Egg Baskets & Rising Tides — Chapter 1: Bridging the Goldschmidt Break
The slow changes of a million years of evolutionary time must yield to the rapidly fluctuating requirements of ecological time.
Stephen Jay Gould, 1977
Gould’s Hierarchy
For any student of paleontology in my time, you’d have to be living in a hole if you’d never heard of Stephen Jay Gould. As evolutionary biologist John Maynard-Smith (1984) famously invoked, Gould helped bring paleontology to the “high table” of evolutionary theory. A major contributor to the paleobiology revolution (Sepkoski, 2012), Gould’s academic and public involvement in science in the latter half of the 20th Century helped set the tone for paleontology ever since.
In many ways, it all begins with punctuated equilibria, an idea first formalized by Gould’s classmate and colleague, Niles Eldredge (Eldredge, 1971; Eldredge and Gould, 1972). Punctuated equilibria is the abrupt appearance of fossil taxa in the rock record whose morphology remains stable thereafter. Eldredge and Gould called this morphological stability stasis.
They contrasted punctuated equilibria with “phyletic gradualism,” the formation of new species “from the slow and steady transformation of entire populations” (p. 82; Figure 1.1).
Gould saw punctuated equilibria as foundational for paleontology’s contribution to evolutionary theory. Two consequences fall out of it. Both rely on the prevalence of stasis in the rock record.
1) Because of the abrupt appearance of new lineages relative to their duration in stasis, most evolution happens during lineage formation.
2) If lineages remain static, they may be susceptible to sorting. In turn, sorting can lead to trends. In Homo, brains get bigger. In the life cycle of land plants, the prevalence of seed-to-adult phase surpasses the pollen-to-seed phase.
Gould spent many hours advocating for ideas and frameworks born from fossils, much of these efforts wrested from these two arguments. One effort resulted in his “Tiers of Time” (Gould, 1980; 1985; 2002). This is Gould’s attempt to synthesize paleontological ideas with biological evolution. Figure 1.2 paints a picture.
Gould made the first tier the domain of microevolution. These are processes like mutation, natural selection, genetic drift, gene flow, and so on. This tier concerns “variation within populations” (Gould, 1980, p. 119).
The second tier is where speciation happens. Gould considered this tier the domain of macroevolution. Punctuated equilibria reigns supreme. A different kind of evolution happens at this level. Whatever processes happen here, the second tier “undoes” the changes occurring at the first (Gould, 2002, p. 1329). “Macroevolution is decoupled from microevolution…” (Stanley, 1975, p. 648).
Finally, the third tier is the domain of mass extinction and the reorganization of whole biotas. For Gould, this was, I think, the leading edge, the exciting contribution paleontology really made to evolutionary biology.
With mass extinctions, a lost biota is replaced by another. Before mammals were trotting around the grasslands, dinosaurs were racing through the cycads. And with estimates that 96% of all marine species went extinct 250 million years ago at the end of the Permian, it’s easy to imagine you can never go home again.
This is also where evolutionary trends enter the fray. Paraphrasing the early 20th Century population geneticist Sewall Wright, Gould writes, “Just as mutation is random with respect to the direction of change within a population, so too might speciation be random with respect to the direction of a macroevolutionary trend” (Gould, 1980, p. 126).
Between these tiers Gould wrote of breaks.
The first break is the Goldschmidt break. Richard Goldschmidt was a geneticist who did not share all of the views brought together in the modern evolutionary synthesis (see Huxley, 1942). In particular, he considered it possible for major organismal changes to arise from tiny genetic changes. These are much shorter durations relative to the orthodoxy of slow, cumulative, and gradual evolution prevailing in his time. He called them “hopeful monsters.”
The Goldschmidt break is a discontinuity that emerges from top-down forces of the second tier.
The next break — the one between the second and third tiers — Gould called the Wright break. If the mechanisms of speciation are independent of the mechanisms of microevolution, the mechanisms of these lower tiers are independent of the mechanisms that determine trends. To Gould trends were initiated by mass extinction and the role of historical contingency.
Since Gould’s hierarchical framework, Eldredge has proposed another (Eldredge and Salthe, 1984; Eldredge, 1985; 2003). This is a dual hierarchy which considers something Gould maintained a “respectful disinterest” in (Allmon et al., 2009): ecology.
Eldredge’s hierarchical framework has four main levels: organismal, population, biotic, and planetary. The economic side is the ecological side. Energy-matter transfers scale from somatic cells to the biosphere of the planet. The genealogical side is the informational side. Information transfers scale from the germ line cells to the highest taxonomic levels, and, if we’re taking things to the extreme, even life itself. Together these dual hierarchies form a dynamic evolutionary model Eldredge calls “the sloshing bucket” (2003).
There’s a bucket of water. A small bump jostles the bucket a little. It disturbs the once calm surface only slightly. The sloshes only go so high. But disturbances of greater force create a greater sloshing of the water in the bucket. A lot of evolution happens in the resettling of the water surface following the larger sloshes, like mass extinctions.
