Introduction 1997 1965 1980 1980 1988 1996 2006 2003 1859 1997 1999 2004 1996 1950 1997 1997 1974 1997 1976 2002 1974 1988 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 cannot be taken for granted by teachers. Sadly, many text books indirectly and implicitly promote developmental thinking. This further aggravates the problem. Given the importance (even necessity) 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 (at Universitetet i Tromsø, Norway). 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 (Høgskolen i Bodø, Norway). 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. 1 1 “How many classes of vertebrates and angiosperms are there? Fill in the scientifically correct answer:...“(There is no scientifically correct answer to this question.) “What are the differences between species and genera, or between orders and classes? Give one or more keywords that describe these differences. (a) Differences between species and genera:...(b) Differences between orders and classes:...“(The correct answer was that species differ from genera [and orders and classes], 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.) “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. 1 n  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 (median 1.5, range 0.3–4.0 years). Four were graduate students, the remainder were undergraduates. A further ten students of the regional university college returned the questionnaire. 1 Of 20 answers to question 2, 18 consisted of numbers. One student entered strokes, and 1 left the answers blank. n n 2 Fig. 2 Phylogenetic 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. Homo sapiens H. sapiens 2008 P −8 H. sapiens Vanessa atalanta Veronica fruticans W P −5 2008 P −4 Discussion 1997 1 3 3 after after Fig. 3  a  b i  a  a  c  g  h  a  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. 1978 2008 1984 1990 1997 1989 2008 2008 4 1 Fig. 4 1  1 1974 1977 1956 1969 Systematic Zoology 1 related 1979 1978 1979 1992 2001 1994 2001 2002 1998 something 1992 1997