Systematics — The goal of systematics is to have classification reflect the evolutionary relationships of species. 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. Phylogeny is the evolutionary history of a group of related organisms. A clade is a group of organisms that includes an ancestor and all of its descendants.
Clades are based on cladistics. This is a method of comparing traits in related species to determine ancestor-descendant relationships. Cladograms give a hypothetical picture of the actual evolutionary history of the organisms. Phylogenetic trees give an actual representation of the evolutionary history of the organisms. All the branches in a cladogram are of equal length as they do not represent any evolutionary distance between different groups.
Phylogenetic trees do show patterns of descent. Phylogenetic trees do not indicate when species evolved or how much genetic change occurred in a lineage. Systematics - The goal of systematics is to have classification reflect the evolutionary relationships of species. Phylogeny leads to classification Fig Phylogenetic systematics 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. In Figure 1 , morphological change is represented along the horizontal axis. The three columns show grades of anagenetic modification, with parts of the phylogeny occupying each grade.
Grades are easy to recognise, because they are based upon raw similarities between species, but they may be misleading as far as the reconstruction of phylogeny is concerned. Why may some grades contain more than one branch of a phylogenetic tree as in the central column of Figure 1? Convergent features may have evolved independently in separate lineages. A grade grouping of such species based on these features as in the central column of Figure 1 would thus exclude their latest common ancestor which remains in the left-hand column in Figure 1.
Thus the central grade grouping in Figure 1 does not comprise a single branch from a phylogenetic tree. A grouping which assembles species with independently evolved similarities is said to be polyphyletic. However, it has long since been recognised that this feature is convergent in these animals, and so the name is no longer used for systematic purposes.
Modern taxonomists attempt to avoid using polyphyletic taxa because they are misleading in phylogenetic reconstruction. Numerous other lines of evidence indicate. Not all grades are polyphyletic, however, and grade groupings which do include their common ancestor as in the left and right columns in Figure 1 make up modem classifications. In Figure 1 , for example, birds and mammals represent groupings considered to differ sufficiently from their reptilian ancestors to be recognised as distinct grades.
If, alternatively, the pattern of cladogenesis i. A clade represents a single whole branch from a phylogeny, and, because it is derived from a single common ancestor, it is said to be a monophyletic grouping. Yes, despite Simpson's earlier reservations about their possible polyphyletic origins, morphological and molecular data now strongly suggest that they are all indeed derived from a single ancestral mammalian species. So the mammals are both a clade and a grade grouping as are also the birds.
If the taxonomic hierarchy is to give an unambiguous reflection of phylogenetic relationships, then the recognition of clades is the most desirable objective of systematics: members of any taxon, so recognised, will be more closely related to each other than to any member of any other taxon, by definition. A classification based purely on a hierarchy of clades is the objective of cladistics.
Because cladistic hierarchies reflect only increasing levels of inclusiveness of the branchings in a phylogeny, they cannot reflect the different amounts of evolutionary change between ancestral and descendent organisms.
In other words, they ignore the anagenetic component of pattern, upon which grades are based. No, because despite the reptiles being derived from a common ancestor, two descendent groups — the birds and the mammals — have been removed from them.
The reptiles therefore do not include all the descendants of the primordial reptile species and so are not a complete monophyletic taxon. A taxon which thus comprises a single branch from which one or more clades have been removed is called a paraphyletic taxon as in the left column of Figure 1 : the reptiles are therefore paraphyletic.
So much for the nature of higher taxa, but how are the constituent species grouped together in the first place? Because organisms are so complex and so highly integrated, the identification of separate aspects to be treated as taxonomic characters has to be arbitrary, and is thus a subjective issue which presents problems whatever the approach to phylogenetic reconstruction.
A particular problem with most morphological characters is that the genetic controls on their development are complex and often poorly understood. As with the selection of variates i. Those features which are reasonably consistent within each species, but which differ sufficiently in expression from species to species so as to permit degrees of similarity between species to be noted, tend to be used.
One fundamental consideration must be mentioned here. Features showing similarities due to convergence are said to be analogous, and a prime objective of modern systematic methods is to avoid the confusion they can cause in classification.
Other features, in contrast, are interpreted as being of similar construction because they have been inherited from a common ancestor. These features are said to be homologous. If homologies could be recognised as such, then the relationships between species could be inferred from their shared homologies. Unfortunately, however, homologies and analogies cannot always be unambiguously distinguished in practice.
The risk of confusion is especially great when closely related species are compared, because similarities in their morphology and ecology make the parallel evolution of analogous features in separate lineages quite likely. As with other statements concerning history, homologies must themselves be inferred.