So what eliminates an individual may not eliminate a population. And what eliminates a population might not eliminate a community. And what eliminates a community does not eliminate a biota. But at the extremes are omnipotent individuals impervious to destruction and life itself.
The Ephemerality Thesis
The bumper sticker for punctuated equilibria came out of a paper in 1977 by Gould and Eldredge:
Stasis is data.
The rock record may be blind to rapid evolutionary transitions, but lineages present in the rocks are a quantifiable source of information. So how is it that fossil lineages show so little directional change? This was, as Gould noted, paleontology’s “trade secret” (Gould, 1987, p. 14).
Eldredge (1971) thought speciation was the result of small populations evolving in geographic isolation (aka, allopatry). Stasis emerged through stabilizing selection maintaining the bell curve’s shape (subsequently evaluated over the decades; e.g., see Lieberman and Dungeon, 1996; Estes et al., 2007; Haller and Hendry, 2014). Because stasis was present — was data — neontological and paleontological scientists attacked it with gusto.
Stasis was a challenge to progress in evolution. If neontologists didn’t think about this before, they did after interpreting that Gould suggested the modern synthesis was “effectively dead” (Gould, 1980, p. 120; they also evaluated evolutionary rates (Gingerich, 1983) and founder effects (i.e., peripheral isolates/peripatry; Barton and Charlesworth, 1984)).
One result arrived in 1987. Its author, evolutionary biologist Douglas Futuyma, had an idea. He thought about evolving local populations and the overall spatial structure. How long can a local population remain in isolation from other, larger populations? Environments fluctuate, climates change, and organisms migrate. Barring extirpation, interbreeding with dominant genetic variants from elsewhere may remove any divergence in a local population. And this may maintain stasis.
It’s an episodic dynamic in population structure tied to gene flow. Divergence in local populations is ephemeral. Futuyma (1987) called it the “ephemeral divergence hypothesis.”
Isolation is the key. Underpinning much of it is geographic variation (Dobzhansky, 1937; Mayr, 1942), the theory of island biogeography (MacArthur and Wilson, 1967), and meta-population ecology (Levins, 1969; Hanski and Gaggiotti, 2004). These theories put the isolation and structure of populations in a more central role in macroecology and macroevolution. They are the foundation of the second tier.
Thus, population isolation is episodic and ephemeral. Other ideas attest to ephemerality.
Consider two first tier formulations. One is resource pulses (Ostfeld and Keesing, 2000; Nowlin et al., 2008; Yang et al., 2008) wherein they “occur as episodic events with long interpulse intervals, few consumers are expected to specialize on such an ephemeral food supply…” (Ostfeld and Keesing, 2000, p. 233). Another is extreme events theory (Gutschick and BasiriRad, 2003, 2010) where “the acclimatory capacities of an organism are substantially exceeded. Consequently, after the event passes, the organism (or population) exhibits physiological and developmental responses to the environment that are significantly different from normal acclimation” (Gutschick and BasiriRad, 2003, p. 22).
At the next level up joining the ephemeral divergence hypothesis in explaining stasis is population averaging (Lieberman et al., 1995). Here “different ‘environmental populations’ may evolve, but they will typically do so in several different ‘directions,’ generally producing no net change” (p.15). Another proposal is the ephemeral speciation hypothesis (Rosenblum et al., 2012) that states “…speciation is very common and very rapid but the new species produced almost never persist” (p. 255). At this level of ephemerality, we also get plus ca change (Shelden, 1996), where “the evolutionary pattern in a lineage at a single locality is predicted to be stasis when there is greater environmental fluctuation, and net directional change and/or more widely fluctuating morphologies when environments are more narrowly fluctuating” (p. 209).
In breaking the bonds of stasis and forming new species, we have punctuated equilibria. Third tier considerations follow.
These include the turnover-pulse hypothesis (Vrba, 1980; 1985; 1993) where “…all (or nearly all) speciations and extinctions are initiated by physical environmental changes, with the necessary consequence that all macroevolutionary events across diverse groups of organisms are concentrated in a series of pulses” (1993, p. 427). Between pulses fits coordinated stasis (Brett and Baird, 1995; Brett et al., 1996; Brett, 2012), the creation of “…an empirical pattern, common in the fossil record, wherein groups of coexisting species lineages display concurrent stability over extended intervals of geologic time separated by episodes of relatively abrupt change” (Brett et al., 1996 p. 1). And finally, the sloshing bucket (Eldredge, 2003, 2008, 2015) is an idea where “…minor disturbance leads to recreation of ecosystems with the same players. Disturbance passing the extinction threshold leads to development of similar ecosystems, but with different species. Mass extinction leads to much more radically altered biotas” (2003, p. 28).
All of these ideas fall into what I would call the ephemerality thesis: changes that make a difference in biological populations are fleeting and rare. Stability reigns.
Within this framing, populations exhibit a number of behaviors. They may evolve in a direction, they may meander, they may go extinct, they’re traits may evolve at different rates, and they may anastobranch and anastomose (see Figure 1.3). They may even form a new lineage.
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