Phylogenetic tree which is more closely related

Figure 8: Trees contain information on the relative timing of nodes only when the nodes are on the same path from the root i. In this tree, nodes x and y are not on the same path, so we cannot tell whether the ancestral organisms in node x lived before or after those in node y. Avise, J. Baum, D. The tree thinking challenge. Science , — Phylogenies and tree thinking. American Biology Teacher 70 , — Dawkins, R. O'Hara, R. Homage to Clio: Toward an historical philosophy for evolutionary biology.

Systematic Zoology 37 , — Population thinking and tree thinking in systematics. Zoologica Scripta 26 , — Maddison, W. Origins of New Genes and Pseudogenes. Evolutionary Adaptation in the Human Lineage. Genetic Mutation. Negative Selection. Sexual Reproduction and the Evolution of Sex. Haldane's Rule: the Heterogametic Sex.

Hybrid Incompatibility and Speciation. Hybridization and Gene Flow. Why Should We Care about Species? Citation: Baum, D. Nature Education 1 1 Phylogenies are a fundamental tool for organizing our knowledge of the biological diversity we observe on our planet. But how exactly do we understand and use these devices? Aa Aa Aa. What an Evolutionary Tree Represents. Figure 1. Figure Detail. The Lexicon of Phylogenetic Inference.

A node represents a branching point from the ancestral population. Terminals occur at the topmost part of each branch, and they are labeled by the taxa of the population represented by that branch. Figure 4: A monophyletic group, sometimes called a clade, includes an ancestral taxon and all of its descendants. A monophyletic group can be separated from the root with a single cut, whereas a non-monophyletic group needs two or more cuts.

How to Read an Evolutionary Tree. Figure 6: Types of phylogenetic trees. These trees depict equivalent relationships despite being different in.

Figure 7: Relationships on a phylogenetic tree can be depicted in multiple ways. These trees depict equivalent relationships despite the fact that certain internal branches have been rotated so that the order of the tip labels is different. The Importance of Phylogenetic Trees.

Science , — Baum, D. American Biology Teacher 70 , — Dawkins, R. Systematic Zoology 37 , — O'Hara, R. Zoologica Scripta 26 , — Maddison, W. Tree Thinking Group homepage, Article History Close.

Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend. Submit Cancel. This content is currently under construction. Explore This Subject. Genome Evolution. Topic rooms within Evolutionary Genetics Close. No topic rooms are there. Or Browse Visually. The initial phylogenetic tree homework featured a short series of open-ended questions designed around a phylogenetic tree of chordates.

In addition to prompts about recent common ancestors, synapomorphies, and monophyletic groups, one question regarding taxa relatedness appeared on the group homework Figure 3.

Poor group performance for this question compelled the instructor to revisit phylogenetic tree interpretations during class. The question was presented to students again and debated through directed, small-group discussions.

A subsequent whole-class discussion acknowledged most recent common ancestry as an appropriate reasoning strategy for determining taxa relatedness on phylogenetic trees. After the initial homework was revisited during class, taxa relatedness was specifically targeted through two additional letter card questions. Instruction specific to phylogenetic trees and evolutionary relatedness occurred across three consecutive course meetings, ending in week 5.

We therefore include each student's average attendance across these 3 d in subsequent analysis as a reflection of the potential impact of instruction on student reasoning with phylogenetic trees. Figure 3. Phylogenetic tree and taxa-relatedness question from the initial homework. Phylogenetic trees and taxa-relatedness questions similar to the initial homework were placed on three subsequent assessments, which followed the end of instruction by 1, 10, and 12 wk, respectively Figure 2.

Such prompts were included on both the individual and group components of the evolution unit exam in which students completed the individual component before the group component Supplemental Figures S1 and S2. A phylogenetic tree was provided for the individual component, but the group component required students to construct a phylogenetic tree from data before answering a taxa-relatedness question.

Students were never asked to construct phylogenetic trees before completing the evolution unit exam. A phylogenetic tree and taxa-relatedness questions were also placed on the review homework 2 wk before the final exam Figure S3 and on the individual component of the final exam Figure S4. The prompt structure for the review homework and final exam was changed slightly from a two-choice prompt with open-ended reasoning to a four-choice prompt with open-ended reasoning.