In some cases, this may seem easy enough. Figure 2 shows the structure of a human arm, a bird's wing and an insect's wing. We readily recognise the first pairing as being homologous and the second as being analogous, but why? In spite of the differences of their superficial form, they share the same basic construction: corresponding bones, with the same spatial relationships, though with differing proportions, may be recognised indicated by different shadings in the figure.
The similarity of the wings of the bird and the insect, in contrast, is only superficial reflecting their common adaptation to flight ; they are of markedly different construction, the insect's wing, of course, having no bones at all. Thus the mode of construction seems to offer a clue, and a useful concept in this respect is that of the information content of features. The more numerous the points of resemblance between structures being compared, in terms of the elements making them up, their positional relationships with respect to each other and their pattern of development, the more likely they are to be homologous.
In other words, there would be an improbably large amount of detailed similarity to explain away as coincidental convergence. The appendages in Figure 2 are all features of high information content, and so the numerous structural similarities of the first pairing strongly imply homology, while the lack of them for the second pairing implies analogy.
Many other examples are less easy to resolve, however, and continue to create systematic problems to this day. This is a common problem with many fossil taxa. Many of the morphological features of simple fossil shells, for example, have a very low information content, and so homology and analogy are readily confused.
Other approaches to the problem of distinguishing homology from analogy can be adopted, but these vary according to the different systematic methods, which must be considered. The late Dr. Colin Patterson was a palaeontologist at the Natural History Museum, and an authority on systematic methods. He played a prominent role in promoting cladistics, the method now most widely employed for phylogenetic analysis.
Three different schools of thought have arisen, and are illustrated using quotations from leading proponents: evolutionary systematics is explained through the words of George Gaylord Simpson, phenetics through those of Ernst Mayr, and cladistics through those of the founder of that school, Willi Hennig. Of these, cladistics has now become the preferred method for phylogenetic analysis, for reasons explained by Dr.
The rest of the sequence is devoted to explaining how the method is employed, with reference to both morphological and mocecular data on the higher primates, especially the apes, or hominoids including our own species. The inference of cladistic relationships, the erection of a classification fimagerom them, and the analysis of biogeographical patterns are all illustrated. The following sequence consists of a set of audio clips. Each of the clips relate to the image, or images, presented on the page.
Click to view a PDF containing all the images that are referred to in the audio sequence. Click to view a PDF containing the full transcripts of the video clips used in this course. In the first clip, Dr. This was the only image included in his book, Origin of Species. The second clip begins with some background into the system of hierarchy formalised by Linnaeus in the eigthteenth century.
Patterson looks at the second of his three systematists, Ernst Mayr. In this clip, Dr. Patterson introduces his third systematist, a German entomologist named Willi Hennig. At the end of the clip, Dr. This figure is repeated below the clip. This clip explores the three kinds of relationships that have been explained so far, in terms of the work of Simpson, Mayr and Hennig, which are referred to as Simpsonian, Mayrian and Hennigian relationships. Patterson links each of the systematists with a particular school of classification — phenetics, cladistics and evolutionary systematics, or eclectics, and establishes which one of these most directly matches the ideas of molecular systematists.
This clip refers to Figures 4, 5 and 6. You may want to review these diagrams before listening to the clip. This clip addresses the question of how one might go about building a tree, or inferring relationships of common ancestry, by recognising evolutionary novelties, or shared derived characters, or synapomorphy.
This is Figure 7. This clip builds on the idea that development recapitulates systematic hierarchy, by trying it out with the wrist bones of hominoids. This clip looks at conflicting morphological characters and at how it is possible to resolve them, with the aid of a table of molecular characters Figure 9.
Dr Patterson also explains how to determine whether a nucleotide shared by two or more species is derived or primitive. Patterson uses a diagram showing alternative cladograms for humans, chimpanzees and gorillas Figure 10 to summarise evidence supporting the hypothesis that chimps are our closest relatives.
He also provides two reasons why this theory should be accepted. This clip refers back to the table of molecular characters, which is shown again here Figure 9. This clip begins with a diagram by Ernst Haeckel published in Figure This is an illustration of how little ideas on the relationships of higher primates have changed in over a century. The diagram is taken from an article by Dr. Patterson, which featured in New Scientist in the early s. It shows three cladograms.
The first of these a matches the pattern shown in the Andrews and Martin tree diagram Figure 7 , which you will be reminded of below the clip. An explanation of each of the cladograms show how they can be translated into a classification. Patterson introduces the concept of systematics and biogeography.
He uses a diagram showing two cladograms Figure 13 — one representing the higher primates that have been discussed in the course, and the other showing where they are found. This free course provided an introduction to studying Science. It took you through a series of exercises designed to develop your approach to study and learning at a distance and helped to improve your confidence as an independent learner. Except for third party materials and otherwise stated see terms and conditions , this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.
