What is the difference between evolutionary taxonomy and cladistics

Cladistics classifies organisms according to the order in time that branches arise along a phylogenetic tree, without considering the degree of divergence how much difference.

Groups subordinate to other groups in the taxonomic hierarchy should represent finer and finer branching of phylogenetic trees.

Molecular clocks - In the past used morphological characters still do , but now can also use molecular techniques :. A phylogenetic tree should have an outgroup that is a closely related taxon recently ancestral to the organisms for which the phylogeny is being constructed.

Ingroups share derived characteristics synapomorphies that the outgroup does not have. Linnaeus, and most other systematists before Darwin, saw this natural hierarchy as an expression of an abstract natural order, the creator's plan. But Darwin saw it in material, or concrete, terms, as the inevitable result of descent with modification, and as something predicted by and so explained by his theory. That's why he said that all true classification is genealogical.

So now, having worked through Darwin's diagram, and his comments on it, we might be ready to answer some questions. The first answer's by George Gaylord Simpson, who lived from to Here's what he said in his book, "Principles of Animal Taxonomy": "Is a man more closely related to his father, son, or brother? The degree of genetical relationship to father and son is invariably the same, 0.

Genetical relationship to a brother is variable, from 1. Unfortunately, relationships among taxa do not have such fixed a priori expectations. The same two kinds of relationships nevertheless exist among successive taxa in an ancestral-descendant lineage, and among contemporaneous taxa of more or less distinct common origin.

The former relationships are called vertical, and the latter horizontal," end of quote. Now we can easily picture Simpson's two kinds of relationship by looking back at Darwin's diagram. Simpson goes on to say that one kind of relationship is obviously just as objective as the other, that classification by either vertical or horizontal relationships alone is absolutely impossible, and that the art of taxonomy is in using your taste and ingenuity to effect a compromise between the two kinds of relationship.

That's Simpson's answer to the question "what is relationship", and it leads him to see classification as an art, a matter of taste and ingenuity. Darwin, C. Figure 3: From Darwin, C. Hypothetical evolutionary lineages derived from 11 ancestral species A-L , are shown over successive time intervals I-XIV. Simpson Activity 2 Timing: 0 hours 5 minutes. Figure 4 The relationship between phylogeny and higher and lower taxa, according to Simpson, G.

Audio clip 3. Skip transcript: Audio clip 3 Transcript: Audio clip 3. We can understand and criticise Simpson's solution better with the help of a diagram, which shows his idea of incorrect and correct ways of looking at a phylogenetic tree. In the left hand tree, the successive levels are equated with taxa, and the result's a family containing two genera, each with two species.

Mayr Activity 3 Timing: 0 hours 5 minutes. Figure 5 Relationships according to 'inferred percentual difference from ultimate ancestor A ', following Mayr, E. Mayr states, 'Taxon C is more closely related to B than to D, even though it shares a more recent common ancestor with D'. Audio clip 4. Skip transcript: Audio clip 4 Transcript: Audio clip 4.

Again, we can understand Mayr's concept better with the help of a diagram. It comes from a paper by Mayr, and it shows four species - an ancestral species A, and three descendants, B, C, and D. Mayr says, "Taxon C is more closely related to B than to D, even though it shares a more recent common ancestor with D". So that's Mayr's solution - relationship means genes in common, or genetic similarity.

Hennig Activity 4 Timing: 0 hours 5 minutes. I shows a cladistic classification and Ia shows the classification a pheneticist would adopt. Audio clip 5. Skip transcript: Audio clip 5 Transcript: Audio clip 5. Hennig's definition sounds formal and Germanic, but it's easy enough to follow with the help of a picture, which comes from the page facing the definition in Hennig's book. I've added the letters X, Y and Z so that we can match his definition to the diagram. He says that X is more closely related to Y than to Z, because X and Y share a stem species - which I've labelled C - which is not also a stem species of Z.

Above Hennig's tree are two Venn diagrams. It's a feature of Hennig's view of classification that the tree and the classification should be exact images of each other. His upper Venn diagram, labelled 1, is an exact match with the relationships shown in the tree. His lower Venn diagram, labelled la, shows a different pattern, a pattern we should get if we followed Mayr's definition of relationship - shared genes - because the distance between stem species A and B is much less than that between B and C.

The shared similarity of species W and Z would make Mayr classify them together, with the result shown by the dotted line in the tree, and by the dotted ellipse in the lower Venn diagram.

The pattern of relationship shown by the tree can't be recovered from the classification shown by the lower Venn diagram, which gives a different tree. As for Simpson's concept of relationship, his right-hand diagram, number 2B, is presented both as a tree and a Venn diagram, and you might try copying out the Venn diagram part of it with these seven species, and seeing what tree you recover from it. Audio clip 6.

