Tree-Thinking Skills in Japanese First-Year University Students in Relation to High School Biology Textbooks

ABSTRACT

Although tree-thinking skills are crucial for understanding biology, research on this topic remains scarce in Asian countries. This study assessed tree-thinking skills among first-year university students in Japan using BETTSI (Basic Evolutionary Tree-Thinking Skills Instrument 3.0). Additionally, it analyzed the presentation of evolutionary diagrams in the biology textbooks used by these students during high school. The results showed that while students with high school advanced biology had more experience and confidence with phylogenies, their overall BETTSI scores did not differ. Furthermore, Japanese university students’ overall BETTSI scores were comparable to those reported in previous studies of American university students majoring in STEM fields. Item-by-item analysis revealed that Japanese university students face challenges concerning interpreting phylogenetic relatedness and comparing phylogenies compared to their American counterparts. The textbook analysis suggests that the instruction aimed at developing these skills is likely not being provided.

INTRODUCTION

The principle of “common ancestry” is the core concept of evolutionary theory. Phylogenies¹⁾ are the most direct representation of this principle (Baum et al., 2005). Phylogenies are used in research and biology textbooks to explain evolution (Catley and Novick, 2008). Therefore, students must learn what kind of information can be extracted from phylogenies (Sandvik, 2008).

Interpreting phylogenies requires a skill set known as “tree-thinking.” Tree-thinking skill is the ability to accurately interpret the relationship depicted in an evolutionary tree (O’Hara, 1997; Baum et al., 2005). It covers all activities interpreting, analyzing, and reasoning with a given tree (Halverson, 2011a). Although tree-thinking is sometimes divided into tree-reading and tree-building (Halverson, 2011a), in this study it refers to the former. After Baum et al. (2005) raised concerns about students' tree-thinking skills, several assessment scales were developed. These studies revealed common misconceptions in interpreting phylogenetic relatedness on cladograms, such as “reading the tips,” “node counting,” and “similarity equals relatedness” (Kummer et al., 2016). However, these reports are concentrated in the United States (Meir et al., 2007; Blacquiere and Hoese, 2016; Kummer et al., 2016; Novick and Catley, 2016) and Germany (Schramm et al., 2021a; Gutowski et al., 2025). While there is one example of research in Indonesia (Rahmania et al., 2023), reports from Asia are scarce. Consequently, while differences in understanding of natural selection have been reported across countries and curricula (Pinxten et al., 2020; Yamanoi et al., 2026), it remains unclear whether such differences exist regarding tree-thinking skills.

Few instruments offer both confirmed multi-skill evaluation of students' tree-thinking and validity/reliability, despite the development of several tools to date (Baum et al., 2005; Smith et al., 2013; Blacquiere and Hoese, 2016; Kummer et al., 2016; Dees et al., 2017). For example, tree-thinking skills are divided into five categories: identifying structures, handling apomorphies, identifying relationships, comparing trees, and arguing and inferring (Schramm et al., 2021a)²⁾. The recently developed Basic Evolutionary Tree-Thinking Skills Instrument 3.0 (BETTSI; Jenkins et al., 2022) is a valid and reliable tool designed to assess multiple tree-thinking skills among students taking introductory biology courses. Studies using BETTSI have been conducted with undergraduate students taking introductory biology courses in the United States (Jenkins et al., 2022; Daniel et al., 2024) enabling comparison with findings from studies in the United States.

Therefore, this study used a translated version of BETTSI to conduct a survey of first-year university students in Japan and compared the results with those from the United States. First-year university students were selected because they were considered a comparable group to university students taking introductory biology courses in the United States. This study further analyzed the presentation of phylogenies in high school biology textbooks used by the surveyed students during their high school years to examine the relationship with the results of the BETTSI survey. While analyses of the number and types of evolutionary diagrams (all forms of diagrams depicting evolutionary relationships, both in treelike formats and in other styles)³⁾ in biology textbooks have been conducted in countries such as the United States, Germany, and the Czech Republic (Catley and Novick, 2008; Machová, 2021; Schramm et al., 2021b), no research reports have been found for Asia, including Japan. Based on these findings, this study explores countermeasures to improve the tree-thinking skills of Japanese university students.

