Chain of being
Before Darwin’s revolutionizing work on the evolution of species (1859), (natural) philosophers tried to structure the diversity of life. These earliest concepts of natural systems usually followed the idea of the “Great Chain of Being” or “Scala Naturae” with a vertical representation of the system of species. At the lower part, “simpler” species are depicted; following the order up, the complexity of presented organisms increases, typically climaxing with humans or God at the top of the natural order (Gregory 2008; Matuk 2007). In the historical context, these representations were based on the idea that life was created by a divine being in an event of full creation. Therefore, all of the different forms of life were seen as having the same age and as not changing over time (Matuk 2007; Mogie 2000). This form of presentation is highly questionable from a modern perspective as it implies the problematic concept that evolutionary development culminates in a small number of advanced species and thus, it is a teleological perspective (Catley et al. 2012). Furthermore, it implies that different species are more or less evolved or complex. Today, no scientifically justifiable basis can be applied to rank species with regards to their complexity or their “rank” in the natural system (e.g., Gould 1996). Nevertheless, the general idea of the Chain of Being is not only present today, but is actually a frequently reported and widespread misconception (Gregory 2008; Kummer et al. 2016; Nee 2005; Philips et al. 2011).
A more recent description of thinking in line with the Chain of Being is what has been termed “developmental thinking” (O’Hara 1997), where evolution is seen as stories of individual development while investigating the developmental history of one species from a root to the present without taking into account the branching nature of lineages.
Considering evolutionary processes and concepts, representations in the linear style of the chain of being (or developmental thinking) do not show relevant concepts such as: the splitting of populations leading to speciation, extinctions, the relative relationships of different species, new evolutionary traits (apomorphies), or any other indicators of a modern understanding of evolution.
Pop-cultural influence on teleological reasoning
Probably the most prominent iconographic depiction of the concept of evolution in popular culture follows the idea of the chain of being. The typical form of this well-known iconography (see Fig. 1) shows a linear progression from left to right in varying compositions. Sometimes starting from unicellular life, with different steps leading to modern humans on the left, but most prominently, from a crooked ape on the left, to an upright human on the right (Baum and Smith 2013; Clark 2009; Gould 1995).
This kind of iconography can convey different ideas not supporting the concepts of evolution. The direct line from a “simple” or “primitive” ape to the upright, developed human could imply that evolutionary processes strive to increase the complexity of living beings and that humans are the pinnacle of the living world. This notion could be further supported by the typical increase in heights of the presented organisms from left to right. Furthermore, one could interpret this image to support the idea that humans developed from apes but are not apes themselves. Thus, this kind of image represents the idea of the chain of being as a valid depiction of evolutionary processes and thus fosters teleological thinking (Werth 2012).
Complexity idea
As biologists explored the diversity of life and tried to organize living things into different groups, they expanded the concept of the chain of being. Instead of a linear progression of complexity, branching depictions were used, often in the form of actual tree-like pictures. In these early organizational works, classification was mainly based on morphological structures and different groups were typically seen as more or less “primitive” or “complex.” In these tree-like-depictions, “lesser” groups were presented closer to the stem of the tree, whereas “highly complex” organisms or groups, including men, were presented on the branches at the top of the tree (Matuk 2007; Mogie 2000). Evolutionary trees in this form of the tree of life are still used in educational literature and only in a minority of the cases are these diagrams put in an appropriate historical context (Catley and Novick 2008).
The idea that different species are more or less evolved, or the notion of “higher” or “lower” species is problematic as it nourishes the idea that evolution pursues a rise in complexity over the course of time as a type of purpose, thus being deeply teleological (Kummer et al. 2016). In the context of tree-reading, it can be seen as the most problematic form of teleological interpretations, as multiple common misconceptions in reading evolutionary trees are heavily based on this idea (Baum et al. 2005; Dees et al. 2014; Gregory 2008; Mogie 2000; Omland et al. 2008). In the following text, we will call this concept the “complexity idea”.
The complexity idea can be an indication of teleological thinking in evolution, as it backs the concept that evolutionary processes are “aimed” at inevitably bringing up more complex forms resulting in modern humans (Mogie 2000). As it is the core of many different (teleological) misconceptions, with regards to evolution in general and depictions of evolutionary processes in specific, we lay out different factors promoting the complexity idea in the context of evolutionary biology below.
Reading across the tips
A common misconception in reading evolutionary trees based on the complexity idea is called “reading across the tips” (Gregory 2008). It describes the concept that students tend to ignore the branching pattern of a given tree and rather read the presented organisms in the order they are presented, as a direct line of rising complexity, depending on the orientation of the tree, from left to right or from bottom up (Blacquiere and Hoese 2016; Gregory 2008; Kummer et al. 2016). This leads to students largely ignoring the branching pattern of the tree and thus drawing wrong inferences about the information presented. As the organisms are seen as being arranged in a continuum of rising complexity, this misconception reveals the notion that species can be arranged according to their “higher” or “lower” development.