This alteration was made for several reasons. First, students had seen several taxa-relatedness questions throughout the semester; to avoid retest concerns, we created prompts that were familiar to students but offered a somewhat new opportunity to interpret relatedness. Second, the multiple-choice foils prevented students from feeling obligated to select one taxon or the other, providing students with the option to identify taxa as equally related or unrelated.

In both the review homework and final exam, the taxa involved were equally related. The phylogenetic tree on the final exam was also the only phylogenetic tree used as part of this investigation that did not include labeled synapomorphies.

The initial rubric for coding student responses to taxa-relatedness questions was developed using a grounded theory approach Glaser and Strauss, This reflected the nature of the project as developing in real time in response to classroom experiences and student learning difficulties.

Existing literature on phylogenetic tree interpretations Table 1 was then used to confirm and refine some categories for the final rubric Supplemental Material and to identify two new reasoning strategies.

Specifically, we found evidence that students determine relatedness by counting synapomorphies taxa relatedness is determined by counting synapomorphies between the taxa on phylogenetic trees and by using negation reasoning reasoning includes descriptions of how not to interpret taxa relatedness on phylogenetic trees; in all cases, this reasoning occurs concurrently with other reasoning; see the Supplemental Material.

In addition, we found evidence of students using monophyletic grouping taxa in the same monophyletic group are more closely related to each other than to a taxon outside the monophyletic group to reason about relatedness. While some research has identified monophyletic grouping as a possible reasoning approach, no one has provided evidence to show that students actually use monophyletic grouping. Two independent raters coded the initial responses and reached consensus through discussion.

Student responses often included more than one form of reasoning and consequently fell into multiple rubric categories, resulting in total reasoning codes assigned to group and individual responses. Coding was partially blind, in which one rater was aware of group and individual identities while the second rater was not.

Due to high agreement between independent raters, we do not believe rater bias was a significant issue for this investigation. The taxa-relatedness questions used throughout the course required students to choose an answer and provide reasoning for their selection. Because answers selected by students were not always consistent with their reasoning, responses were coded again for answer correct or incorrect and reasoning used to support the answer correct, incorrect, or mixed, i.

The categories of most recent common ancestry and monophyletic grouping were considered correct reasoning, while negation reasoning always appeared with other forms of reasoning and was considered neither correct nor incorrect. All other rubric categories were deemed incorrect reasoning for taxa relatedness. This coding procedure identified students who guessed correct answers correct answer with incorrect reasoning , and students who memorized correct reasoning without understanding its application incorrect answer with correct reasoning.

Only responses with both correct answers and correct reasoning demonstrated understanding of taxa relatedness on phylogenetic trees. Following the suggestion of Theobald and Freeman , we constructed statistical models to test various hypotheses regarding student reasoning about phylogenetic trees.

To assess hypotheses related to reasoning and answer selection, we constructed statistical models that accounted for variables affecting reasoning and answer selection.

In addition, random effects were used to capture repeated measurements on the same groups and individuals on multiple assessments. Specifically, mixed-effect ordinal logistic-regression models were used to analyze taxa-relatedness reasoning, while mixed-effect logistic-regression models were used to analyze correct answers.

For group reasoning, group assignment was modeled as a random effect, and assessment was a fixed effect. For individual reasoning, student was modeled as a random effect, while assessment, class attendance, year in school, and academic major were fixed effects. For group correctness, group assignment was modeled as a random effect, and assessment and reasoning correct, incorrect, or mixed were fixed effects.

For individual correctness, student was modeled as a random effect, while reasoning, assessment, class attendance, year in school, and academic major were fixed effects. F -tests were used to determine significance of batches of explanatory variables e. Additional details of the statistical analyses e. To assess student understanding of phylogenetic trees, we performed four separate analyses.

Referencing the analyses by their response variable, the analyses are group reasoning, individual reasoning, group correctness, and individual correctness. The following sections report both summary statistics for reasoning Table 3 and correctness Table 4 along with model-based analyses. Table 3. Reasoning used by students to determine taxa relatedness from all four assessments.