MATERIALS AND METHODS

Survey Using BETTSI

BETTSI is a 14-item multiple-choice scale, can be administered and scored quickly. Other features of BETTSI include using rectangular rather than diagonal trees⁴⁾ with time-direction arrows to make answering easier for students, and limiting each phylogeny to just two questions to minimize respondent burden. Of the 14 items, 11 (Q4–14) assess tree-thinking skills, with each item corresponding to a single concept (Table 1). Additionally, for each item, consideration is given to what alternative concepts are likely to be held by the respondents who select an incorrect answer. The remaining three items (Q1–3) examine the experience of seeing or using phylogenies, experience being taught how to interpret them, and confidence in interpreting them. The BETTSI is available at https://qubeshub.org/publications/2106/1.

Table 1. Concepts corresponding to each BETTSI item

	Concept/Tree Interpretation and Reading Skill	Hypothesized Alternative Concepts
Q4	The relatedness of two species is determined by how recently they shared a common ancestor.	Grouping by similarity / Reading across the tips / Taxa are biologically unrelated
Q5	All contemporary species have been evolving from LUCA (last universal common ancestor) for the　same amount of time and are “equally” evolved.	Equates complexity with “higher” evolution / Equates complexity with time
Q6	Trait changes occur along the branches and represent a change in the species.	Faulty answers are likely due to errors in defining lineage
Q7	The relatedness of two species is determined by how recently they shared a common ancestor.	Reading across the tips
Q8	Nodes represent the last common ancestral population of the lineages descended from that node.	Any directional change along a branch corresponds to a node
Q9	Lineages represent ancestor-descendant relationships of populations, organisms, or genes.	Faulty answers are likely due to errors in defining lineage
Q10	The relatedness of two species is determined by how recently they shared a common ancestor.	Reading across the tips / Grouping by similarity
Q11	The traits of species living at a point in time are those that accumulate during their evolution up until that point in time.	Lacking recognition that nodes represent hypothetical common ancestry
Q12	A tree diagram shows the order in which lineages diverged from one another.	Reading across the tips
Q13	Different tree designs contain the same information for　any given evolutionary hypothesis.	Tree shape influences relationships shown
Q14	Different tree orientations contain the same information for any given evolutionary hypothesis.	Orientation impacts relationships shown / Time is static

This table was created based on the “Supporting Docs” section of BETTSI website ( https://qubeshub.org/publications/2106/supportingdocs/1#supportingdocs).

BETTSI was translated from English to Japanese. The three authors (TY, SM, and AS), specializing in evolutionary biology and evolution education, translated the instruments to ensure that they were faithful to the original English version and intelligible to Japanese university students. Thereafter, university students associated with the authors’ laboratory were asked to complete the questionnaire and indicate any unintelligible expressions, which were revised appropriately. Subsequently, a web-based questionnaire was created based on the translated material.

The survey was conducted among first-year students enrolled in the faculties of education or life and environmental sciences at five universities (three national and two private). It was administered online between June 2024 and January 2025 as part of the biology and science education courses taught by the authors. In addition to BETTSI’s 14 items, students were asked about their course history in biology-related subjects during high school. Notably, the students surveyed received high school science education based on the 2009 revision of the Course of Study (Ministry of Education, Culture, Sports, Science and Technology [MEXT], 2009). High school biology consists of Basic Biology and Advanced Biology. Since Advanced Biology covers evolution in detail (Nakamichi and Katayama, 2018), the study focused on the effects of taking that course. In Japan, students enter university immediately after high school graduation. Consequently, it is believed that the academic performance of first-year university students is strongly influenced by their high school education.

In analyzing the survey results, the overall BETTSI scores (total number of correct answers for Q4–14) and the correct response rates for each item were examined. Furthermore, the overall scores and item-level scores were compared between students who had taken only Basic Biology (hereinafter, basic biology group) and those who had taken both Basic Biology and Advanced Biology (hereinafter, advanced biology group). When comparing the two groups statistically, the Mann-Whitney U test was used to compare BETTSI scores, and the chi-square test was used to compare the correct response rates for each item (Fisher’s exact test was used when more than 20% of the cells had expected frequencies of less than 5).

