Tree thinking can not taken for granted : challenges for teaching phylogenetics Tree thinking is an integral part of modern evolutionary biology , and a necessary precondition for phylogenetics and comparative analyses . Tree thinking has during the 20th century largely replaced group thinking , developmental thinking and anthropocentricism in biology . Unfortunately , however , this does not imply that tree thinking can be taken for granted . The findings reported here indicate that tree thinking is very much an acquired ability which needs extensive training . I tested a sample of undergraduate and graduate students of biology by means of questionnaires . Not a single student was able to correctly interpret a simple tree drawing . Several other findings demonstrate that tree thinking is virtually absent in students unless they are explicitly taught how to read evolutionary trees . Possible causes and implications of this mental bias are discussed . It seems that biological textbooks can be an important source of confusion for students . While group and developmental thinking have disappeared from most textual representations of evolution , they have survived in the evolutionary tree drawings of many textbooks . It is quite common for students to encounter anthropocentric trees and even trees containing stem groups and paraphyla . While these biases originate from the unconscious philosophical assumptions made by authors , the findings suggest that presenting unbiased evolutionary trees in biological publications is not merely a philosophical virtue but has also clear practical implications . Introduction Modern evolutionary biology relies on population thinking and tree thinking -LRB- O'Hara 1997 -RRB- . Both ways of thinking have replaced earlier philosophies that turned out to be incompatible with the empirical basis -LRB- Hull 1965 ; Simberloff 1980 ; Sober 1980 -RRB- . The transition from essentialism to population thinking has been described by Ernst Mayr -LRB- 1988 , 1996 ; but see Levit and Meister 2006 ; Winsor 2003 -RRB- . This transition followed directly from the Darwinian revolution -LRB- Darwin 1859 -RRB- : it is organisms with unique trait combinations that make up populations , that constitute biodiversity , and that are the basis of evolutionary change . The earlier paradigm , which viewed organisms as blueprints of unchangeable essences , was incompatible with these observations . Equally important to modern biology is tree thinking . Each and every species occupies one specific place in the tree of life . Consequently all species have shared certain parts of their history with all other living and extinct species . The degree to which this history has been shared , allows us to predict the extent and way in which species differ from and resemble each other . A description of biodiversity is impossible or at least incomplete without taking this knowledge into account . It is necessary both for the idiographic activity of reconstructing evolutionary relationships between species , and for the nomothetic activity of testing hypotheses about biological laws of nature -LRB- Ghiselin 1997 ; Hull 1999 -RRB- . Phylogenetic reconstruction -LRB- Felsenstein 2004 -RRB- and the comparative method -LRB- Martins and Hansen 1996 -RRB- are thus entirely unthinkable without tree thinking . Tree thinking became widely accepted somewhat later than population thinking , as it can be said to have originated with phylogenetic systematics -LRB- Hennig 1950 -RRB- . However , this does not mean that tree thinking has superseded population thinking -LRB- O'Hara 1997 -RRB- . Both perspectives are compatible , and both are necessary in biology today . As Robert O'Hara -LRB- 1997 -RRB- has shown , what has been replaced by tree thinking , are the earlier paradigms of group thinking and developmental thinking in evolutionary biology . Group thinking had equated systematics with classification , i.e. it perceived taxonomic groups as units that could be defined rather than entities that had to be discovered . Group thinking thus represented essentialism in systematics , because it failed to understand that taxa are evolving entities and thus , ontologically speaking , individuals -LRB- Ghiselin 1974 , 1997 ; Hull 1976 -RRB- . Developmental thinking , on the other hand , had assumed that organic evolution was a teleological process -LRB- Ghiselin 2002 ; Hull 1974 ; Mayr 1988 -RRB- . It was thus based upon the ancient idea that change in nature has an ultimate goal . This is incompatible with the irregular and divergent nature of evolution . One might presume that the replacement of group and developmental thinking by tree thinking is of merely historical interest -- a subject to be dealt with