Skip transcript: Audio clip 6 Transcript: Audio clip 6. Now the three kinds of relationship I've been talking about have been called Simpsonian, Mayrian and Hennigian relationship. And they can be fitted very neatly to the three different schools of classification that developed during the s, and were much disputed through the s. These three schools are called phenetics, cladistics and evolutionary systematics, or eclectics. Phenetics relies on overall similarity as a measure of relationship, and so it classifies similar organisms together.

This matches Mayr's definition of relationship as shared genotype. Cladistics aims to classify by inferred recency of common ancestry, and so it matches Hennig's definition of relationship. And eclectics, or evolutionary systematics, classifies by a mixture of similarity and inferred common ancestry, using taste or judgement as to when one criterion's given precedence.

And so it matches Simpson's discussion of relationship, and how one ought to classify. But notice that there are only two criteria of relationship - the phenetic one of similarity, and the cladistic one of inferred common ancestry. Eclectics merely uses a mixture of the two. Well, which school is correct, or is best? The overwhelming consensus, after twenty years of argument, is that cladistics is best, and it's unusual these days to find a systematist, who has given any thought to the fundamentals, who isn't a cladist.

Cladistics has won through, I think, for two main reasons. First, it has a consistent and coherent philosophy, and second, it developed at the same time as the early work in molecular systematics, when protein sequences and other molecular evidence was first brought to bear on problems of relationship and classification.

Let me explain quickly why this is so. A molecular biologist can only sample living taxa. He ends up with a set of data, let's say DNA sequences, from which he wants to recover a tree, or a classification. The sequences are necessarily seen as terminals of the tree. No-one would dream of seeing a DNA sequence from one species as ancestral to a sequence from another species. And, in a tree, it's ancestors that occupy the internal nodes and branches. So molecular systematists, from the beginning, worked with samples from the tips of the tree, and tried to reconstruct the tree by one method or another.

Now, if we look back to the diagrams explaining Simpson's, Mayr's and Hennig's ideas about relationship, we see that Hennig's is the only one that deals just with terminals of the tree. He's trying to classify species W, X, Y and Z, four terminals of the tree in his diagram. Simpson's trying to classify ancestors, as we can see from his diagram recommending combining three ancestral species into a genus ancestral to the four living or terminal species.

And, in Mayr's diagram, the three terminals, B, C and D, are labelled with their percent genetic difference from A, the ancestor. So among these three, Hennig is the only one whose definition of relationship treats the terminals of the tree as real, and the internal part of the tree as hypothesis or conjecture. And that's why his system directly matches the ideas of molecular systematists. Figure 7 Cladogram for the Hominoidea, from Andrews, P. Human Evol. Audio clip 7. Skip transcript: Audio clip 7 Transcript: Audio clip 7.

If we can agree that Hennig's is the only theoretically justifiable definition of relationship, and the one we should accept, how do we set about building a tree, or inferring relationships of common ancestry?

If you think about the problem, you'll realise that the ideal way of building the tree would be by recognising evolutionary novelties, the innovations that characterise different lineages or groups of species. In an ideal tree, each node would be marked by one or more novelties, characters unique to the group of species stemming from that node.

I've put an example as number 5, one dealing with familiar animals, the apes, or hominoids. That tree has one peculiar feature, the way the chimpanzee is linked to two different places. But that's done to emphasise a particular problem, that we'll get to in a minute, the fact that there are two different sets of characters - the ones labelled 7a and 7b, and each suggests different relationships for the chimpanzee.

The question I want to tackle, at the moment, is how this tree was built up by recognising evolutionary novelties, or shared derived characters. Synapomorphy is the technical term for a shared derived character. The authors of this tree of hominoids, Peter Andrews and Lawrence Martin, gave a list of characters for each of the numbered branches of the tree. As an example, let's take branch 5, the one distinguishing African apes and us from the orang-utan.

They listed about 10 characters for that branch, but I'll just mention three of them. The first is fusion of the os centrale, the second is that the frontal sinus is developed, and the third relates to mutations in DNA. And I want to ask how you might decide that these features are innovations or synapomorphies. Take the os centrale first. It's a bone in the wrist, one of the carpals. In orangs and gibbons, there are nine bones in the wrist - nine carpals - but in African apes and us there are only eight.

Given that information, either state might be primitive or derived, so how do we decide that eight is derived? In this case it's easy, because in the embryo of all these animals, there are nine carpals, but in us, and in African apes, two of them - the centrale and the scaphoid - fuse together. So we begin life with no carpals, then we develop nine carpals, and we end life with eight of them.

Now, in using this developmental sequence as evidence for evolutionary transformation, we aren't just appealing to the theory of recapitulation - the idea that ontogeny recapitulates phylogeny.

We're using a much older theory, or law - one proposed by the embryologist von Baer, in the Os. Von Baer's Law says that development proceeds from the general to the particular. The most general characters appear first, and the most particular, or restricted, appear last.