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Printable page generated Friday, 12 Nov , Use 'Print preview' to check the number of pages and printer settings. Print functionality varies between browsers. Printable page generated Friday, 12 Nov , An introduction to biological systematics Introduction This course is concerned with macroevolution the patterns and processes of evolution above the species level. It is important to appreciate the differences between the three methods of phylogenetic analysis that are described, namely evolutionary systematics phenetics cladistics A further illustration of these concepts is provided by a sequence of audio clips featuring the late Dr.
Learning outcomes After studying this course, you should be able to: understand the patterns and processes of evolution above the species level appreciate the differences between the three methods of phylogenetic analysis: evolutionary systematics, phenetics, cladistics. Answer One is the relationship of descent — as in the parent—child relationship — and the other is that of shared parentage, as with brothers and sisters.
Figure 1 Morphological change. Long description. SAQ 2 Why may some grades contain more than one branch of a phylogenetic tree as in the central column of Figure 1? Answer Convergent features may have evolved independently in separate lineages. Answer Yes, despite Simpson's earlier reservations about their possible polyphyletic origins, morphological and molecular data now strongly suggest that they are all indeed derived from a single ancestral mammalian species.
SAQ 4 What aspect of evolutionary pattern is missing from such a scheme of classification? Answer Because cladistic hierarchies reflect only increasing levels of inclusiveness of the branchings in a phylogeny, they cannot reflect the different amounts of evolutionary change between ancestral and descendent organisms. Answer No, because despite the reptiles being derived from a common ancestor, two descendent groups — the birds and the mammals — have been removed from them.
View larger image. Figure 2 Human arm, insect's wing. SAQ 6 What aspect of the human arm and the bird's wing would suggest that they are homologous? Answer In spite of the differences of their superficial form, they share the same basic construction: corresponding bones, with the same spatial relationships, though with differing proportions, may be recognised indicated by different shadings in the figure.
Download this audio clip. Audio clip 1. Skip transcript: Audio clip 1 Transcript: Audio clip 1. Colin Patterson. Darwin's diagram is bound in with his chapter on natural selection, and he used nine pages of that chapter to explain it. He gave the diagram another three pages in the chapter on geological succession, and then another five pages in the chapter on classification. Of course we don't have time to work right through Darwin's seventeen pages, but the format of the diagram is probably familiar to you.
It has a vertical time-scale of fourteen periods, which Darwin says might each represent a thousand generations, or ten thousand generations, or a million, or even a hundred million. The capital letters A to L, at the bottom of the diagram, represent, "the species of a genus large in its own country," in Darwin's words. If you take a species capital A as an example, the diverging and branching dotted lines represent its offspring, and the lower case letters and superscript numbers, going from a1 to a14, m1 to m14, and so on, represent well-marked varieties, with distance along the horizontal axis indicating amount of divergence.
So, if each time period represents a thousand generations, then after 14, generations, at the top of the diagram, species A has produced eight descendant species, numbered a14 to m And Darwin says, "Thus, as I believe, species are multiplied and genera are formed".
Among those eight species, Darwin says that the three on the left, a14, q14 and p14, "will be nearly related from having recently branched off from a10, whereas b14 and f14 will be more distinct from those three, and o14, e14 and m14, the three on the right, will be nearly related to each other, but having diverged at the first commencement of the process of modification, will be widely different from the other five species, and may constitute a distinct genus," end of quote.
Darwin goes on to say that the six species descended from species I at the top right of the diagram, will have to be ranked in a different subfamily from the species descended from A.
And then he says that he sees no reason to limit this kind of descent with modification to species and genera alone. If the amount of change in each time period was greater, we might end up with two different orders, represented by the descendants of species A and I. In his chapter on classification, Darwin uses the diagram to show how his theory of descent with modification predicts and explains what he calls, quote, "the grand fact in natural history of the subordination of group under group, which, from its familiarity, does not always sufficiently strike us".
And then he goes on, "propinquity of descent - the only known cause of the similarity of organic beings - is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications," end quote.
And then Darwin goes on, "that the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking".
Show transcript Hide transcript. Interactive feature not available in single page view see it in standard view. Audio clip 2. Skip transcript: Audio clip 2 Transcript: Audio clip 2. I shall quote no more from Darwin, but I want to emphasise the fact that he saw classification as one of the most important pieces of evidence bearing on his theory.
Since antiquity, naturalists have found that animals and plants fall into a hierarchy, a system of groups and subgroups. And this system was formalised by Linnaeus in the 18th century, into classes containing orders, orders containing families, families containing genera and so on. 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.
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