The results of this study were compared with previous studies using BETTSI. In Daniel et al. (2024), a survey using BETTSI was administered to 884 students (294 STEM majors and 590 non-STEM majors) enrolled in introductory biology courses at universities in the United States before their evolutionary biology lectures, and the mean total score was reported. Furthermore, Jenkins et al. (2022) conducted a study using BETTSI with 89 university students enrolled in an introductory biology course in the United States and reported the correct answer rates for each item using a three-tier scale (Easy: 0.85-1.0, Medium: 0.51-0.84, Hard: 0.0-0.50). The results of the present study were compared to both these studies.

Textbook Analysis

The study summarized how “evolution” was addressed in the science education received by Japanese university students who participated in the BETTSI survey (MEXT, 2008, 2009; Nakamichi and Katayama, 2018). Evolution-related content is introduced in junior high school. The evolution of vertebrates is covered there, and some textbooks include phylogeny. However, since this topic is not explicitly mandated by the Course of Study and is treated as an advanced topic, some teachers choose not to cover it in class (Yamanoi et al., 2022). High school Basic Biology covers the concept of common ancestry for all living organisms and phylogenies are included as part of the standard curriculum. Furthermore, Advanced Biology covers topics such as the mechanisms of evolution and macroevolution, and phylogenies are used more frequently. However, no textbook analysis focusing on phylogenies has been conducted.

Therefore, this study analyzed the number and types of evolutionary diagrams appearing in the chapters dealing with evolution in the Basic Biology and Advanced Biology textbook specifically the first chapter in Basic Biology and the final chapter or the chapter preceding it in Advanced Biology.

The textbook analysis was conducted according to the following procedure. To begin, we recorded the total number of pages in Basic Biology and Advanced Biology textbooks published by each of the five publishers⁵⁾, as well as the number of pages in the chapters that primarily covered evolutionary diagrams. Next, we counted the number of evolutionary diagrams featured in those chapters and compared the results between the two textbooks. Furthermore, the types of evolutionary diagrams were classified based on a similar method to Catley and Novick (2008). It confirmed whether the evolutionary diagrams were cladograms (phylogenies constructed using cladistic principles of most recent common ancestry that depict evolutionary relationships among a set of taxa as nested levels of common ancestry). If they were cladograms, this study recorded whether they were rectangular trees (called tree format) or diagonal trees (called ladder format) (Figure 1). If the diagram was not a cladogram, it was classified (a) through (e) (Table 2, Figure 1). Although Catley and Novick (2008) further subdivided the categories based on several criteria, this study did not do so because its objective was to investigate the general trends in the forms of evolutionary diagrams found in the textbooks. It further conducted a preliminary survey to determine whether the figure legends for the evolutionary diagrams included explanations to interpret the phylogenies, whether there were activities involving the comparison of phylogenetic trees, and whether the trees included descriptions of changes in traits or genes. Finally, we compared the number and types of evolutionary diagrams with American high school biology textbooks reported by Catley and Novick (2008).

Figure 1. Classification of evolutionary diagrams

Table 2. Classification of evolutionary diagrams that are not cladograms

Category		Classification criteria
a	Almost-a-tree cladogram	A diagram that appears largely like a tree but has some irregularities or violations (e.g., side branches or terminal nodes ending at more than one level) that preclude its classification as a valid tree.
b	Almost-a-ladder cladogram	A diagram that looks like a ladder but has irregularities or violations precluding its classification as a valid ladder.
c	Tree of life	Haeckel's tree and Haeckel-like diagram showing a progression from “simpler” to more “complex” taxa, as one moves from lower to higher on the diagram. Diagrams with a main line in the center that branches out were classified into this category. Some diagrams within this category include those with arrow-shaped branch tips.
d	Anagenesis	A diagram showing a clear linked, linear progression of taxa along a single branch, suggesting that one taxon “turned into” the next. Some diagrams within this category include those with arrow-shaped branch tips.
e	Other links	A diagram showing taxa that are connected in some way not covered by any of the previous four categories. The rootless phylogeny depicting three domains was classified into this category.

Note. This table was created based on Catley and Novick (2008).