Order of teaching evolution
The complexity idea might be unintentionally supported by many scientists and educators by the way they present information. Multiple books about evolution, some by well-known scientists, are structured in support of the idea of increasing complexity by following an arrangement similar to the Chain of Being (Nee 2005). Often, they begin with describing the origin of life and the first living organisms and continue with events like the first eukaryotic cell, multicellularity, leaving the water, first mammals, and in the end, human beings. This common order of how to present evolutionary history is suited to nourish the idea that evolution works on increasing the complexity of life and that humans are the pinnacle of biological life (Nee 2005). By sustaining the notion that evolution is a process producing beings that are ever more complex and that one can follow a “complexity ladder” through the domains of life, the interpretation of evolution being a process with the purpose to spawn ever more complex beings might be enticing. Teaching this way could support the complexity idea and associated teleological views about evolution.
Teleological wording
The way an educator chooses his or her wording can have a considerable influence on the way students understand a topic. This is especially true in the case of evolutionary biology, as many professional biologists and educational textbooks regularly use teleological formulations, despite not intending to teach teleological interpretations (González Galli and Meinardi 2011; Hanke 2004; Ruse 2009; Werth 2012). For example, one could say that the heart evolved to pump blood, and the underlying meaning of this statement is that the heart underwent an evolutionary process, adapting and shaping it, which resulted in a better-adapted morphological structure. The statement is not meant in a way that the heart was originally designed as a blood-pump. Another example is the statement that a species lives in flocks to defend against predators. Here, the reason of living in flocks is an evolutionary adaptation and not the idea of an individual organism that living in a group could be a good way to repel predators. Typically, educators assume that their students understand their teleological formulations as metaphors and do not want to foster teleological reasoning. This raises the risk that students might become used to teleological wording and adopt it without regarding the metaphorical nature or might directly adopt teleological reasoning (Kelemen 2012; Werth 2012). Furthermore, textbooks often focus on the functions of biological systems or processes without shedding much light on the causes. This can lead to students analyzing biological concepts from a teleological perspective (Abrams and Southerland 2001). The issue of teleological wording is not only present in educational textbooks or oral presentations, but also in peer-reviewed publications, especially with the use of the classification into “higher” and “lower” species, and therefore, in the form of the complexity idea.
Effects of representational elements and styles
Darwin’s publication of the concept of evolution as descent with modification introduced trees representing the order of life: Lines did not represent some similarity between different groups any more, but a direct relationship by descent. These diagrams came in various different forms, and conventions on how to best present evolutionary relatedness had yet to be found (Pietsch 2012). In modern phylogenetics, three forms of evolutionary tree diagrams are most prevalent: rectangular trees (also called tree diagrams), diagonal trees (also called ladder diagrams or trees), and circular trees (Fig. 2), although the latter are rather scarcely used (Catley and Novick 2008; MacDonald and Wiley 2012). Different representational styles can lead to different interpretations of a given tree, especially with novice learners (Catley et al. 2012), although all styles carry the same informational value. Furthermore, the absence or presence of certain elements of a tree can change the difficulty of reading the tree or influence the way it is read. In the following, we will present different aspects of the representational style of evolutionary trees that can foster teleological ideas while working with them.
Diagonal trees
Diagonal trees can especially support misconceptions in reading evolutionary trees. For example, students tend to interpret diagonal trees by seeing long lines as an entity, determined by its terminal taxon (see taxon A or F in Tree b in Fig. 2). This interpretation leads to interpreting branching events as other groups developing “away from the main branch,” and thus, implying the idea of a main line of development, usually culminating in humans, with different other, inferior developmental lines (Gregory 2008; Halverson and Friedrichsen 2013; Omland 2014). This interpretation is especially prominent as students follow the Gestalt principle of good continuation. Depending on which species are presented at the end of one long, continuous line, the representation can support teleological interpretations. For example, if humans are presented at the end of the uninterrupted long line at one side of the tree, it could be seen as a directed evolutionary process culminating in the development of humans, with other taxa as mere side branches of this development.
Anagenetic diagrams
Anagenetic diagrams are largely used when representing the evolutionary history of modern humans and in the case of the evolutionary history of the horse (Catley and Novick 2008). These diagrams typically put different species into perspective based on the fossil record, and at least two different species are linked directly and successively along a single branch of the tree without a bifurcation event (Catley and Novick 2008). This form of presentation can easily be interpreted as the representation of anagenetic processes, one species turning into another, without a branching event occurring (Philips et al. 2011). The idea that evolutionary processes are transformation processes, turning one species into another, is seen as being linked to the teleological interpretation of the presented data (Novick et al. 2014; Philips et al. 2011; Scott 2010). Here again, we can find the complexity idea, as one species seems to develop into a new and “higher developed” species, leading to extant species we know today (Novick et al. 2014).
Orientation of the tree
Besides the style of the tree, the orientation of the trees can have an influence on the way learners interpret the presented information. If extant species in a tree are presented along a vertical axis (so the flow of time is presented horizontally), novice learners tend to interpret the tree in a teleological way by ascribing species at the top to be highly developed. The interpretation with the tree rotated and extant species presented along a horizontal line is less common (Philips et al. 2011).