Values are the number of responses that received a particular code with percentage of responses in parentheses. Italics indicate correct forms of reasoning. Table 4. Answers and reasoning used by students to support answers from all four assessments. Values are number of responses that received a particular code with percentage of responses in parentheses. Table 3 shows that group performance was poor on the initial homework: only two groups applied correct reasoning, while 12 groups used the incorrect reasoning of counting synapomorphies and six used the incorrect reasoning of counting nodes.

On the subsequent assignment, 17 groups used most recent common ancestry and seven groups used monophyletic grouping. Counting synapomorphies was still prominent with six groups.

Two groups used counting nodes. Apart from a decrease in monophyletic grouping, individual reasoning largely persisted from the evolution unit exam through the review homework 9 wk later Table 3.

Individuals were less likely than groups to use correct reasoning on the evolution unit exam, and counting nodes was more common than counting synapomorphies among individuals.

Reasoning varied somewhat between the review homework and final exam. Most categories increased as counting synapomorphies decreased to only two responses on the final exam, in which the phylogenetic tree did not include synapomorphies. Interpreting taxa relatedness on phylogenetic trees requires both knowledge of correct reasoning, as indicated by student reasoning descriptions Table 3 , and application of correct reasoning, as indicated by selecting correct answers for taxa-relatedness questions.

Coding for answer correct or incorrect and reasoning used to support the answer correct, incorrect, or mixed revealed six different combinations of knowledge and application Table 4. Two groups selected the correct answer on the initial phylogenetic tree homework, and two other groups offered correct or mixed reasoning, but not a single group provided a correct answer coupled with correct reasoning. Unlike other taxa-relatedness questions, groups were required to build a phylogenetic tree from data for the group component of the evolution unit exam Figure S2 , and all but one group constructed a phylogenetic tree that was sufficient to correctly answer the question i.

Taxa-relatedness questions completed by individuals had lower rates of correct answers coupled with correct reasoning compared with the prior group component of the evolution unit exam Table 4 , excluding the individual component of the evolution unit exam due to poor question structure see Discussion.

Phylogenetic trees are an essential component of undergraduate biology education that remain difficult for students to interpret. Our in situ research documents common reasoning patterns used by students in an introductory biology course. Counting synapomorphies and nodes between taxa on phylogenetic trees were the most common forms of incorrect reasoning for determining taxa relatedness.

Students independently generated an alternative form of correct reasoning using monophyletic groups, but the popularity of this approach decreased over time. Further, after multiple learning opportunities, including broad instruction on phylogenetic trees, targeted instruction for evolutionary relationships, textbook readings, and homework, slightly more than half of groups and less than half of individuals provided correct answers coupled with correct reasoning for taxa-relatedness questions.

Many students appeared to have memorized correct reasoning without understanding its application, and of the variables we measured, attendance was the only predictor of student performance on taxa-relatedness questions.

This investigation has important implications for instruction and research on student interpretations of phylogenetic trees. As previously mentioned, results from coding student responses for answer correct or incorrect and reasoning correct, incorrect, or mixed are problematic for the individual component of the evolution unit exam due to flawed question structure Figure S1.

According to the phylogenetic tree and using most recent common ancestry or monophyletic grouping reasoning, bears are more closely related to sea lions than cats. Some incorrect reasoning, such as branch tip proximity and external insights rare among our students , led to incorrectly choosing cats instead of sea lions.

However, incorrect strategies of counting synapomorphies and nodes most common among our students led to correctly choosing sea lions. Although responses from the individual component of the evolution unit exam are unreliable for determining student understanding of taxa relatedness, we included the results for two important reasons.

First, student reasoning alone, regardless of its application, provided valuable insights into how students approach phylogenetic trees and aided development of the taxa-relatedness reasoning rubric Supplemental Material. Second, the flawed prompt is an informative example of what not to do when assessing student understanding of phylogenetic trees. Taxa relatedness is understood by biologists in terms of most recent common ancestry, similar to family trees of humans Baum et al.

Following the initial phylogenetic tree homework in which all groups struggled, a majority of students were aware that most recent common ancestry determines taxa relatedness Table 3 , although far fewer students applied the reasoning correctly Table 4.