by historians of science , just as the replacement of essentialism by population thinking . To the contrary , I here argue that the transition of both group thinking and developmental thinking to tree thinking is still happening , and that it deserves more attention by those who teach phylogenetics and who communicate phylogenies . The transition is still happening in the sense that students of biology have to accomplish it in the course of their `` academic ontogenies '' : tree thinking is an ability that has to be acquired and can not be taken for granted by teachers . Sadly , many text books indirectly and implicitly promote developmental thinking . This further aggravates the problem . Given the importance -LRB- even necessity -RRB- of tree thinking in order to fully appreciate the meaning of evolutionary results , this problem deserves attention . Materials and methods I devised a short questionnaire which was handed out during the first lesson in several classes of a university course in systematics and evolution -LRB- at Universitetet i Tromsø , Norway -RRB- . The course was intended for students who had studied biology for several terms , but this was not a formal requirement . The students who followed normal progression had been taught evolutionary principles and processes in earlier courses . The same questionnaire was used in a similar course at a regional university college -LRB- Høgskolen i Bodø , Norway -RRB- . The students were asked to answer the questions , using the time they needed , and were assured anonymity . They did not have access to any books or notes . The questions -LRB- and correct answers -RRB- were : `` Given the following evolutionary tree -LSB- reproduced in Fig. 1 -RSB- , where letters symbolise species : which species is -LRB- are -RRB- most closely related to species B ? '' 1 -LRB- The correct answer was `` C , D , E and F '' . -RRB- `` How many classes of vertebrates and angiosperms are there ? Fill in the scientifically correct answer : ... `` -LRB- There is no scientifically correct answer to this question . -RRB- `` What are the differences between species and genera , or between orders and classes ? Give one or more keywords that describe these differences . -LRB- a -RRB- Differences between species and genera : ... -LRB- b -RRB- Differences between orders and classes : ... `` -LRB- The correct answer was that species differ from genera -LSB- and orders and classes -RSB- , for instance by being populations within which gene flow can occur . The difference between orders and classes is merely one of definition , none of nature . -RRB- `` What does the ` tree of life ' look like ? Please draw the evolutionary tree which describes the relationships between groups of organisms . Include as many groups as you are able to recall '' . Fig. 1Example phylogeny , where letters symbolise species . Not a single student -LRB- n = 31 -RRB- was able to give the correct answer to the question `` which species is/are most closely related to species B '' Results Twenty one university students returned questionnaires that were at least partly answered . On average they had previously taken biology courses for 1.6 years of full time study load -LRB- median 1.5 , range 0.3 -- 4.0 years -RRB- . Four were graduate students , the remainder were undergraduates . A further ten students of the regional university college returned the questionnaire . Not a single correct answer was given to question 1 . In most cases , species A -LRB- cf. Fig. 1 -RRB- was the whole answer or part of the answer -LRB- 17 of 20 university students , and 8 of 9 college students -RRB- . Of 20 answers to question 2 , 18 consisted of numbers . One student entered strokes , and 1 left the answers blank . In response to question 3 , 5 of 18 respondents answered that species were fundamentally different from genera , while there was no such difference for orders and classes . The remainder answered that there was no important differences at all -LRB- n = 8 -RRB- or that there were important differences between both species and genera , and between orders and classes -LRB- n = 5 -RRB- . As an answer to question 4 , 20 university students produced a drawing , however , 7 of them did not enter any names , and 1 entered one name only . The remainder provided between 4 and 23 taxon names . See Fig. 2 for an example . Fig. 2Phylogenetic tree drawn by a university student . The question posed was `` What does the ` tree of life ' look like ? Please draw the evolutionary tree which describes the relationships between groups of organisms . Include as many groups as you remember '' . Note the prominent position of mammals and the unresolved `` side branches '' . Taxon names provided by the student were translated verbatim into English , the drawing itself is unchanged Seven of 13 students depicted 1 to 9 extant taxa as stem groups in their drawings . A total of 14 % of all names provided were drawn as stem taxa . 