RESULTS

BETTSI Score among Japanese Students

Responses were collected from 180 first-year Japanese university students. The overall correct answer rate for the 11 items assessing tree-thinking skills was 46% (Figure 2). By item, Q10 (interpreting the relatedness of two species) had the lowest correct answer rate at 9%, followed by Q13 (comparison of different tree designs), which had a correct answer rate of 28%. The correct answer for Q10 was “(c) a seal is equally related to a horse and a whale.” However, 68% of the students selected “(a) a seal is more closely related to a horse than to a whale,” suggesting that they held the alternative concept of reading across the tips/grouping by similarity (Table 1). Although the correct answer for Q13 was “(d) all trees are the same,” the high selection rates for “(b) tree 2” and “(c) tree 3” (31% and 23%, respectively) suggested that there were challenges in comparing different designs (diagonal tree, rectangular tree, and almost-a-tree cladogram). On the other hand, Q8 (meaning of nodes) had the highest correct answer rate at 69%, followed by Q14 (comparison of different tree orientations) at 68%.

Figure 2. Correct answer rates for total and each item of BETTSI

Regarding the effects of taking Advanced Biology, no significant difference was observed in the overall BETTSI scores between the advanced biology (N = 96) and basic biology groups (N = 84) (U = 3389, p = 0.063). The results of the comparison of response rates between groups for each item in BETTSI are presented in the appendix 1. When comparing correct response rates by item, a significant difference between the groups was observed only for Q5 (all contemporary species are equally evolved) (advanced biology group: 45%, basic biology group: 19%, χ² = 13.475, p < 0.001). On the other hand, responses to Q1–3 differed across groups. There was a significant difference in the frequency of responses to Q1 between the groups (χ² = 63.543, p < 0.001), suggesting that the advanced biology group had more experience viewing or using phylogenies (Q1) (Figure 3a). Regarding the experience of being taught how to interpret phylogenies (Q2), the advanced biology group had a higher percentage of “yes” responses than the basic biology group (advanced biology group: 89%, basic biology group: 40%, χ² = 46.197, p < 0.001). In Q3 as well, there were significant differences in the frequency of responses among the groups (Fisher’s exact test, p < 0.001), and the advanced biology group had a higher proportion of positive responses regarding confidence in interpreting phylogenetic trees (Q3) compared to the basic biology group. However, the proportions of respondents who answered “dead sure of myself” and “confident” were low at 2% and 3%, respectively (Figure 3b).

Figure 3. Responses to Q1 and Q3

Comparison of the BETTSI Score with Other Countries

Japanese university students’ overall BETTSI scores were higher than those of non-STEM undergraduates taking introductory biology in the United States. However, the scores were comparable to their STEM-major counterparts (Table 3a). Table 3b shows the scores by item. There were five items for which Japanese and American university students fell into different accuracy categories. The only item for which Japanese university students were placed in the category with a higher accuracy rate was Q6 (trait changes occur along the branches). For the other four items (Q4, 7, 12, and 14), American university students were placed in the category with a higher accuracy rate. Of these items, Q4 and Q7 concern the interpretation of the phylogenetic relatedness of two species, while Q12 and Q14 concern the comparison of different tree designs (Table 1).

Table 3. Comparison of the BETTSI score with other countries

a) Total score

	Mean	S.E.
America, introductory biology course, non-STEM majors, Daniel et al. 2024	4.13	0.86
Japan, first-year students, current study	5.09	0.16
America, introductory biology course, STEM majors, Daniel et al. 2024	5.10	0.14

b) Correct answer rate of each item

	Easy (0.85-1.0)	Medium (0.51-0.84)	Hard (0.0-0.50)
America, introductory biology course, Jenkins et al. 2022	Q14	Q4,7,8,9,12	Q5,6,10,11,13
Japan, first-year students, current study		Q6,8,9,14	Q4,5,7,10,11,12,13

Note. S.E. denotes the standard error. The numbers shown in bold indicate that the categories differed between Japanese and American university students.

Textbook Analysis

The number of evolutionary diagrams included in textbooks for Basic Biology and Advanced Biology was 1.8 and 16.8 per textbook, respectively (Table 4a). Since Basic Biology textbooks included only a few phylogenies, the results of the phylogeny type classification are shown along with those from Advanced Biology. After classifying the evolutionary diagrams, cladograms accounted for 58% of the total, while non-cladogram trees accounted for 42%. The proportion of rectangular trees was high at 47%, while the proportion of diagonal trees was low, at around 11%. Of the 39 evolutionary diagrams other than cladograms, 15 were classified as type b (Almost-a-ladder cladogram), which was the most common (a: 7, c: 4, d: 4, e: 9).