Use of the alternative correct reasoning, monophyletic grouping, was a novel outcome for this study. Monophyletic groups were discussed at length during the course and in relation to phylogenetic trees, but neither the instructor nor the textbook Freeman, directly suggested using monophyletic groups to determine taxa relatedness.

Our students generated this alternative reasoning on their own, either spontaneously or from outside materials. Over time, however, students used this reasoning less frequently, perhaps in response to direct feedback on the unit exam that highlighted most recent common ancestry reasoning. While branch tip proximity, contemporary descent, and external insights are the most common forms of incorrect reasoning cited in the literature Table 1 , these forms of reasoning were rather uncommon in responses from our students.

Counting synapomorphies, and counting nodes were by far the most common forms of incorrect reasoning used by our students to determine taxa relatedness Table 3. Determining taxa relatedness by counting synapomorphies has not previously been described in the literature to our knowledge but proved to be a persistent approach. Two students even applied this reasoning on the final exam, in which the phylogenetic tree did not include synapomorphies Figure S4.

Both students suggested that seals are equally related to horses and whales which is correct , because there are no trait differences between the three taxa. Such responses are illogical and demonstrate the persistence of incorrect reasoning strategies. The existence and frequency of synapomorphy counting among students presents a pedagogical dilemma.

A previous investigation concluded that labeled synapomorphies on phylogenetic trees encourage comprehension of evolutionary relationships Novick et al. Investigators used translation exercises between two common phylogenetic tree styles Figure 1 , and students were significantly more accurate when synapomorphies were present. The researchers suggested that labeled synapomorphies improve translation performance due to a combination of cognitive psychology and biological understanding.

Phylogenetic trees are constructed from nested groups of taxa, and from a cognitive perspective, synapomorphies help identify points along continuous lines where hierarchical levels begin. From a biological viewpoint, synapomorphies help identify common ancestors and monophyletic groups, which are maintained during translation from one style of phylogenetic tree to another.

Although useful for translating between phylogenetic trees, synapomorphies are problematic for interpreting a single phylogenetic tree, as our students often misused them to determine taxa relatedness.

In one case, synapomorphies act as guides, while in another case, synapomorphies act as distractors. This apparent conflict between phylogenetic tree translation and interpretation tasks warrants further investigation. As cited in the literature and supported by the present study, introductory biology students use many forms of incorrect reasoning when interpreting phylogenetic trees, especially before instruction.

Thus, it was not surprising that attendance during targeted, active-learning instruction on evolutionary relatedness was a significant predictor of correct taxa-relatedness reasoning. Interpreting phylogenetic trees is an ability acquired through instruction and practice rather than an intuitive ability Sandvik, If formal instruction is the most important factor for understanding phylogenetic trees, it should not be surprising that year in school and major were not correlated with correct taxa-relatedness reasoning.

Phylogenetic trees are difficult for introductory biology students to interpret without instruction, regardless of their college experience or interest in biology. Because the purpose of phylogenetic trees is to visually represent evolutionary relationships, taxa-relatedness questions exemplify tree thinking Novick and Catley, This investigation used such prompts to measure understanding of phylogenetic trees by combining results from answers and reasoning used to support answers.

Responses that provided both correct answers and correct reasoning demonstrated understanding of taxa relatedness on phylogenetic trees. After the initial homework and targeted instruction on evolutionary relationships, and disregarding the individual component of the evolution unit exam unreliable for correctness coding , approximately half of the students demonstrated such understanding across multiple assessments Table 4.

With the ability to share interpretations, groups were expected to outperform individuals on taxa-relatedness questions. Although based on only three data sets excluding the initial phylogenetic tree homework, which was completed before targeted instruction on evolutionary relationships, and the unreliable individual component of the evolution unit exam , results align with expectations for cooperative work Table 4.

However, another explanation is that students performed better on the group component of the evolution unit exam versus the review homework and final exam due to building the phylogenetic tree before answering the taxa-relatedness question Figure S2. Phylogenetic tree construction could have required students to concentrate on taxa relationships or simply forced students to spend more time on task, and this alternative explanation cannot be ruled out.

Because the only two studies examining benefits of phylogenetic tree construction before interpretation disagree with each other Halverson, ; Eddy et al. Some answer and reasoning combinations other than correct answers with correct reasoning offer valuable insights into student understanding of phylogenetic trees. Correct answers coupled with incorrect reasoning indicate students who guessed correctly without understanding phylogenetic trees.