11 of 13 students drew 1 to 11 branches which they named with paraphyletic groups . A total of 31 % of all names provided referred to paraphyla . As regards the placement of our own species , or the taxon containing Homo sapiens , the placement of our taxon seemed arbitrary in four drawings , and in two cases no names provided referred to taxa that contained our species . Our taxon was placed at top of the drawing in five cases , and at the outermost position in two cases . This means that 7 of 13 respondents gave a special position to our taxon . Counting the number of nodes at which the path to H. sapiens leads to the uttermost or uppermost position , a continuous `` human utterness score '' can be calculated -LRB- Sandvik 2008 -RRB- . Combined for the eleven drawings to which it was applicable , the average score was 0.90 -LRB- 43 of 48 nodes -RRB- , which is highly significantly biased in favour of man -LRB- P < 10 − 8 -RRB- . The resolution of the tree also favoured our species . I calculated scores of the relative attention the tree drawings devoted to three species , viz. . H. sapiens -LRB- Mammalia -RRB- , Vanessa atalanta -LRB- Lepidoptera -RRB- and Veronica fruticans -LRB- Lamiales -RRB- , in the following manner : a score of 1 was assigned if the species was named , of 2 if at least the `` order '' was named , of 3 if at least the `` class '' was named , of 4 if at least the `` phylum '' was named , of 5 if a higher taxon was named , and of 6 if no named branch contained the species . The average attention score was 3.6 ± 1.5 -LRB- SD -RRB- for man , 4.8 ± 1.2 for the butterfly , and 4.6 ± 1.3 for the flower . A one-tailed Wilcoxon two-sample tests showed that man received more attention than the other two species -LRB- W = 155.5 , P < 10 − 5 -RRB- . If using the `` human attention score '' defined elsewhere -LRB- Sandvik 2008 -RRB- , a continuous measure that expresses the degree to which tree resolution favours our own species , the average for the 13 tree drawings was 0.60 , which is highly significantly biased in favour of man -LRB- P < 10 − 4 -RRB- . Discussion O'Hara -LRB- 1997:327 -RRB- remarked that `` beginning students in biology should be taught how to read trees -LSB- ... -RSB- just as beginning students in geography need to be taught how to read maps '' . My findings drastically support this opinion . None of the undergraduate or graduate students in my -LRB- admittedly small -RRB- sample was able to correctly interpret a simple cladogram -LRB- Fig. 1 -RRB- . The cladogram was certainly not the first evolutionary tree that the students had met , since most had studied biology for several terms , including introductory courses in evolutionary biology . It does not require many sentences to explain how a cladogram should be read . The concept of evolutionary relatedness is not hard to grasp once it is explained . If it is not , however , erroneous conceptions seem to be able to survive for several years during prospective biologists ' educational careers . At the end of my course all university students were able to give correct interpretations of cladograms . Matters differed for the students of the regional university college . These students were asked which of the small cladograms in Fig. 3b -- i were compatible with the large cladogram -LRB- Fig. 3a -RRB- . This latter question was posed after the meaning of cladograms had been explained and after a considerable amount of time had been spent practicing the reading of cladograms . Still , only one answer out of fourteen correctly identified all of the three compatible cladograms . The remainder failed to identify at least one compatible cladogram and/or indicated at least one incompatible tree as compatible . This sobering result might be related to the fact that many of the college students simply needed some biology courses in order to study , e.g. fishery . They might thus not have been genuinely interested in biology per se , so that these findings would need to be replicated with university students . Still , it is independent evidence that reading cladograms is very much an acquired ability rather than one that can be presupposed . Fig. 3Nine phylogenetic trees over Mollusca with different topologies . a was used as a reference phylogeny . Students were asked which of the eight small trees were compatible with the reference tree . The eight topologies b -- i differ from a in either the phylogenetic relationships displayed -LRB- making them incompatible with a -RRB- , resolution of