The idea here is that development recapitulates not phylogenetic history, but the systematic hierarchy, so that characters of the largest groups appear first, and characters of the most restricted groups appear last. Figure 8 Venn diagrams for the classification of the Homininae. Audio clip 8. Skip transcript: Audio clip 8 Transcript: Audio clip 8.

We can try out that idea with the wrist bones of hominoids. The first condition, in the egg and the very early embryo, is to have no bones or cartilages in the wrist - the skeleton hasn't yet started to develop.

The next stage is to develop nine carpals. And the final stage, in African apes and us, is to fuse two of them, leaving eight carpals. Given those three conditions, we could convert them into a Venn diagram, with each condition characterising a group.

The group with no carpals is the whole of life, including plants and bacteria. The group with nine carpals happens to be mammals. And the group with eight is African apes and us, the subfamily Homininae.

Now the hominoids have another character that behaves like this, with ontogenetic or developmental evidence on transformation. The character is reduction of the tail. All adult apes have just a rudiment of a tail, but during embryonic life they develop a long tail, like all other vertebrates, and then it becomes reduced to a vestige by differential growth. So we can get another Venn diagram from the tail, and I've combined it with the carpal diagram.

I'm sure you get the idea. I mentioned one other feature shared by us and African apes, the frontal sinus, which is a space in the bones of the face developed during ontogeny, as an outgrowth or expansion of the ethmoid sinus.

And again, we could use development from the general, absence of sinuses, to the more particular, presence of an ethmoid sinus, to the still more particular, presence of the frontal sinus.

And so we could get another series of groups to add to the Venn diagram. Now all this probably seems much too simplified and, in real life, characters are often much more difficult to sort out. There's a good example in the hominoid tree, where the chimp is linked to both the human and gorilla lines.

This is a case where there are two conflicting sets of characters, the ones labelled 7a and 7b on the tree. Just to take a couple of examples, chimps are linked to us by having the premaxilla and maxilla fused in the adult. But chimps are linked to gorillas by having six vertebrae involved in the sacrum, instead of the five that we have and orangs have. And chimps and gorillas also share a number of features of the arm and hand, associated with their habit of walking on the knuckles. Figure 9 Nucleotides at selectived positions left column in sequences of non-coding DNA in the region of the beta haemoglobin family of genes in various higher primates.

Asterisks denote gaps in the sequences of the species concerned. Based on Williams, S. Audio clip 9. Skip transcript: Audio clip 9 Transcript: Audio clip 9. So here we've got conflicting morphological characters, and all of them seem to be derived. They can't both be true, so how do we resolve the conflict? These are selected positions in an alignment of over ten thousand nucleotides, from non-coding DNA in the region of the beta haemoglobin genes.

We don't yet know the DNA sequence for this region in gibbons, but the table includes the other four apes, and also a couple of monkeys - the Rhesus monkey, Macaca Mulatta from the Old World, and the Spider monkey, Ateles, one of the New World monkeys from South America.

I want to use this table as an example in tackling the particular question of the conflict on chimpanzee relationships, but also in tackling the more general question of deciding on primitive and advanced characters. The method I've just been describing, using ontogeny or development to resolve general and special features in morphology, obviously won't work with DNA, because DNA has no development.

Barring accidents, we're born and we die with the same DNA sequences in our chromosomes. So how can we determine whether a nucleotide shared by two or more species is derived or primitive? Take the first row in the table, the one numbered You'll see that five of the animals have G, Guanine, at that position, but human and gorilla share A, Adenine.

Derived character groups are those found in the study groups but not the outgroups. Clades are groups that share derived characters and form a subset within a larger group.

A clade is a unit of common evolutionary descent. A synapomorphy is a derived character that is shared by all the members of the clade. Using synapomorphies to define clades will result in a nested hierarchy of clades.

Ancestral character states for a taxon are called plesiomorphic. Symplesiomorphies are shared ancestral characters. Symplesiomorphies do not provide useful information for forming a nested series of clades.

The nested hierarchy of clades can be shown as a cladogram that is based on synapomorphies. A valid clade is monophyletic , it consists of the ancestor species and all its descendants. A paraphyletic clade consists of an ancestral species and some, but not all , of the descendants. Cladistics , also called phylogenetic systematics , is a taxonomic theory that is based on cladograms.

All taxa must be monophyletic! Traditional Evolutionary taxonomy is based on common descent and the amount of evolutionary change to rank higher taxa. Sometimes this type of classification includes paraphyletic groupings.

Humans, chimpanzees, gorillas, and orangutans are now all included together in one monophyletic family - Hominidae. A sister group is a pair of taxa that are most closely related to each other. Gorillas form a sister group to the clade containing humans and chimpanzees.