Evolution is the central organizing principle of biology. One of the most fundamental concepts of the theory of evolution since it was developed by Charles Darwin is that species share a common origin —commonality of descent from a single ancestor—and have subsequently diverged through time. To many, evolutionary trees are the embodiment of the theory, of the historical narrative of life. They summarize the evolution of life: Darwin’s principle of the non-fixity of species, common descent, and diversification by a branching process from ancestral forms to modern ones. Thus, trees are the icons of evolution. It is almost impossible to think of the ancestry and relationships of living beings without them.
From the first decades of the twentieth century, evolutionary trees have been used in textbooks and museum displays to illustrate the process of evolution. These diagrams and images represent the emergence of species from common ancestors and their subsequent diversification or extinction. Interestingly enough, since the early twentieth century, publications for general audiences have tended to use literal representations of trees to represent human or animal evolution, as opposed to scientific publications, where the evolutionary history of organisms has normally been represented by line diagrams. Thus, in popular visual culture,Footnote 1 the pictures of evolutionary trees are usually filled with images of animals and plants to show how each branch tip represents a group of organisms.
Amazing Varieties
Visual representations of evolutionary trees can be amazingly varied. Regarding their form, they can differ in shape, design, and orientation. The crucial evolutionary information of a tree is given by the branching order. As long as it remains the same, diagonal, curved, straight, or circular lines can be used (Gregory 2008). Trees may be oriented up or down, to the left or to the right, depending on the position of the root or point of origin. They also can be symmetrical (or balanced) if the number of species at each side of the nodes is the same, or asymmetrical (unbalanced) when not all of the groups represented contain the same number of species. For example, placental mammals are richer in species than monotremes (Crisp and Cook 2005).
Concerning the representation of organisms, evolutionary trees may represent the hypothetical relationships among the entire biodiversity or among members of a kingdom such as the animals. They can be about phyla such as the mollusks or a class such as that of the cartilaginous fishes or chondrichthyes. Orders like primates, families like composite plants, and molecules such as DNA or hemoglobin can also be represented (when using molecular data, scientists use modelling approaches like maximum likelihood and Bayesian methods (Tassy 2011)).
Regarding the method used to build them and the kind of information conveyed, evolutionary trees can be cladograms when composed by species and speciation events and constructed based on cladistic analysis. They can be phylograms when presenting non-equal branch lengths to convey the inferred degree of relatedness among the groups presented. Or they can belong to other types of phylogenies (non-cladistic) if they include different components of the evolutionary history of organisms such as estimates of diversity, enrichment or depletion of species, absolute time, biogeographical relations, and the direction of evolutionary change, in addition to genealogy. These factors provide richer information about organisms than a cladogram sensu stricto. Museums extensively use these representations.
From all this startling variety of representations of the evolution of lineages, we are mostly concerned with “traditional trees of life,” mainly because many of these images are far from Darwinian (moreover, the shape of a real “tree” can unwittingly misrepresent evolutionary relationships) as well as modern images whose design might inadvertently lead to non-scientific notions.
Where Did the Image of the “Tree of Life” Come From, How Did It Become so Widespread, and What Kind of Notions Does It Convey?
One of the most fundamental concepts of Charles Darwin’s argument was that species share a common origin from a single ancestor and have subsequently diverged through time. In the fourth chapter of the Origin, Darwin included a picture that he used as an argument and as an evidence of this notion. The resulting line-diagram is an abstract and irregular design, not a tree in a literal sense and not described by Darwin as such. In his subsequent books, Darwin never included a depiction of this kind. Then, why is it that, for popular accounts of evolution, the chief image in use from late nineteenth century to represent the process is that of a tree?
Darwin came to use the metaphor of a great tree to illustrate his notion of descent with modification as early as 1837 in his Notebook “B”. There, he describes an abstract tree with a main trunk to represent links of ancestry for every species and side branches which ramify further or die out to represent species divergence and extinction. Therefore, to many, the “tree of life” has its roots in one of Charles Darwin’s diagrams and thus represents one of his legacies. “Charles Darwin sketched the branches of a tree in one of his notebooks to illustrate the idea that all life forms branched out from a common ancestry. Since then, the tree of life image has persisted as an aid to understanding evolution”Footnote 2.
Yet history tells us that Darwin’s image was not the one in everybody’s mind until the middle of the twentieth century, as it was only in 1960 that his personal notes began to be printed.
It was Ernst Haeckel, a German naturalist, who published in 1874—in his book Anthropogenie—the image that became iconic of evolutionary theory in the popular realm (Fig. 1). Haeckel’s now famous motif—Systematischer Stammbaum des Menschen—explores the evolutionary history of man. The most fascinating aspect of this image is that it is a real tree—a twisted European Oak with bark and leaves. Popularly, it is assumed to represent an upward journey of life from the most primitive beings at the bottom to the most advanced at the top—man at the summit of all life—even though, to Haeckel, it was a direct chain of human ancestors.
Thus, Haeckel’s Stammbaum des Menschen is the source image for most of the subsequent representations of evolutionary trees in the popular sphere. Not Darwin’s line-diagram of the Origin—in which no single species at the end of the lines can count as a clear culmination—and especially not Darwin’s 1837 ink sketch—which many scholars have described as Darwin’s first evolutionary tree.