some of the branches , placement of taxa along the left -- right axis , or a combination of those -LRB- only c , g and h are compatible with a . In the tree distributed to students , all names were given as unabbreviated Norwegian vernacular names -RRB- In their answers , the students proved to pay more attention to how the taxon names at the tree tips were ordered along the left -- right axis , than to the topology of the cladogram . Given that the information of cladograms is conveyed in the branching order of taxa , while the left -- right ordering is arbitrary , this wrong focus necessarily leads to wrong conclusions . The main problem is presumably that students simply have not been taught how to read cladograms . Teachers may often assume that cladograms are self-explicatory graphic devices to illustrate phylogenetic relationships , when in fact their interpretation is not trivial . As Halstead -LRB- 1978:760 -RRB- put it : `` Cladograms are difficult enough for experts in the field to comprehend fully '' . Ironically , what he was advocating as the obvious alternative was phylograms -- which were meant to not only express phylogenetic relationships , but , in addition , phenotypic similarity , distribution through geologic time and species number . As such , phylograms are not only harder to interpret for untrained readers than cladograms . They may also convey incorrect information on any one of the aspects because they represent a two-dimensional compromise of several multidimensional measures . Regrettably , the presentation of systematics in text books has been lagging behind the cutting edge of systematic research by several decades . It is thus only quite recently that most biological text books have replaced phylograms with cladograms . A less appreciated fact is that even displaying cladograms does not guarantee that the information provided is unbiased , as many cladograms are drawn in an anthropocentric fashion -LRB- Sandvik 2008 -RRB- . It might thus well be that one source of confusion for students is the ambiguity of evolutionary tree drawings in text books . Drawings of evolutionary trees can not be clearer than their authors ' thoughts about evolutionary processes . And the latter have long been rather muddled , as can be exemplified by the occurrence of `` stem groups '' in many text book trees -LRB- e.g. Villee et al. 1984 : Fig. 19.6 ; Willmer 1990 : Fig. 14.2 , which has even been reprinted by a number of other text books -RRB- . However , stem groups have never existed in nature . A taxon above the species level can not possibly give rise to other taxa -- nor to anything else -LRB- Ghiselin 1997 -RRB- . One might say , therefore , that stem groups are just another way of expressing -LRB- or , worse still , trying to hide -RRB- ignorance . That some taxonomists have chosen to even give scientific names to their ignorance -LRB- e.g. `` Procoelomata '' ; Bergström 1989 -RRB- , does not really make the matter more transparent for biologists under education . No wonder , then , that stem groups occurred in 54 % of the student drawings . One problem is thus that many tree drawings which students meet during their studies are biased and thereby distort the evolutionary understanding of their readers . Another is that this distorted understanding even impinges on the interpretation of correctly drawn phylogenetic trees . In other words , both problems re-enforce each other . I have demonstrated elsewhere that even cladograms in phylogenetic text books are biased in an anthropocentric way -LRB- Sandvik 2008 -RRB- . In terms of ordering of taxa and differential resolution of branches , otherwise correct cladograms may distort the understanding of evolution . These findings make the evidence presented here even more alarming : If taxa are ordered in an anthropocentric manner in most cladograms , and if students rely more on ordering than topology , this affects the interpretation of relationships . The taxon containing our own species is most often placed in the rightmost position in cladograms -LRB- Sandvik 2008 -RRB- . This can be illustrated with Fig. 4 , which displays the phylogeny of Osteognathostomata in an anthropocentric manner . The topology of the cladogram is the same as in Fig. 1 . Accordingly , many untrained student of biology can be expected to conclude that coelacanths are more closely related to ray-finned fishes than to mammals . Fig. 4Cladogram depicting the phylogeny of bony vertebrates -LRB- Osteognathostomata -RRB- . The cladogram is anthropocentric in that both the resolution of its branches and the ordering of taxon names are biased in favour of mammals . Note that the tree topology is identical to Fig. 1 . See text for further details The