Lines represent no change
Many learners possess the misconception that lines in an evolutionary tree represent no alterations and only nodes represent evolutionary changes; therefore, this can be seen as a concept similar to the idea of main branches, described in the context of diagonal trees discussed above. The concept that lines represent no change can be interpreted as teleological fallacy, as it can imply that evolutionary changes occur for the reason of speciation (Kummer et al. 2016). A consequence of this idea can be the well reported misconception that students attempt to evaluate relative relations of species by counting nodes along the lines between different species (Gregory 2008; Meir et al. 2007).
Unbalanced trees
The number and arrangement of taxa in a given tree can have a major influence on the way learners read a tree. When presented with a tree where different clades or parts of the tree feature very unequal numbers of presented species, students tend to interpret the clade with less groups as a more primitive one (Gregory 2008; Meisel 2010). Just as the general idea of different species being differently “developed” or more or less primitive is not supported by evolutionary reasoning, this concept is also flawed. As every evolutionary tree shows a selection of species or groups, typically any branch can be provided with a number of additional branching events to tip the proportion of the number of presented species in the interpreted clades.
Age of groups
A form of evaluating relative relationships in a tree that is tightly linked to the complexity idea is the notion that different extant groups in a tree are older or younger; this implies that the older group is less evolved or more basal than the younger group. The idea of this concept is that different taxonomic groups emerged at different times throughout the history of the earth; for example, the first organisms later classified as birds lived before the first organisms later classified as humans (see Fig. 3). Novices in reading evolutionary trees tend to attribute groups with early branching events as being less evolved because of the time of divergence in a presented tree (Gregory 2008).
In a given tree, all extant species are the same age. The time from the root of the tree to the tips is the same for all presented species (extinct species not factored in), and therefore, their total time of evolution is the same (Baum and Smith 2013). As trees presented in publications can only show parts of the complete evolutionary tree of all living beings, the number of branching events along a given lineage cannot be interpreted meaningfully. If a tree is presented with a scaled time axis, branching events on different lineages can be set into relations by stating that one branching event occurred before the other. Nevertheless, this does not justify any interpretation of less or more advanced clades in a tree (Baum and Smith 2013; Mogie 2000), which could be the basis for teleological interpretations, following the complexity idea.
Interpreting focal taxa
The positioning of humans in evolutionary trees is a common feature in teaching tree-reading (Sandvik 2009). Presenting humans (or the taxonomic group containing humans, like mammals or vertebrates) in a tree in a position at the outer side of the branching pattern can support interpretations of the species being presented in an ascending order of complexity (complexity idea). Such a prominent position can also corroborate anthropocentric tendencies and the interpretation that evolutionary processes culminated in modern humans (Sandvik 2009; Philips et al. 2011). Both interpretations can be seen as a feature of teleological reasoning (Meisel 2010; O’Hara 1997; Philips et al. 2011). Often, humans are presented in a prominent position in evolutionary trees, at the edge of the diagram, depending upon the general orientation at the far right or the top of the tree. By positioning humans at the edge of the tree diagram, their position implicitly places focus on them, implying that humans deserve focus or are a type of endpoint. Interpreting a tree this way can lead to the conclusion that evolutionary processes took place in order to lead to humans as a special group of organisms, in an extreme form, to humans as the pinnacle of life on earth (Meisel 2010; Philips et al. 2011; Sandvik 2009). The same principles hold true if the tree shows another focal taxon instead of humans.
Different levels
Another factor influencing the perception of different species as “higher” or “lower,” is whether extant species in an evolutionary tree are presented at the same level in the diagram. In a cladogram, the length of the branches has no informational value, and therefore, all extant species are typically arranged on a line orthogonal to the direction of the flow of time and only extinct species are presented at lines ending earlier (Catley and Novick 2008). In special trees such as phylograms or chronograms, the length of the branches represent evolutionary changes, mutation rates, or time. However, in modern educational literature, extant species are frequently presented in diagrams on the same level without the branch length showing any metric logic (Catley and Novick 2008). Diverting from the design that extant species are presented at one level can lead to students interpreting the tree in a way that different species are seen as being less evolved or more primitive, which could lead to teleological interpretations of the tree. If the species are presented at different levels on purpose, the design should be clearly highlighted and explained in the diagram.
Evolutionary trees are a highly context-specific form of representation in evolutionary biology, and unfortunately, numerous teleological pitfalls exist in constructing or presenting them in a learning context. In a modern learning context, we can still find interpretations and concepts based on scientific interpretations that are many centuries old. In fact, one of the most fundamental hurdles in reading evolutionary trees is that learners tend to attribute different complexities to presented organisms and see them as more or less developed. In an extreme form, learners think of a linear order for the complexity of species and use this as a basis for evolutionary explanations. This hurdle—the complexity idea—can be corroborated in the context of evolutionary trees by different teaching practices and diagrammatic properties. Furthermore, an evolutionary tree is comprised of numerous elements, whose absence or presence can influence teleological interpretations.