When Charles Darwin published On the Origin of Species in 1859, he included just one figure—the now-famous “Diagram of Divergence of Taxa.” At first glance, it looks like a branching tree of life: lines splitting and diverging, tracing common ancestry. Many have treated it as a simple visual of common descent.
Juan L. Bouzat’s 2014 article in The Quarterly Review of Biology argues something bolder: Darwin’s diagram is not merely a representation of evolutionary pattern but also a causal model—one that places natural selection at the heart of the diversification process. Bouzat shows that for Darwin, the diagram was a conceptual tool linking mechanism (selection) with pattern (common descent), embedding it into his overarching “one long argument.”
Main Argument of the Paper
Bouzat’s thesis is that Darwin’s Tree Diagram:
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Unifies natural selection and common descent into one explanatory model, rather than treating them as logically independent processes.
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Embodies Darwin’s causal reasoning under the 19th-century scientific principle of vera causa—requiring a cause to be shown to exist, to be competent to produce the effect, and to be responsible for the phenomenon.
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Functions as a hypothetico-deductive model, capable of generating predictions testable with geological, geographical, and taxonomic evidence.
This reframing challenges the modern textbook habit of presenting “common descent” and “natural selection” as two separate pillars. Bouzat insists that for Darwin, selection was the engine that drove the branching—without it, common descent would be a static genealogy without an explanation.
Key Analytical Points
1. The Vera Causa Framework
Bouzat uses M.J.S. Hodge’s reading of Darwin:
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Existence: Darwin first establishes natural selection as a real process (Chapters I–III of Origin).
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Competence: In Chapter IV, he shows it can create new, well-marked species.
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Responsibility: In later chapters, he connects it to actual patterns in nature—fossils, biogeography, and classification.
The Diagram visually integrates these steps: divergence, extinction, and gradual change all emerge from selection.
2. Why the Diagram is a Causal Model
Bouzat dissects the elements:
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Dotted lines = incipient varieties under selection.
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Horizontal “time” lines = generational accumulation of change.
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Branching fan = divergence in character, favoring survival.
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Extinctions = natural pruning of less fit forms.
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Hierarchical groupings = taxonomic patterns as a byproduct of descent with modification.
Rather than just showing that species are related, the figure explains why they become different—by linking small variations to long-term diversification through selection.
Below is a stylized reproduction of Darwin’s original figure with Bouzat’s causal insights marked:
3. Predictive Power
Bouzat stresses the diagram’s role as a predictive model. From it, Darwin could forecast:
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Gradual, not abrupt, morphological change.
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Variable rates of change among lineages.
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Extinction as a pervasive, selection-driven process.
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Geographic clustering of related species.
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Nested taxonomic hierarchies as natural outcomes of branching divergence.
These predictions were then checked against:
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Fossil record patterns (gradualism, succession, extinction).
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Geographic distribution (regional affinities, island endemism).
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Morphological affinities (hierarchical classification, unity of type).
4. Historical Positioning
Bouzat contrasts Darwin’s contribution with:
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Pre-Darwin tree diagrams (Buffon, Lamarck, Wallace) which depicted relatedness but lacked a causal mechanism.
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Wallace’s 1855 paper—which had the branching-tree analogy but no explanation for divergence.
Darwin’s originality lay in marrying the tree pattern to a generative process.
Inferences and Broader Implications
Bouzat’s analysis suggests:
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Darwin’s scientific method was not purely inductive (“Baconian”), as he sometimes claimed, but a blend of induction and deduction.
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The Diagram can be seen as a working hypothesis—an early systems model of evolution.
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Viewing the figure only as a static “tree of life” misses its role in Darwin’s argumentative strategy.
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Modern portrayals that separate common descent and selection may obscure Darwin’s own framing of the theory.
Critical Reflections
Bouzat’s reading is persuasive, but it also invites some questions:
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Did Darwin always see natural selection as the sole driver of divergence, or did he sometimes allow for other mechanisms (sexual selection, environmental pressures without selection)?
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By focusing on causal integration, does Bouzat underplay the extent to which common descent could stand as an accepted idea independently of selection (as Wallace, Lamarck, and others entertained)?
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Modern evolutionary theory includes mechanisms Darwin didn’t foresee—how might the Diagram be updated today without losing its causal elegance?
Conclusion
Juan L. Bouzat’s paper revitalizes our understanding of Darwin’s lone figure in Origin of Species. The Diagram of Divergence of Taxa, he argues, is not a decorative aside—it’s the conceptual heart of Darwin’s theory, uniting process and pattern, and serving as a predictive causal model grounded in natural selection.
By restoring this integrated view, Bouzat not only clarifies Darwin’s original intent but also reminds us that the visual models we use in science are not just summaries of data—they are arguments in themselves
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