Key Features of a Cladogram. Constructing Cladograms. Cladograms can be constructed based on either a comparison of morphological structural features or molecular evidence. Using Structural Evidence. Step 1: Organise selected organisms according to defined characteristics. Step 2: Sequentially order organisms according to shared characteristics to construct a cladogram.
Using Molecular Evidence. Step 1: Select a gene or protein common to a range of selected organisms. Step 2: Copy the molecular sequence DNA or amino acid for each of the selected organisms. Step 3: Run a multiple alignment to compare molecular sequences DNA or amino acid.
Step 4: Generate a phylogeny tree cladogram from multiple alignment data. Brent Cornell. Cell Introduction 2. Cell Structure 3. Membrane Structure 4. Membrane Transport 5. Origin of Cells 6. Cell Division 2: Molecular Biology 1.
Metabolic Molecules 2. Water 3. Protein 5. Enzymes 6. Cell Respiration 9. Photosynthesis 3: Genetics 1. Genes 2. Chromosomes 3. Meiosis 4. Inheritance 5. Genetic Modification 4: Ecology 1. Energy Flow 3. By doing so, Hennig removed human opinion and authoritative decisions from evolutionary biology. The phylogenetic or cladistic way of systemising threw out the arbitrariness of classification and replaced it by the scientific rigor of phylogenetics.
It might not be intuitively clear why this should be relevant for anthropocentricism. The relevance lies in the fact that there is not a single paraphyletic taxon that incorporates the human species. On the other hand, quite many paraphyletic groupings denote taxa that would have been monophyletic if they had not been erected to the exclusion of a taxon containing man. Obviously, one unwritten rule for the erection of paraphyla was that they must not contain Homo sapiens.
Inventing paraphyletic taxa that include our species may be a useful and funny exercise. One might have thought that with the acceptance of Darwinian evolution and cladistic methodology, anthropocentricism was finally banned from evolutionary narratives, at least in scientific publications. This was the background for limiting my case study to textbooks that explicitly adopted a cladistic perspective.
As one aim of phylogenetic systematists is to present phylogenetic history in a way that is independent of the opinion of authorities, one might expect that they are more conscious about avoiding anthropocentricisms than the average author of evolutionary textbooks.
However, as my results show, this is merely wishful thinking. Two distorting devices can still occur even in state-of-the-art phylogenetic cladograms, and at least one of them is prevalent in textbooks: the taxon containing Homo sapiens is quite consistently placed at the rightmost position of each cladogram.
These two narrative devices—i. All trees in this figure are cladograms of the same taxon, viz. Figure 2 a is perfectly balanced, thus having eliminated device no. Four out of more than 36, possibilities to represent gnathostome phylogeny using eight tips. See text for further explanations. Figure 2 b is attempting to distribute taxa along the left—right axis in an objective way, thus eliminating device no.
This places the taxon containing man, Theria, somewhere in the middle of the tree. On the other hand, all taxa not including man are collapsed into only one single terminal taxon, resulting in a perfectly comb-like tree topology, again at the disadvantage of non-human taxa. Of course, it is possible to combine the advantages of both cladograms in one figure, just turning some branches of Fig. Figure 2 d shows a fourth out of the over 36, possibilities to depict gnathostome phylogeny using 8 tips.
In this last illustration, the cladogram is maximally unbalanced, but this time to the advantage of the beluga or European sturgeon Huso huso and to the disadvantage of Sarcopterygii including Homo sapiens and five other side branches.
Such presentations are quite uncommon, even though ichthyocentric cladograms are as justified as anthropocentric cladograms but see Scott for a papiliocentric tree of life. Finally, Fig. Figure 3 also illustrates that there are natural constraints to perfectly balanced trees: the tree of life is not itself perfectly balanced Guyer and Slowinski ; Mooers ; Mooers and Heard , and several of the branches in Fig.
Phylogeny of gnathostomes using a maximally balanced representation and resolving up to seven bifurcations per branch. The phylogeny follows Sandvik and references therein. The sorting criterion for taxa along the vertical axis is their species number see text. One might argue that the use of these narrative devices has certain advantages, i. As regards device no. This depends of course on the aim of the cladogram. When one wants to tell the history of man or any other taxon , and nothing else, comb-like trees are, of course, perfectly suited.
However, in zoology textbooks this is normally not the intention, instead the reader is expected to get an impression of zoological diversity. This may be better achieved by using balanced trees. It might also be argued that, because the tree of life itself is not balanced cf. In most cases, however, cladogram resolution is not constrained by the topology of the tree of life.
Moreover, even in the cases where it is, it may be very useful heuristically to visualise that some tips in fact are species or species-poor taxa. This makes readers of the cladogram understand that biodiversity is indeed not distributed uniformly across the tree of life as implicitly suggested by the straitjacket of Linnean categories, see below. That Choanata includes lungfishes Dipnoi and terrestrial vertebrates Tetrapoda is only known to specialists many of whom would not even use the scientific name Choanata, but prefer other names such as Rhipidistia sensu lato.
Likewise, that Callorhinchus and Chimaeroidea together constitute the chimaeras Holocephalii , or that Galea and Squalea together represent the rays and sharks Elasmobranchii , will not be visible from the cladogram, unless names of higher taxa are displayed above the tips as is shown in Fig. In such cases, perfect tree balance does not necessarily seem to outweigh the loss of information. A compromise would be to display one more branching in the rightmost branch of Fig.
However, this argument seems to be mostly a question of habit and practice , as it also could be applied to device no. In fact it has been applied, by claiming that evolutionary systematics is more in accord with commonsense than phylogenetic systematics; Halstead ; Mayr and Bock After all, taxa have to be sorted in one way or another, and as all sorting criteria are equally correct, it does not really seem to matter whether the criterion chosen is the phylogenetic distance to Homo sapiens.
