“Molecular and anatomical evidence converge on a single tree of life.”
§1 The claim, as taught
Phylogenetic trees built from different kinds of data, such as anatomy and DNA sequences from different genes, generally agree. This independent confirmation is strong evidence that all life is related through a single branching tree of common descent.Representative summary of introductory-text treatments (e.g. Campbell Biology, “Phylogeny and the tree of life”; Futuyma & Kirkpatrick Evolution). PLACEHOLDER: to be replaced with exact quotations and page numbers.
§2 The strongest evidence offered
These are the examples most often cited. We present each one fairly and say what it actually shows.
Five proteins, similar trees
Penny, Foulds and Hendy built trees for eleven mammals from five different proteins. The trees were far more similar to one another than random trees would be.A formal test of common ancestry
Theobald applied a statistical test to universally conserved proteins and reported overwhelming support for one universal common ancestor over several independent origins.More genes, one tree
In eight yeast species, single genes gave more than twenty conflicting trees. Analysing 106 genes together gave one fully resolved tree with maximum support.§3 The grade: two claims hiding in one
The claim as taught combines a modest claim with a sweeping one. Graded separately, they look very different. Hover or tap a rung to see why it got its score. How the grading works →
Trees from different data often agree in broad outline
BStrongly supportedAll the evidence converges on a single universal tree
DExtrapolation§4 What it would take
Here is what would move the claim up a grade. These are not impossible demands. They are the observations the claim itself implies.
- Explain conflict independentlyWhen trees disagree, explain why with independent evidence, not by adding a mechanism after the fact.
- Test the right alternativeShow that common ancestry beats alternatives that also produce similarity (such as common design), not only random, independent origins.
- Reconcile the microbesAccount for the prokaryote record, where horizontal gene transfer blurs or replaces a single branching tree.
§5 Another reading of the same data
Every author cited on the “critical” side of this card is a mainstream evolutionary biologist. None of them doubts evolution. That is what makes their findings worth weighing: they come from inside the field.
W. Ford Doolittle argued in 1999 that horizontal gene transfer among microbes means there may be no single tree at the root of life at all. Dávalos and colleagues wrote that conflict between trees built from anatomy and from molecules, and between different sets of genes, “has become pervasive” as datasets grow. Koonin and Wolf, responding to Theobald's well-publicised test, ran simulations of sequences with no tree-like history at all and found the test still favoured common ancestry. They concluded it could not by itself establish universal common ancestry. Theobald has defended his test, and that exchange is worth reading in full.
None of this proves that common descent is false. It shows that the textbook picture, independent lines of evidence converging on one tree, is a summary that leaves out a great deal of disagreement. Pooling more genes does produce resolved trees. But a method that resolves conflict by outvoting the dissenting data is not the same as data that agree.
Read through a design lens, a patchwork of similarities, where a feature or gene points one way and another feature points another, is what we would expect if a designer reused components across creatures according to function rather than strictly by lineage. That is a hypothesis too, and it makes its own predictions; future cards will set them out.
§6 Reasoning check
The grade weighs the evidence. This check looks at the reasoning: the step from the evidence to the conclusion. The same tests are applied to both readings. How the reasoning check works →
- Possible
“Generally agree” summarises the agreeing results. It leaves out the conflict that Dávalos and colleagues describe as pervasive as datasets grow.
- Possible
Conflicts between trees are explained by gene transfer, incomplete lineage sorting or methodological artefacts. These processes are real; the question is whether each is tested independently in each case, or simply invoked.
- PossibleIn a source: Theobald DL.
Koonin and Wolf argued that the test also favours common ancestry for sequences made similar by convergence. If so, it cannot rule out the main rival it was meant to exclude. Theobald disputes this.
- Possible
A patchwork of similarities is said to be what a designer reusing components would produce. The card itself calls this a hypothesis with its own predictions. Until they are tested, it is a fit, not a test.
§7 Questions to ponder
- If dozens of genes must be pooled to get one tree, what do the individual genes that disagree mean?
- Is “one tree versus independent random origins” the right test? What other models produce similar sequences?
- Some leading evolutionary biologists now speak of a “forest” rather than a tree for microbes. What does that do to the textbook picture?
§8 Sources
- Penny D, Foulds LR, Hendy MD. “Testing the theory of evolution by comparing phylogenetic trees constructed from five different protein sequences.” Nature 297:197–200 (1982).B
- Theobald DL. “A formal test of the theory of universal common ancestry.” Nature 465:219–222 (2010).C
- Theobald DL. “On universal common ancestry, sequence similarity, and phylogenetic structure: the sins of P-values and the virtues of Bayesian evidence.” Biology Direct 6:60 (2011).C
- Rokas A, Williams BL, King N, Carroll SB. “Genome-scale approaches to resolving incongruence in molecular phylogenies.” Nature 425:798–804 (2003).B
- Doolittle WF. “Phylogenetic classification and the universal tree.” Science 284:2124–2128 (1999).B
- Dávalos LM, Cirranello AL, Geisler JH, Simmons NB. “Understanding phylogenetic incongruence: lessons from phyllostomid bats.” Biological Reviews 87:991–1024 (2012).B
- Koonin EV, Wolf YI. “The common ancestry of life.” Biology Direct 5:64 (2010).B
- Puigbò P, Wolf YI, Koonin EV. “Search for a ‘Tree of Life’ in the thicket of the phylogenetic forest.” Journal of Biology 8:59 (2009).C
Source grades are provisional. In production each links to a full published evaluation that readers can challenge.