Disregarding the individual component of the evolution unit exam, guessing correctly was rare during this investigation Table 4. On the other hand, incorrect answers with correct reasoning indicate students who memorized taxa-relatedness reasoning but did not understand how to apply the reasoning to phylogenetic trees.

Shallow learning strategies are very common in the sciences Elby, ; Pungente and Badger, ; Tomanek and Montplaisir, and can be attributed at least in part to assessment practices Momsen et al. Similar to correct reasoning, attendance was a significant predictor of choosing correct answers for taxa-relatedness questions, while major and year in school were not significant factors.

For example, many readers confronted with the tree in a might be tempted to infer an evolutionary trend toward increased body size in snail species over time or, in Fig. Unfortunately, misinterpretations such as this can be found even in the primary scientific literature. Once again, this can be corrected simply by rotating a few internal nodes, as has been done in b , in which the topology is the same but where the supposed trend is no longer apparent.

The important consideration is internal branching: In this case, there is information about ancestral states e. Despite this being a clear evolutionary trend, there is no pattern evident across the terminal nodes. Thus, reading across the tips can create apparent trends where there are none and can mask real trends that are strongly supported by historical information.

The modern science of taxonomy is built upon the foundation laid by Carolus Linnaeus in the mid-eighteenth century. His system, which long predated the widespread scientific acceptance of common descent inspired by Darwin, categorized organisms on the basis of physical similarity.

Notably, in the first edition of his Systema Naturae of , whales were grouped with fishes—an oversight that he corrected in the tenth edition in by placing them with the other mammals. Today, the primary criterion for scientific classification is evolutionary relatedness, whereas differences in the degree of physical similarity across lineages are often a confounding variable.

This can be so for two major reasons: First, as with whales and fishes, adaptation to similar environments can lead to a superficial convergence of physical appearance. By way of example, consider the phylogeny presented in Fig. This tree shows one of the more prominent hypotheses regarding the relationships of major groups of nonmammalian tetrapods. Frogs are given as the outgroup in this tree, with turtles being the next most distantly related lineage to the others.

Snakes are the sister group to lizards, and in fact, both modern lizards and snakes may be descended from a more ancestral lizard lineage. Although physical similarities would seem to suggest otherwise, crocodiles are more closely related to birds than they are to lizards.

The reason for this is that the bird lineage has experienced significant modification, whereas crocodilians have remained largely unchanged for tens of millions of years. Birds are, in fact, descended from a lineage of theropod dinosaurs, making Tyrannosaurus rex far more similar to the last nonavian ancestor of modern birds than anything resembling a crocodile see Prothero Evolutionary relatedness and physical similarity are not necessarily linked.

The rates at which physical features change can differ among lineages Fig. As a result, close relatives may look different from one another or distant relatives may look misleadingly similar. Although they look very different, birds and crocodiles are actually more closely related to each other than either is to any other group of reptiles. The similarities between birds and mammals e. Mistaken assumptions that the ancestor of two modern groups must have been very similar to, or perhaps even was, one of the modern groups extend well beyond the case of crocodiles and birds.

For example, the hypothesis that whales and hippopotamuses are sister groups e. Not surprisingly, the fossil record of whales, which is becoming increasingly extensive, shows that the early ancestors of whales e. Nowhere is this misconception more pronounced than in discussions of human evolution. Figure 14 a shows a ladderized phylogeny of the anthropoid primates.

Humans and chimpanzees are sister taxa whose next equally close relatives are the gorillas, then the orangutans. Humans and chimpanzees share a common ancestor that lived around 5—7 million years ago. This ancestor was neither chimpanzee nor human, and as with whales, the increasingly detailed fossil record of the hominin lineage shows the extensive changes that have taken place since this divergence.

Although the fossil record of chimpanzee ancestors is currently sparse, it can be presumed that a great deal of change characterized the evolutionary history of that branch as well. Cousins are not ancestors, and humans are not descended from chimpanzees. All are absolutely false. This becomes clearer if a few internal nodes are rotated, as in b , which is an equally accurate depiction of primate relationships.