What, then, are the disadvantages of anthropocentric cladograms, and do they weigh more than the advantages? Sandvik, unpublished manuscript.
If students are asked to draw the tree of life, they generally produce drawings that place Homo sapiens in a prominent position either on top of the drawing or on its right-hand side. This in itself does not tell us whether the choice to draw a tree in this way is based on misconceptions about evolutionary processes, or just a matter of habit.
However, there is also overwhelming evidence that many students are unable to read cladograms: even many graduate students of biology pay more attention to the left—right ordering of taxon names on the cladograms tips, than to the topology of the cladogram displayed beneath the taxon names. A further problem is that students often regard Linnean categories i.
They assume that the different levels in the Linnean hierarchy have distinct meanings and that the assignment of a given category to any one taxon is either right or wrong. Of course, Linnean categories are entirely arbitrary and do not carry any information whatsoever de Queiroz and Gauthier ; Ereshefsky , , ; Donoghue This misconception is further reinforced by the fact that many textbook authors tend to adjust the resolution of cladograms to Linnean categories for a refreshing exception, see Ax — In other words, taxa that got Linnean categories attached to them are overrepresented among the tips of cladograms.
In some textbooks, this is even aggravated by the pruning of less-known and supposedly less important branches i. Cladograms therefore often only display the taxon names that the students are supposed to memorise, which is a somewhat biased sample of the taxa that in fact exist in nature. These problems re-enforce each other in misleading at least untrained readers of cladograms. To be sure, none of those problems is solved by avoiding anthropocentricism in cladograms alone.
However, stating this point somewhere in the vicinity of a tree diagram does not prevent the readers of the diagram from unconsciously noticing the implicit sequencing of taxa. In addition to increased awareness about the fact that tree thinking is an ability that has to be acquired, authors and teachers should become more aware of how they present cladograms.
It they are not, it turns out that more or less unconsciously the result becomes anthropocentrically biased.
This seems to suggest that other modes to determine taxa positions along the left—right axis should be considered by authors and teachers. Several such alternatives to the anthropocentric mode have been proposed. Secondly, several authors seem to use a kind of implicit complexity measure: at each node, the taxon having acquired more, or more complex, autapomorphies is placed to the right of the taxon that has undergone less evolutionary change.
This is evidently how e. A third alternative, which has been carried through in a Norwegian textbook of mine Sandvik , is alphabetic order: at each node the two taxa are ordered simply by their scientific names. There are advantages and disadvantages to all those methods. The latter one is deliberately arbitrary. It is perhaps best suited to illustrate that left—right order does indeed not carry any meaning whatsoever.
The apomorphy-based method is somewhat more in accordance with intuition. However, it accepts the horizontal orientation of a cladogram as a narrative axis, at which most change occurs. This, and the fact that there is no objective measure of the complexity of autapomorphous or any other traits Johnson ; Ghiselin ; Griffiths , suggests that this method has its weaknesses.
The method based on species numbers might be the best in order to express the importance of taxa in terms of biodiversity. As such, this method might be recommended for most uses. This is most likely explainable by an innate psychological bias to think anthropocentrically. If this disposition is not kept in check by conscious decisions, it obviously introduces systematic biases in how we choose to represent evolutionary history.
These findings do of course not relate to the validity of the scientific results displayed by the cladograms. To the contrary, it is exactly because the graphical representation of the results is irrelevant to their correctness that scientists are entirely free to choose whatever resolution and ordering of taxa they like.
Only because of this freedom, unconscious biases can affect the graphical result in the way they do and may thereby reveal some underlying expectations or worldviews of the scientists. Given that textbook authors—particularly cladistic ones—should be expected to be especially aware of issues that can confuse or mislead readers, this subconscious bias seems to be strong indeed. In conclusion, the cladistic revolution has indeed largely removed three of the narrative devices that distort accounts of evolutionary history.
However, two devices can also be found in cladistic depictions of phylogenetic relationships, viz. The latter narrative device has been found to be present even in textbooks that explicitly adopt a cladistic perspective.
The current paper illustrates that having the right intentions does not suffice to produce unbiased evolutionary narratives. Obviously, the subconsciousness even of trained cladists is more anthropocentric than we would like to acknowledge. It can be envisaged as being inversely proportional to the number of nodes between a taxon and the root of the tree. This measure is relative in such a way that sister taxa always have the same rank, while subordinated superordinated taxa have lower higher rank.
Linnean categories are sometimes referred to as ranks, too, but those two concepts are unrelated. National Center for Biotechnology Information , U. Biol Philos. Published online Dec Hanno Sandvik. Author information Article notes Copyright and License information Disclaimer. Corresponding author. Received Jun 29; Accepted Nov 5. This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author s and source are credited.
This article has been cited by other articles in PMC. Abstract Both written and graphic accounts of history can be biased by the perspective of the historian. Introduction Just as history can be represented by fundamentally different narratives, depending on the background of the historian i. Open in a separate window. Two numeric measures of anthropocentricism In order to look for the presence of narrative devices in published cladograms, one needs some sort of metric to represent the degree of anthropocentricism.
A case study using phylogenetic textbooks The algorithms developed above were applied to two textbooks in animal phylogenetics, Das System der Metazoa Ax — and Animal evolution Nielsen Table 1 Anthropocentricisms in two cladistic textbooks of animal evolution as measured by the human attention score.
Table 2 Anthropocentricisms in two cladistic textbooks of animal evolution as measured by the human rightness score. Discussion: why anthropocentricism matters The past years have seen a gradual reduction in the anthropocentricism in biological systematics.
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