Table 4. Evolutionary diagrams in Japanese high school biology textbooks

a) Numbers

		Basic Biology			Advanced Biology
		Number of diagrams	Number of pages		Number of diagrams	Number of pages
			Chapter	Total		Chapter	Total
Publisher	A	2	40	248	19	88	480
	B	2	42	241	21	82	398
	C	3	36	232	15	84	440
	D	1	41	232	13	78	464
	E	1	38	238	16	70	400
	ave.	1.8	39.4	238.2	16.8	80.4	436.4

b) Types

				Cladogram			Other evolutionary diagrams
				Trees	Ladders	Other
Japanese high school textbooks (current study)				0.47	0.11	－	0.42
American high school textbooks (Catley and Novick, 2008)				0.18	0.26	0.09	0.47

Note. The cladograms featured in Japanese textbooks were either rectangular or diagonal; no other types of cladograms were found.

In 38% (35 out of 93) of all evolutionary diagrams, changes in traits or genes were noted. On the other hand, only one diagram included a clear explanation of how to interpret the relationships depicted on the cladogram (degrees of phylogenetic relatedness among taxa). Furthermore, only seven examples included comparisons of the topologies of multiple phylogenies. All of these were comparisons between rectangular trees or diagonal trees, and no comparisons of the topologies of rectangular and diagonal trees were found. Nevertheless, all textbooks included three types of evolutionary diagrams (rectangular trees, diagonal trees, and other evolutionary diagrams).

　 These results were compared with an analysis of American high school biology textbooks by Catley and Novick (2008). The number of evolutionary diagrams per textbook was higher in Japanese textbooks than in American textbooks (16.8 and 12.3, respectively). Regarding topology, Japanese textbooks had a higher proportion of rectangular trees (Japan: 47%, United States: 18%) and a lower proportion of diagonal trees (Japan: 11%, United States: 26%) compared to the American textbooks (Table 4b). Furthermore, while the proportion of “other evolutionary diagrams” was nearly the same in both countries’ textbooks (Japan: 42%, United States: 47%), the proportion of type b (almost-a-ladder cladogram) among “other evolutionary diagrams” was higher in Japan (Japan: 38%, United States: 25%).

DISCUSSION

Japanese students’ BETTSI scores exceeded those of American non-STEM majors, yet aligned with STEM majors among undergraduates taking introductory biology. Although the advanced biology group reported having more experience viewing and using phylogenies (Q1, Figure 3a) and having been taught how to interpret them (Q2) compared to the basic biology group, no difference was observed in their overall scores (Q4–11). The advanced biology group further reported higher confidence levels in interpreting phylogenetic trees than the basic biology group; however, even within the advanced biology group, the proportions of students who answered “Dead sure of myself” and “Confident” were low at 2% and 3%, respectively (Figure 3b).

Textbook analysis revealed that the Advanced Biology textbooks contained more evolutionary diagrams than the Basic Biology textbooks (Advanced Biology: 16.8, Basic Biology: 1.8). The number of evolutionary diagrams included in Advanced Biology textbooks was higher than that in American high school biology textbooks. However, similar to American high school biology textbooks, the proportion of evolutionary diagrams other than cladograms was high (42%) in all Advanced Biology textbooks. Of all evolutionary diagrams (93 examples), only one included a clear explanation in the figure legend regarding interpreting the relationships (degrees of relatedness among taxa) on the cladogram, and only seven included topological comparisons. Hence, to improve the tree-thinking skills among first-year university students in Japan, it is necessary to enhance phylogenetic instruction in Advanced Biology courses.