In the popular realm, Haeckel’s tree took over Darwin’s 1859 descent diagram. But it was taken out of context. It was assumed to represent a complete evolutionary tree—therefore the popular term “Tree of life.” From this approach, Haeckel’s Stammbaum des Menschen is clearly embedded with non-Darwinian accounts of evolution and also prejudices assumed to be inspired by the prevailing social and philosophical beliefs. The image of a real tree mightily growing upwards offered a view of evolution as a progressive and goal-oriented process that culminates in the appearance of human kind. Moreover, the conflation of religious connotations powerfully embedded in the tree image led to interpretations that modern living beings can be less and more evolved and that the whole process was intended to “produce” human beings.
In the early twentieth century, due to the continuing effort to promote the idea of evolution in layman, evolutionary trees inspired by Haeckel’s started to appear in works aimed at a more popular audience, and their inclusion eventually became standard in biology textbooks (Figs. 2 and 3). Some natural history museums also used them as a new way to explain the organic world through the lens of evolution (Fig. 4). Thus, at least in their beginning, evolutionary trees in the popular sphere were not just embodiments of interpretations of the theory of evolution. They enfolded deeply embedded Western preconceptions of how the world was thought to be according to religious views and other pre-evolutionary views of organic relationships.
By the turn of the century, natural history museums witnessed great changes. For many, a crucial event was the celebration of Darwin’s bicentenary in 2009. Several brand new exhibits of evolution were opened, and outdated ones were overhauled. Today, evolutionary theory is at the heart of most of them. Thus, scientific illustrators and museum displays are making real efforts to change the public perception of evolution. As the metaphor of branching and rebranching to represent the complexity of the natural world proved to be incredibly fecund for practicing biologists and for the general public, in order to distance evolutionary diagrams based on scientific evidence from traditional “tree of life” connotations, the use of cladograms became a common practice from the decade of 1990, and alternative geometries are beginning to appear.
However, even though today it is difficult to find a traditional “tree of life” (inspired by Haeckel’s) in natural history museums, they are still present. Also, the design of many evolutionary trees is still prone to confuse the audience. Below, we revise the most fundamental misunderstandings about trees. Some of them are caused by a lack of phylogenetic literacy, but others are a direct consequence of the design of evolutionary trees (shape, orientation, and organisms presented). Later, we will discuss our findings in five important Western natural history museums.
Problems of Interpretation
Being the direct representation of evolution, evolutionary trees or phylogenies are a central element of modern biology. Tree thinking (the ability to understand evolution as a process of branching and rebranching) among the public in general, however, is not as accepted and widespread as one might expect. “Tree thinking is very much an acquired ability which needs extensive training” (Sandvik 2008), and even graduate students and professors of biology find it difficult to correctly interpret a simple tree drawing (Baum et al. 2005; Bishop and Anderson 1986; Diamond and Scotchmoor 2006; Meir et al. 2007a). There are at least three reasons for this absence of tree thinking: a general misunderstanding of the theory of evolution; a lack of familiarity with phylogenies; and a visual evolutionary culture which sustains an eminently progressive discourse full of prejudices that leads to wrong interpretations of the process (i.e., the widespread presence of traditional “trees of life”).
According to many authors, but summarized by Gregory (Gregory 2008), the following ten misunderstandings about trees are the most significant and pervasive among non-specialists:
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1.
Evolution is a goal-oriented process and a linear transformation from less to more evolved organisms: Homo sapiens. This means that lay audiences tend to read trees as if they were a march of progress depicting less evolved organisms either at the bottom or at the far left side and more evolved organisms at the top/far right side.
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2.
There is a main line in evolution. Progressionist interpretations of evolution are hard to eradicate, and even the diagram of a tree is normally read as representing a main story (normally the one of human being or primates) with all other lineages branching off from this line sometime in the past. This problem arises specially in unbalanced and ladderized trees.
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3.
Reading across the tips. No matter how meticulous and elaborate a ramified diagram is, most people tend only to look at the tips and derive wrong conclusions on what is represented: The group of species at the far left is thought to be more primitive and closely related to the next at its right and so on.
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4.
Similarity versus relatedness. Non-specialists normally do not identify the clades depicted in trees and pay attention only at terminal nodes to conclude that trees convey relations of similarity instead of evolutionary relationships.
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5.
Sibling versus ancestor. The ancestor of two modern groups was or must have been very similar to one of the modern groups.
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6.
Long branches imply no change or an ancestral species. Many people tend to misinterpret the length of branches as if they mean the extent of change. Long branches therefore are thought to represent “ancestral” species or species similar to the ancestor who did not diversify or “kept evolving.”
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7.
Different lineage ages for modern species. To evolutionary biologists, any modern species have been evolving for exactly the same amount of time since their divergence from a distant common ancestor. However, there is a tendency to interpret that there are lineages “more” or “less” evolved than others.
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8.
Backwards time axes. For lay audiences, it is common to interpret the time in the wrong way, whether from left or right (therefore assuming species at the far left to be less evolved or primitive) or from the leftmost tip to the root.
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9.
More intervening nodes equals more distantly related. Some people think the higher the number of internal nodes between groups of species the more distantly related they are instead of considering the number of shared ancestors.
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10.
Change only at nodes. Nodes represent an event of speciation but change can occur before, during, and after that particular time.