Humans and chimpanzees are more closely related to each other than either is to gorillas, orangutans, or any other living primates. Humans are not descended from chimpanzees any more than chimpanzees are descended from humans; rather, the two share a common ancestor U that lived some 5—7 million years ago and that was neither a human nor a chimpanzee. Old World monkeys share a more recent ancestor with apes Y than either does with New World monkeys Z , which means that apes including humans and Old World monkeys are equally related to New World monkeys.

Monkeys are not ancestral to humans: The two lineages are related as distant cousins, not as grandparents and grandchildren. The notion that other primates should have disappeared now that humans have evolved is based on a false understanding of species formation.

Chimps continue to exist because they are part of a separate branch that formed through cladogenesis when an ancestral population of a species, which was neither chimp nor human, split into independent lineages.

Being confused about the coexistence of humans and chimpanzees is akin to being puzzled by the coexistence of Canada and Australia. Once again, rotating some internal nodes Fig. When viewing unbalanced trees such as those presented as Figs.

First, it is sometimes assumed that this species, although actually a contemporary of all others on the tree, is ancestral to the other lineages or at least is more similar to the root ancestor than any of the other species included in the tree Crisp and Cook Second, this long branch is often taken to imply that no further branching has occurred along this lineage. Figure 15 exposes the fallacy of both interpretations. In this case, humans are accurately included as the outgroup—the so-called basal lineage—to the echinoderms.

It should go without saying that the branch leading from the common ancestor of echinoderms and vertebrates to modern mammals such as humans has not been devoid of additional divergence. In actuality, there have been hundreds of thousands, if not millions, of branching events along that lineage. The corollary of this observation, that humans do not resemble the ancestral echinoderm, should be even more obvious.

A straight line does not mean that no change has occurred. In this case, humans are accurately used as the outgroup to the echinoderms, which includes sea lillies, brittle stars, sea stars, sea cucumbers, and sea urchins. Of course, humans do not resemble the common ancestor of echinoderms, and there has been an enormous amount of branching among vertebrates since the very distant split of these two lineages from their common ancestor.

It must be borne in mind that even if the unbalanced nature of a phylogeny reflects real differences in species diversity which it often does not, as most trees include an incomplete sample of species , the relative diversity of major lineages can change over time, with one being the most diverse now and the other having been so in the past Crisp and Cook Any other interpretation runs the risk of invoking the fallacy of a progressive evolutionary scale.

Moreover, as Crisp and Cook put it,. Once two lineages have separated, each evolves new characters independently of the other and, with time, each will show a mixture of plesiomorphic [inherited largely unchanged from the ancestor] and apomorphic [newly evolved and thus not possessed by the ancestor] character states.

Therefore, extant species in both lineages resemble, to varying degrees, their common ancestor. Consequently, whereas character states can be relatively ancestral plesiomorphic or derived apomorphic , these concepts are nonsensical when applied to whole organisms. However, the overall lineage leading to any modern species is of exactly the same age as that leading to any other modern species with whom an ancestor is shared Fig.

This is a fundamental consequence of the principle of common descent, but there nevertheless can be a tendency to conflate taxon age with lineage age. For example, the group identified as teleost fishes is thought to be older—that is, to have appeared as a recognizable taxonomic group earlier—than mammals. Similarly, the first organisms that would be recognized as rainbow trout Oncorhynchus mykiss probably lived and died before the first individuals that would have been classified as Homo sapiens were born.

However, rainbow trout and humans are contemporary species, meaning that the lineages of which they are currently terminal nodes have been evolving for exactly the same amount of time since their divergence from a distant common ancestor.

The lineages leading to contemporary species have all been evolving for exactly the same amount of time. Rates of morphological change may vary among lineages, but the amount of time that separates two living lineages from their common ancestor does not.

This figure shows the relationships among a sample of vertebrate lineages, all of which have been evolving for exactly the same amount of time, even if some lineages have undergone more change or more branching than others or if some taxonomically identifiable subsets of those lineage e. Note, however, that this is a cladogram rather than an ultrametric tree, such that one cannot assume that any or all of G , H , E , F , C , and B are equal, only that the total amount of time between root and tip is the same along each of the lineages.