Improvement of the “Identifying Relationship” Skill

Among Japanese university students, the BETTSI item Q10 had the lowest correct response rate, while Q4 and Q7 had lower correct response rates than those of American university students. All these items required students to infer phylogenetic relatedness between two species from a phylogeny (Table 1). Regarding Q10, option a was selected most frequently (68%). For Q4, options c (32%) and a (21%) were selected most frequently (appendix 1). For Q7, options c (24%) and b (22%) were selected most frequently. This suggests that students possess alternative concepts, such as “reading across the tips” and “grouping by similarity.” According to Schramm et al. (2021a), the “identifying relationships” skill level (the ability to identify clades and evaluate evolutionary relationships among taxa and their most recent common ancestors) was required and considered central to tree-thinking skills (Blacquiere and Hoese, 2016; Schramm et al., 2019). This skill is considered difficult to improve even after instructions compared to other skills (Dees et al., 2014, 2018). Daniel et al. (2024) also reported that the correct response rate for Q10 remained below 20% even after intervention. Although the advanced biology group reported having been taught an approach for interpreting phylogenies more than the basic biology group (Figure 3a), since only one phylogeny illustrating this skill was included in the textbook, it is possible that they were hardly taught this skill. To improve this skill, an understanding of the nested structure of phylogenies is necessary. Furthermore, instruction incorporating circle-in-circle diagrams (Schramm and Schmiemann, 2019) may lead to improvement.

Improvement of the “Comparing Trees” Skill

Q13 in the BETTSI test, which had the second-lowest correct response rate among Japanese university students, along with Q12 and Q14, which had lower correct response rates than American university students, were all problems involving the comparison of phylogenies. According to Schramm et al. (2021a), this corresponded to the “comparing trees” skill level (ability to interpret relationships across tree rotations and subtrees). Of the 93 evolutionary diagrams, only 7 included comparisons of tree topology, and not a single example compared the topologies of rectangular and diagonal trees. This suggests that even students in the advanced biology group had not received sufficient instruction for this skill.

Compared to American high school biology textbooks, Japanese high school biology textbooks had a lower proportion of diagonal trees, yet a higher proportion of almost-a-ladder cladograms. It has been reported that diagonal trees may promote the misconception of teleology (Schramm and Schmiemann, 2019). Phylogenetic relatedness is harder to discern than in rectangular trees (Novick and Catley, 2007; Dees et al., 2017). Hence, the use of diagonal trees has decreased in recent years in university biology textbooks in the United States and Germany, with rectangular trees accounting for the majority (approximately 80%) (Schramm et al., 2021b). It would be desirable for Japanese textbooks to reduce the use of diagonal trees and almost-a-ladder cladograms and increase the use of rectangular trees. Additionally, to improve students’ “comparing trees” skills, it would be effective to incorporate activities that compare the topologies of diagonal and rectangular trees, and using activities that use pipe cleaners (Halverson, 2011b) to help students understand that the meaning of a phylogenetic tree remained unchanged even when nodes were rotated.

The Strengths of Biology Education in Japan

Although modest, the positive effects of high school biology education in Japan were confirmed. Since the advanced biology group had a higher correct response rate on Q5 than the basic biology group, it appeared that enrolling in Advanced Biology promoted an understanding that “all living organisms have evolved equally over the same period of time from a common ancestor.” Additionally, Japanese students had a higher correct response rate for Q6 (trait changes occur along the branches) than their American counterparts. Given that the advanced biology group reported having been taught how to interpret phylogenetic trees more than the basic biology group (Q2), and 38% of the evolutionary diagrams in the textbooks described changes in traits or genes, it can be inferred that the content of Q6 was covered in Advanced Biology classes as part of instruction on how to interpret phylogenetic trees. In future, studies need to clarify what Japanese high school biology teachers are teaching regarding phylogenies, and explores why even students in advanced biology groups lack sufficient confidence in interpreting phylogenies.

CONCLUDING REMARKS

A survey on tree-thinking skills conducted among first-year university students in Japan revealed that their scores were comparable to those of American university students majoring in STEM fields who were taking introductory biology courses. An item-by-item analysis indicated challenges in the skill levels related to “identifying relationships” and “comparing trees” (Schramm et al., 2021a). Furthermore, a textbook analysis confirmed that content corresponding to these skill levels was rarely found in Advanced Biology textbooks. Hence, there is a need to enhance education (including textbook content and teaching methods) aimed at fostering tree-thinking skills in Japanese high school biology.

In the future, as surveys using BETTSI and analyses of evolutionary diagrams featured in textbooks are conducted in various countries, the progress of tree-thinking education in each nation will become clearer. The Japanese high school biology curriculum recently moved the topic of evolution to Chapter 1 (MEXT, 2018). How this shift has affected both presentations of phylogenies in textbooks and students' tree-thinking skills will be investigated.