students ' inability to answer question 1 -LRB- Fig. 1 -RRB- might be argued to have other reasons . An obvious alternative explanation would be that the error was caused by a simple misunderstanding of the word `` relationship '' , and that it is sufficient to tell students that relationship , as used in evolutionary biology , is a technical term meaning `` the relative recency of common ancestry '' -LRB- Mayr 1974 -RRB- . However , the question to the students was posed in Norwegian , were the word `` slektskap '' only has the latter meaning . In other words , while knowledge of the precise meaning of `` relationship '' is necessary -LRB- especially in English -RRB- , it is not sufficient for students to understand cladograms . In passing I would like to mention a speculation on the reason why phylogenetic systematics and cladistic methodology was rather quickly accepted in Germany -LRB- Ax 1977 ; Remane 1956 ; Schlee 1969 -RRB- , but provoked intense debates in English-language journals -LRB- verifiable with almost any issue of Systematic Zoology from the 1970s -RRB- . I suspect that part of the problem was semantic . The German word for `` relationship '' is `` Verwandtschaft '' , but while the English word has all kind of abstract and symbolic connotations , including overall similarity , the German term is reserved for true , genealogical bonds -LRB- as is the Norwegian `` slekt '' , see footnote 1 -RRB- . The statement that for instance the lungfish is more closely `` verwandt '' to the cow than to the salmon is quite uncontroversial in German . On the other hand , the statement that the lungfish is more closely related to the cow than to the salmon , was able to create a heated discussion -- which was only peripherally concerned with the actual phylogeny of the groups concerned -LRB- Gardiner et al. 1979 ; Halstead 1978 ; Halstead et al. 1979 -RRB- . A final observation concerns the importance attributed to Linnean categories -LRB- i.e. labels such as `` family '' , `` order '' , etc. -RRB- by students . It is well-established that Linnean categories above the species level do not carry information , that they are not comparable across taxa , and that it is entirely arbitrary to which taxa they are assigned in the first place -LRB- de Queiroz and Gauthier 1992 ; Donoghue 2001 ; Ereshefsky 1994 , 2001 , 2002 -RRB- . Many taxonomists have abandoned categories altogether , simply referring to taxa by their names . However , few students are aware of the arbitrariness of Linnean categories . In many undergraduate text books , the enumeration -LRB- ! -RRB- of animal `` phyla '' or insect `` orders '' seems to be more important than the discussion of interrelationships between the taxa concerned -LRB- e.g. Barnes et al. 1998 -RRB- . This leaves students with the impression that categories must express something . Unless told otherwise , they tend to make up their own explanations . As my results indicate , only a tiny proportion of students seems to question the existence and reality of Linnean categories . Of course , blank answers to question 2 do not necessarily indicate the deliberate rejection of categories -- the two students may also have wished to give a number , but were unable to `` recall the correct one '' . On the other hand , question 2 was clearly a leading -LRB- not to say , loaded -RRB- question , which may have biased the respondents into thinking that a number was the only acceptable answer . Still , findings from question 3 show that at least 27 % of the students perceive the distinction between `` orders '' and `` classes '' to be a reflection of real differences between natural levels of organisation . The findings presented here re-enforce earlier reports that reading cladograms is an ability that has to be practiced -LRB- O'Hara 1992 , 1997 -RRB- . Even though the transition from developmental thinking to tree thinking is more or less completed in the science of systematics at the collective level , it has to be accomplished anew by every generation of biology students on the individual level . This must not be forgotten when teaching phylogenetics or writing or illustrating text books . Other topics that need to be addressed explicitly are the artificiality of Linnean categories , the non-existence of stem taxa , and the arbitrariness of paraphyla . It does not seem to be sufficient to `` get the facts right '' in teaching and text books . Students tend to fall back to group and developmental thinking unless explicitly told otherwise .

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 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O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O

cladogram
essentialism
evolutionary tree
group thinking