Among the common misconceptions identified by Meir et al. Many students interpreted the location of the terminal nodes as indicating time, for example by reading from left to right or from the leftmost tip to the root. In Fig. Neither is correct, as time extends from the root to the terminal nodes, all of which are contemporary. As indicated in Fig.

The number of intervening nodes does not indicate overall relatedness between lineages. The tree in a is the same in topology as the one used in the study of Meir et al. More generally, because the tree is unbalanced, students may tend to consider birds and mammals separated by four internal nodes on this tree, Z , Y , X , and W as more distantly related than turtles and mammals separated by two internal nodes, X and W.

However, this is simply an artifact of the species chosen for inclusion on the tree. All species descended from ancestor X are equally related to kangaroos, with which they all share the same last common ancestor, W. To demonstrate this, b illustrates the same tree with different patterns for each branch, which are then spliced together in c to reveal the identical total distance from the common ancestor W to all of the terminal nodes.

In the study by Meir et al. The important point in calculating relatedness is not the number of intervening nodes along a given branch but the number of shared ancestors.

By contrast, birds share three common ancestors with crocodilians nodes Z, Y, and X but only two with turtles nodes X and W , which makes birds and crocodilians more closely related to one another than either is to turtles. To illustrate the basic notion that all modern species in a tree are equally distant from their common ancestor, one can plot the same phylogeny as in Fig.

The only difference is the number of branching events that occurred within the lineages, whereas the relatedness of the lineages themselves is not affected by this. There is a legitimate debate among professional evolutionary biologists regarding the patterns of species formation, such as whether it occurs comparatively rapidly in a geological sense or is more gradual.

Proponents of the punctuated equilibrium model of speciation argue that species remain largely unchanged morphologically for the duration of their existence, with most physical diversification occurring concomitant with species formation events Eldredge and Gould ; Gould ; Eldredge If punctuated equilibrium were established conclusively to represent the exclusive mode of species formation in a clade and an accurate and complete phylogenetic tree were available for that clade that included all living and extinct species, then one could reasonably interpret the internal nodes as the points at which most morphological divergence took place among species.

As Meir et al. The fact is that one should not assume that an internal node indicates the exact moment again, geologically speaking when particular physical changes came about, any more than one should interpret a long, node-free branch as indicating that no change has occurred.

More accurately, an internal node represents the time at which a formerly cohesive population diverged into two genetically isolated descendant populations, with morphological change possible both at this time and long afterward Baum et al. Finally, one must bear in mind that terminal nodes can also be misinterpreted if the diversity that they sometimes represent is neglected.

For example, the tree in Fig. The important point is that any given node, whether internal or at the tips, represents a diverse assemblage of organisms with a complex evolutionary history. Two points are abundantly clear when it comes to phylogenetic literacy: 1 It is crucial for an understanding of modern evolutionary concepts, and 2 it is insufficiently common.

Misconceptions abound regarding evolutionary trees—sometimes because of, and sometimes creating, incorrect preconceptions about how, evolution operates. Some, along with widespread misunderstandings of evolutionary mechanisms such as natural selection, undoubtedly contribute to the staggeringly low public acceptance of the principle of common descent in North America Alters and Nelson ; Miller et al. The way forward on this issue is unambiguous. Students, members of the public, and other nonspecialists must be better educated about the information that evolutionary trees do and do not convey.

In addition, freely accessible online resources are making it possible for individuals to learn about and interact with evolutionary trees see Appendix. More generally, lessons at the high school and undergraduate level should de-emphasize the technical aspects of phylogeny reconstruction in favor of a focus on the concepts underlying tree thinking.

In this regard, identifying, confronting, and clarifying misconceptions is perhaps the most important strategy. After all, a misconception corrected is a concept better understood. A discussion of phylogenetic methods is well beyond the scope of this article. Introductions to the technical aspects of phylogenetic analysis are provided by Hillis et al.

The quiz used by Meir et al. Students including many graduate students sometimes exhibit confusion regarding the singular and plural forms of terms such as these. Of course, one must not take this analogy too far.

Human offspring have two parents, four grandparents, and so on, whereas each species in a phylogenetic tree is usually considered to have descended from a single parental species through a branching event speciation.

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