NOTES

1) As in the previous study (Schramm et al., 2019), we will treat terms like “evolutionary tree,” phylogenetic tree,” and “phylogeny” as interchangeable.

2) Tree-thinking skills encompass the ability to identify phylogenetic structures, interpret apomorphies for grouping taxa, evaluate clades and relative relatedness, maintain reasoning across diverse tree representations, and infer evidence-based conclusions beyond the depicted data.

3) This term (evolutionary diagrams) has also been used in related previous studies (Catley and Novick, 2008; Machová, 2021; Schramm et al., 2021b)

4) It has been reported that it is more difficult to interpret phylogenetic relatedness from diagonal trees than from rectangular trees (Novick and Catley, 2007; Dees et al., 2017).

5) Since the use of textbooks approved by the MEXT is mandatory in Japan, the surveyed students would have used one of these textbooks during their high school years.

ACKNOWLEDGMENTS

We would like to express our gratitude to all the students who participated in the questionnaire survey. This work was supported by the Japan Society for the Promotion of Science under Grant [number 23K02819].

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APPENDIXES

Appendix 1. Responses to each BETTSI item by groups

	Group	Selection rates for each option					Statistical comparisons between groups
		a	b	c	d	e	χ2	p
Q1	Basic	0.17	0.56	0.25	0.02		63.54	< 0.001
	Advanced	0.01	0.14	0.73	0.13
	Total	0.08	0.33	0.51	0.08
Q2	Basic	0.40	0.60				46.20	< 0.001
	Advanced	0.89	0.11
	Total	0.66	0.34
Q3	Basic	0.73	0.27	0.00	0.00	0.00	-	< 0.001
	Advanced	0.33	0.47	0.15	0.03	0.02
	Total	0.52	0.38	0.08	0.02	0.01
Q4	Basic	0.18	0.36	0.36	0.06	0.05	0.67	0.41
	Advanced	0.23	0.42	0.28	0.05	0.02
	Total	0.21	0.39	0.32	0.06	0.03
Q5	Basic	0.33	0.14	0.19	0.18	0.15	13.48	< 0.001
	Advanced	0.26	0.13	0.45	0.11	0.05
	Total	0.29	0.13	0.33	0.14	0.10
Q6	Basic	0.04	0.08	0.20	0.61	0.07	1.27	0.26
	Advanced	0.04	0.04	0.19	0.69	0.04
	Total	0.04	0.06	0.19	0.65	0.06
Q7	Basic	0.42	0.19	0.25	0.14		0.96	0.33
	Advanced	0.49	0.24	0.24	0.03
	Total	0.46	0.22	0.24	0.08
Q8	Basic	0.01	0.07	0.15	0.70	0.06	0.13	0.72
	Advanced	0.06	0.07	0.15	0.68	0.04
	Total	0.04	0.07	0.15	0.69	0.05
Q9	Basic	0.01	0.17	0.07	0.11	0.64	0.40	0.53
	Advanced	0.04	0.13	0.07	0.07	0.69
	Total	0.03	0.14	0.07	0.09	0.67
Q10	Basic	0.62	0.12	0.08	0.07	0.11	0.23	0.63
	Advanced	0.74	0.04	0.10	0.06	0.05
	Total	0.68	0.08	0.09	0.07	0.08
Q11	Basic	0.12	0.07	0.42	0.36	0.04	0.15	0.70
	Advanced	0.04	0.07	0.44	0.39	0.06
	Total	0.08	0.07	0.43	0.37	0.05
Q12	Basic	0.07	0.52	0.20	0.13	0.07	0.77	0.38
	Advanced	0.04	0.46	0.27	0.10	0.13
	Total	0.06	0.49	0.24	0.12	0.10
Q13	Basic	0.15	0.32	0.20	0.24	0.08	1.59	0.21
	Advanced	0.05	0.29	0.25	0.32	0.08
	Total	0.10	0.31	0.23	0.28	0.08
Q14	Basic	0.07	0.10	0.12	0.63	0.08	1.58	0.21
	Advanced	0.05	0.11	0.06	0.72	0.05
	Total	0.06	0.11	0.09	0.68	0.07

Note. The numbers in bold in each item indicate the correct answer rate. Since the Fisher's exact test was used to compare responses across groups in Q3, the chi-square values are not shown.