sure this is the case, though it can get very complex. For modern organisms, species can be defined by behavioral or reproductive patterns that are seldom preserved in the fossil record. In paleontology, we rely entirely on the shape and size of fossils to distinguish species. Nevertheless, provided a species is defined carefully, it is useful for the study of phylogenetic relationships.
Phylogeny is dependent on species—whether modern or fossil—being clearly defined so we can explore the similarities and differences among species and work out evolutionary relationships. This is done typically through cladistic analysis, which is essentially a mathematical means of exploring similarity between species, assuming that those more similar are likely to be more closely related (but this is an oversimplification and is a topic of another post).
To even begin to discuss phylogeny, we must understand taxonomy, particularly the concept of species. Thus, taxonomy underpins all study of the evolutionary relationships among organisms.