“Long stasis in fossil species is consistent with gradual evolution.”
§1 The claim, as taught
Many species appear in the fossil record, persist largely unchanged for long periods, and then disappear. The punctuated equilibrium model explains this pattern: change happens rapidly during speciation, often in small populations, followed by long periods of stability.Representative summary of introductory-text treatments (e.g. Campbell Biology, “Speciation rates”; 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.
Stasis is the norm
Fitting more than 250 sequences of fossil traits to three models, Hunt found directional change best supported in only about 5% of them. The rest split almost evenly between random wandering and stasis.Punctuated equilibria
Eldredge and Gould proposed that most change happens quickly, in small isolated populations at speciation, and so is rarely caught in the rocks. Established species then stay stable.“Living fossils”
The coelacanth, known only from fossils, was found alive off South Africa in 1938. Like other “living fossils,” it closely resembles relatives known from very old rocks.Stabilising selection
Estes and Arnold showed that models in which selection holds traits near a slowly moving optimum can reproduce the observed pattern of little net change across many timescales.§3 The grade: two claims hiding in one
Again, the pattern and the theory's account of it are graded separately. Hover or tap a rung to see why it got its score. How the grading works →
Most fossil species show little directional change over their history
BStrongly supportedStasis is consistent with gradual Darwinian evolution
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.
- Say what would count against itState in advance how much stasis, for how long, would be inconsistent with the theory.
- Catch the changeDocument, in living or fossil populations, the rapid change between stable species that punctuated equilibria require, at the scale seen between fossil species.
- Stability through upheavalExplain why species stay the same through major environmental changes, not only in constant conditions.
§5 Another reading of the same data
Stasis was not a prediction of Darwinian gradualism; it was a problem for it. Darwin expected slow, steady change, and paleontologists long assumed that the record would show it once enough was collected. When Eldredge and Gould proposed punctuated equilibria in 1972, they were explaining a pattern paleontologists already knew well.
Gould was candid about it. In a 1977 essay he wrote that “the extreme rarity of transitional forms in the fossil record persists as the trade secret of paleontology.” (Quotation taken from secondary transcriptions; to be checked against the original issue.) Punctuated equilibria moved the change out of view: into small, short-lived populations that would rarely be fossilised.
That is a coherent idea. But it means that the stasis we can see and the rapid change we cannot are both counted in the theory's favour. A theory that can absorb either outcome is not being tested by this evidence.
Read through a design lens, long stability is what we would expect of created kinds with built-in capacity for variation: species that adapt within limits, and stay recognisably themselves. The fossil record's most common pattern fits that expectation without additional explanation.
§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
Punctuated equilibria were proposed to explain a pattern gradualism did not predict, by placing change where it would rarely be fossilised. Whether this is ad hoc depends on whether the model made new predictions that were confirmed, which its defenders argue it did.
- Possible
“Created kinds with built-in capacity for variation” explains stasis well, but a kind needs a definition that could be checked against the record. Without one, it too could absorb any pattern.
§7 Questions to ponder
- If both change and the absence of change count as evidence for the theory, what would count against it?
- Gould described the rarity of transitional forms as “the trade secret of paleontology.” Was it a secret, or an open question?
- What would a world of created, adaptable kinds predict about stasis?
§8 Sources
- Eldredge N, Gould SJ. “Punctuated equilibria: an alternative to phyletic gradualism.” In Schopf TJM (ed.), Models in Paleobiology, pp. 82–115. Freeman, Cooper (1972).C
- Gould SJ, Eldredge N. “Punctuated equilibria: the tempo and mode of evolution reconsidered.” Paleobiology 3:115–151 (1977).C
- Hunt G. “The relative importance of directional change, random walks, and stasis in the evolution of fossil lineages.” PNAS 104:18404–18408 (2007).B
- Estes S, Arnold SJ. “Resolving the paradox of stasis: models with stabilizing selection explain evolutionary divergence on all timescales.” American Naturalist 169:227–244 (2007).C
- Smith JLB. “A living fish of Mesozoic type.” Nature 143:455–456 (1939).B
- Gould SJ. “Evolution's erratic pace.” Natural History 86(5) (1977).C
Source grades are provisional. In production each links to a full published evaluation that readers can challenge.