An exculsive SARG article written by Sam Shonleben firstname.lastname@example.org (June 2009)
Dedicated to the memory of Julia Wycherley for her tireless encouragement
The Reptiles: Why They Don’t Exist
Question: When is a Reptile not a Reptile?
Reptile enthusiasts have an instinctive idea of what a reptile is. It is a snake, a lizard, a tuatara, and so on. Where does this ‘idea’ of what we know as a ‘reptile’ come from? Human beings have a desire to compartmentalise and classify things, perhaps in order to make more sense of them, and nature is no exception. The scientific field that deals with the organisation of life is called Systematics.
One of the earliest and most famous Systematists was the Swede Carolus Linnaeus. His ‘Systema Naturae’ ordered nature in a hierarchical system based on Man as the pinnacle of Gods creation, thus having dominion over all other living things. Organisms were ranked in groups: Species (Linnaeus having invented the binomial system of giving species generic (Genus) and specific (Species) names) within Genera, Genera within orders, orders within classes and classes within kingdoms. With the publication of his ‘Origin of Species’ Charles Darwin (happy 200th birthday!) emphasised the need for a classification system “that must be strictly genealogical in order to be natural” and therefore be based on evolutionary relationships. However, problems arise when we try to use a classification based on creationism to order organisms based on their evolutionary relationships. Here I will use the example of the ‘Reptiles’ in order to highlight this.
As SARG members will know, the Reptiles are grouped together in one of Linnaeus’ hierarchical groups as the ‘Class Reptilia’. What is the basis for this grouping, and does it reflect evolutionary relationships (= phylogeny)? Reptiles were grouped as such based on characteristics that made them superior to the amphibians, locating a place for them in the creationist hierarchy of life. In fact, when one searches for characteristics that merit their grouping as a Class on their own, it is anything but straightforward. Most textbooks give a definition of the ‘Class Reptilia’ as vertebrates with impermeable scaly skin, internal fertilisation and amniotic eggs – features raising them above the amphibian on the ladder of creation. The problem, as the reader will have noted, is that this doesn’t define snakes, lizards, turtles and tortoises, crocodilians, and tuataras in their own separate group. Birds lay eggs, so do monotreme mammals, and both have internal fertilisation. It is this confusion that has led some Systematists to render this grouping as obsolete, as it has little to say about evolution.
Darwin knew that Classification should have its basis in phylogeny, but it wasn’t until the German entomologist Willi Hennig published his work in the mid 1960’s that systematists had a method by which to do this. Cladistics is the search for relatedness among species by assembling them into groups composed of a common ancestor and all of its descendants (= monophyletic group). If, within a group there is inherited variation for a particular characteristic, then there will be organisms possessing this characteristic in its ancestral state, and organisms possessing this characteristic from more recent transformations. The former can tell us nothing about evolutionary relationships within the group, just like the features of the Class Reptilia that deem them ‘higher’ than amphibians. The latter, however, are the result of unique evolutionary events and will be shared by related organisms. These shared, derived characters (= synapomorphies) will indicate a relationship between organisms that form a monophyletic group.
The quest for monophyletic groups using synapomorphies is the reason why the Class Reptilia and the term ‘Reptile’ are not supported by systematists. The Reptiles do not possess any synapomorphies to justify their grouping. They exist as the superiors of the Amphibia and the ancestors of the Birds and Mammals. The Class Reptilia can be said to be ‘paraphyletic’: Its members are more closely related to members outside the group than to members within it. Cladistics has also defined the birds as being more related to Dromaeosaurid Theropod Dinosaurs than to anything else, and Crocodilians as evolutionarily closer to birds than to any other reptile (see diagram).
So what does one do in this situation? Cladists have proposed a new method of classification that abolishes the hierarchical groupings of Linnaeus, and names organisms according to their monophyletic groups. In my mind this is desirable, as this classification has its basis in evolutionary relationships. But the clade (= successive monophyletic group) name for birds and reptiles is ‘Sauropsida’. It is not practical for us to just drop whatever name we have previously been using for these organisms. We should just bear in mind where these names have come from and what they actually mean and represent when we talk about such organisms. Often, cladists obtain a monophyletic group with topography (tree) that differs to another obtained by someone working with the same organisms. So there are often several trees, all interpreting evolutionary relationships differently! The reptile/bird tree I have shown you here is widely accepted. But that doesn’t mean that it is the true story of evolutionary relationships of these organisms. Indeed, no one can ever know the truth, and each tree represents a ‘best guess’ hypothesis of relationships, nothing more. I personally find it comforting that life in all its complexity doesn’t lend itself easily to compartmentalisation.
Answer: When it’s a Bird! Diagram: