Allopatric Speciation

A key element of the modern synthesis assimilated by PE concerns the primary mode of speciation. Before the 1940s, most biologists assumed (as did Darwin) that gradual change within lineages (anagenesis) was largely responsible for the emergence of new species. During the 1940s, however, biological field studies and evidence from genetics showed that a crucial component of speciation was genetic isolation. Large interbreeding populations with their huge gene pools, in the absence of geographic subdivision (vicariance), seldom produced new variants. By contrast, small, isolated populations whose members were prevented from interbreeding with those of other populations often had unusual gene frequencies, and these populations appeared to be the source of most new species observed in nature. Ernst Mayr (Reference Mayr1942, Reference Mayr1963) formally developed this recognition into his model of allopatric speciation (although the idea has a long history stretching back well before Darwin [Reference Darwin1859]; see Lieberman [Reference Lieberman2000]). Mayr (Reference Mayr1942, Reference Mayr1963) argued that small, isolated (allopatric—meaning “different homeland”) populations on the periphery of the main population ultimately become the sources of new species. If the peripherally isolated population diverges sufficiently from the ancestral population(s) such that its members do not interbreed once they come back into contact, that population should be considered a new species. In this circumstance, the lineage splits (cladogenesis), yielding two species where there had been one.

While the importance of allopatric speciation had come to be accepted in the biology literature and textbooks of the 1950s, most evolutionary biologists and paleontologists—then and to some extent even today—did not recognize or even question the implications of this idea for how species’ originations should manifest in the fossil record (Eldredge Reference Eldredge2008a). It is interesting to note that allopatric speciation was first proposed in 1942, yet paleontologists of that generation failed to see its implications for speciation in the fossil record for a full 30 years, until Eldredge and Gould put allopatric speciation at the center of their model. Germane here is that allopatry involves cladogenesis. Yet, before the formulation of PE, many paleontologists had focused on documenting cases of anagenesis, where the entire population of a species transformed into another species across its entire range. This view aligned with Darwin’s (Reference Darwin1859) notion of sympatric (“same homeland”) transformation of species—a viewpoint superseded by the development of the modern synthesis and therefore largely outdated (Eldredge Reference Eldredge2015). PE instead represented a natural evolution from the modern synthesis, with scholars of deep time taking the next step of asking what pervasive allopatry would look like in the fossil record. Eldredge and Gould (Reference Eldredge, Gould and Schopf1972) reasoned that species should arise gradually on ecological timescales (ca. thousands of years), but that this would look “instantaneous in terms of our biostratigraphic resolution” (p. 106) on geological timescales.

Allopatry also predicts that species should arise from small populations in peripherally isolated areas, so the fossil record would be unlikely to capture the process unless that small area fortuitously happened to be preserved, and with exceptionally high stratigraphic/temporal resolution. More likely, strata would only record the presence of the descendant species once it had successfully differentiated, proliferated, and expanded its range over a much broader region, at which point it would be much less likely to show significant genetic, and hence morphologic, change. Thus, Eldredge and Gould (Reference Eldredge, Gould and Schopf1972) made the case that the model of speciation almost universally accepted by biologists should yield a fossil record in which species appear suddenly over their range and without preservation of intermediate forms and then persist for long periods of time with no net morphological change.

Rates of Evolution Vary, and Some Gaps in the Fossil Record are “Real”

Because phyletic gradualism presumes that rates of evolutionary change within species are slow and steady, any intervals of apparently rapid evolutionary change observed in the fossil record were viewed as times when the stratigraphic record of “true” gradual change was incomplete. In contrast, Eldredge and Gould (Reference Eldredge, Gould and Schopf1972) emphasized long periods of species-level stability—the equilibria—broken up by geologically quick episodes of speciation/cladogenesis—the punctuations (Eldredge and Gould Reference Eldredge, Gould and Schopf1972; Gould and Eldredge Reference Gould and Eldredge1977; Eldredge Reference Eldredge1985b, Reference Eldredge2008a; Gould Reference Gould2002). This insight, while novel, shares a connection with elements of the modern synthesis, particularly the work of another of its architects, George Gaylord Simpson (Lieberman Reference Lieberman, Erwin and Anstey1995). One of the central contentions of Simpson (Reference Simpson1944) was that rates of evolution did in fact show tremendous variation. Simpson (Reference Simpson1944) recognized the disconnect between putative slow and gradual evolution and the extremely rapid pace of mammal evolution early in the Cenozoic, when archaic mammal groups quickly (on geological timescales) transitioned into modern mammalian orders. This rapid pace of evolution could easily be distinguished from the relatively pedestrian pace of evolutionary change that has transpired since the early Eocene—modern bats were, after all, still clearly recognizable as the kin of Eocene bats, whereas the connection between bats and archaic mammals was much more challenging to forge. Was the quick transition between archaic and modern mammals indicative of a gap in the fossil record, as proponents of phyletic gradualism would assume? Simpson (Reference Simpson1944) argued against this view for several reasons, including the lack of evidence for such a gap in the well-studied mammalian record. Furthermore, if rates of change were truly uniform and could be characterized by the pace of evolutionary changes in bats since the early Eocene, then the amount of time missing from the mammal fossil record had to be stupendous, with modern mammal groups likely originating in the late Paleozoic. Yet the fossil record was simply not that incomplete.

To be sure, Simpson (Reference Simpson1944) never held that long-term stasis prevailed, especially at the level of species, and he did posit that apparently sudden changes between closely related species in the fossil record were in fact attributable to gaps in the record. He further held that the fossil record could be most informative regarding changes that led to the appearance of new higher taxa, such as families, orders, etc. However, his view on rates of evolutionary change and the existence of real times of rapid evolution that could not be explained away by gaps in the fossil record in an important way set the stage for a key insight of Eldredge and Gould (Reference Eldredge, Gould and Schopf1972): rates of evolution vary, and seeming morphological gaps or disjunctions separating ancestral species from their descendants in the fossil record might not be caused by an incomplete fossil record.

Emerging Hierarchies of Evolutionary Entities

A final crucial aspect from the modern synthesis that helped set the stage for the development of PE was Dobzhansky’s (Reference Dobzhansky1937) acknowledgment that nature was hierarchically arrayed, and processes at a lower level could not necessarily be extrapolated to explain what happened at a higher level (Eldredge Reference Eldredge1985a,Reference Eldredgeb, Reference Eldredge2008b; Gould Reference Gould1985). Dobzhansky (Reference Dobzhansky1937) detailed this view by explaining that evolution at the grand scale was more than just genetic mutation (Eldredge Reference Eldredge1985b, Reference Eldredge2008b, Reference Eldredge2015; Gould Reference Gould2002). Instead, genetic mutations need to spread within the context of populations, and different processes govern the appearance of mutations in organisms and the spread of those mutations through populations. Dobzhansky (Reference Dobzhansky1937) eloquently explained this as the difference between the physiology of organisms and the physiology of populations (Eldredge Reference Eldredge1985b, Reference Eldredge2008b). Further, both Dobzhansky (Reference Dobzhansky1937) and Mayr (Reference Mayr1942) held that species were real entities at any one time, despite being evanescent and continually changing through time. A novel insight of PE was that not only were species real at any one time, but they were also real and persistent through time (Eldredge Reference Eldredge1979, Reference Eldredge1985b, Reference Eldredge2008b; Gould Reference Gould1982, Reference Gould1985, Reference Gould2002; Turner Reference Turner and Delisle2017). Just as the processes affecting mutations within organisms and the spread of organisms within populations mattered, so too did the processes involving the appearance and disappearance of species. Again, this novel insight was connected to past discoveries, and it would have fundamental implications for evolutionary theory.

The existence of hierarchically arrayed biological entities that both underpinned and emerged from the modern synthesis was buttressed by work in phylogenetics, a field profoundly burgeoning around the time of the development of PE (see discussions in Eldredge [Reference Eldredge2011], Wiley and Lieberman [Reference Wiley and Lieberman2011], and Brooks et al. [Reference Brooks, DiFrisco and Wimsatt2021] and references therein). As the field of phylogenetics grew, it became clear that higher taxa could not give rise to other higher taxa. Not only would this violate the principle of monophyly, but there was no process of “familification” or “generification” (Hendricks et al. Reference Hendricks, Saupe, Myers, Hermsen and Allmon2014). It might seem paradoxical today, but the architects of the modern synthesis were operating within a framework that posited that evolutionary change not only produced new species, but also new genera, new families, orders, etc. And these categories were invoked to convey not just the order of evolution but the degree of change. This aspect of the modern synthesis is completely nonsensical today and has been entirely discarded. While phylogenetics as a field in the early 1970s did not provide consensus or unanimity on the nature of species, and still does not, the rejection of higher taxa as evolving entities does align well with PE’s emphasis on the centrality of species.

Eldredge and Gould were well versed in the new developments in genetics, paleontology, and systematics that comprised the modern synthesis when they arrived at Columbia hoping to study evolution in the fossil record (Eldredge Reference Eldredge1985a, Reference Eldredge2008a; Gould Reference Gould1989a, Reference Gould2002; Allmon Reference Allmon, Allmon, Kelley and Ross2008). Yet, as Eldredge (Reference Eldredge1985a) recalled, his research on phacopid trilobites (Eldredge Reference Eldredge1971) did not reveal the expected pattern of slow continuous transformation called for by the old view of phyletic gradualism. Instead, each species appeared at a particular level in the fossil record and then remained largely stable (“stasis” in the parlance of Eldredge and Gould [1972]) through strata spanning millions of years. Putting the pieces together from their understanding of allopatry, the fossil record, and species as the only “real” entities in systematics ultimately led Eldredge and Gould to see the inherent contradiction between contemporary speciation theory and the classic notion of phyletic gradualism and to propose their alternative of PE. Eldredge and Gould (Reference Eldredge, Gould and Schopf1972), then later Gould and Eldredge (Reference Gould and Eldredge1977), and a large number of other studies, showed that only rarely does the fossil record produce “insensibly graded sequences” (Eldredge and Gould Reference Eldredge, Gould, Kauffman and Hazel1977: p. 39) of organisms gradually changing through time as they evolved into new species.

The Connections between Microevolution and Macroevolution

While PE was an idea that focused on the fully formed appearance and persistence of species through time in the fossil record, it also became something more, for several reasons. First, in the paper itself, in a section titled “Some Extrapolations to Macroevolution,” Eldredge and Gould (Reference Eldredge, Gould and Schopf1972: pp. 108–112) attempted to forge broader connections to patterns and processes of evolution. They specifically distinguished their view of macroevolution based on PE from that of the modern synthesis, which maintained that macroevolution was only the product of extrapolated population genetic phenomena that yielded gradual change and “stately unfolding” (Eldredge and Gould Reference Eldredge, Gould and Schopf1972: p. 109) over time. Because PE’s central contention, stasis, entails that species are mostly stable over millions of years, the standard selection processes impacting organisms and populations on ecological timescales generally do not induce much change in species on geological timescales. If species are real and persistent entities over geologically long intervals of time, as Eldredge and Gould (Reference Eldredge, Gould and Schopf1972) posited, then they become important units of evolution. The relative prevalence of PE has important implications for the debate about whether the processes of microevolution (evolution below the species level) can be smoothly extrapolated to explain macroevolution (evolution at and above the species level). (See Erwin [Reference Erwin2000, Reference Erwin, Ayala and Arp2010], Gould [Reference Gould1982, Reference Gould1985, Reference Gould2002], Myers and Saupe [Reference Myers and Saupe2013], and Kearney et al. [Reference Kearney, Lieberman and Strotz2024] for extensive discussions on the relationship between micro- and macroevolution.)

Disparity and Trends

Eldredge and Gould (Reference Eldredge, Gould and Schopf1972) specifically distinguished two important macroevolutionary phenomena that needed to be revisited in light of PE. The first was “‘classes’ of great number and low diversity,” such as major clades of echinoderms that evolved early in the Paleozoic (Eldredge and Gould Reference Eldredge, Gould and Schopf1972: p. 110). In modern parlance, this phenomenon would be treated as a question about the difference between diversity and disparity (Gould Reference Gould1989b). PE informs this discussion by emphasizing that morphological change is concentrated at the time of speciation. In this view, speciation events might each encompass small or large amounts of change, and very large morphological shifts can be accommodated in very little geological time with a quick succession of speciation events. This interesting phenomenon eventually led to Gould’s (Reference Gould1989b) work on the Burgess Shale and the tremendous amount of literature it inspired about the nature and significance of Cambrian soft-bodied faunas. It also produced important interactions between paleontology and the newly emerging field of evolutionary developmental biology (e.g., Erwin Reference Erwin2017). Thus, PE helped create links (although not the only ones) to macroevolutionary processes emanating from developmental systems.

The second important macroevolutionary phenomenon distinguished by Eldredge and Gould (Reference Eldredge, Gould and Schopf1972: p. 111) was the nature of trends. They argued, building on the work of Wright (Reference Wright1967), that “just as mutations are stochastic with respect to selection within a population, so might speciation be stochastic with respect to the origin of higher taxa.” In this respect, macroevolutionary trends are explained by the differential origination and extinction of species. This view is very different from that of the modern synthesis, which saw trends as gradual directional change resulting from consistent selection pressures on individuals within lineages (Eldredge Reference Eldredge1979, Reference Eldredge2015). While Eldredge and Gould (Reference Eldredge, Gould and Schopf1972: p. 112) suggested that such stochastic speciation followed by sorting involved “no ‘new’ type of selection,” Stanley (Reference Stanley1975) reasoned that such trends could indeed involve a new type of selection and coined the term “species selection.” Despite this connection, the original formulation did not require that species selection occur for PE to be valid. (For extensive discussions of species selection, see Vrba [Reference Vrba1984], Gould [Reference Gould2002], Lieberman and Vrba [Reference Lieberman and Vrba2005], and Jablonski [Reference Jablonski2008].)

From Single Species to Regional Biotas: PE, Turnover Pulse, and Coordinated Stasis

Another way that PE expanded its scope beyond evolutionary patterns and processes within individual lineages was in the development of the turnover pulse hypothesis (Vrba Reference Vrba1985) and coordinated stasis (Brett and Baird Reference Brett, Baird, Erwin and Anstey1995; Brett et al. Reference Brett, Ivany and Schopf1996; Morris et al. Reference Morris, Ivany, Schopf and Brett1995; see also Brett et al. [Reference Brett, Ivany, Zambito, Welych-Flanagan and Baird2025] this issue). These related hypotheses of pattern and cause extended PE beyond the evolution of single species to argue that groups of species co-occurring in regional biotas show roughly coeval origins, long periods of coincident relative stability (Miller Reference Miller1997), and roughly coeval extinctions and extirpations. These authors suggested that comparatively large shifts in the physical environment drive coordinated pulses of turnover across regional biotas, and Morris et al. (Reference Morris, Ivany, Schopf and Brett1995) further proposed that aspects of ecological structure helped to maintain relative stasis between turnovers. Elements of these hypotheses were incorporated in Eldredge’s (Reference Eldredge, Crutchfield and Schuster2003) “sloshing bucket” hypothesis, which emphasized the important role abiotic environmental change played in driving evolution.

PE as a More Universal Phenomenon—Extending beyond the Fossil Record

Another fundamental way that PE became distinguished is as a broader, more general theory that might serve as a metaphor for describing, and perhaps helping to explain, the way that change occurred in a variety of systems, not only evolutionary and paleontological, but also, for example, cellular, ecological, and cultural (Gould Reference Gould2002; Atkinson et al. Reference Atkinson, Meade, Venditti, Greenhill and Pagel2008; Valverde and Solé Reference Valverde and Solé2015; Wosniack et al. Reference Wosniack, da Luz and Schulman2016; Blanco et al. Reference Blanco, Calatayud, Martin-Perea, Domingo, Menéndez, Müller, Fernández and Cantalapiedra2021; Duran-Nebreda et al. Reference Duran-Nebreda, Vidiella, Spiridonov, Eldredge, O’Brien, Bentley and Valverde2024; O’Brien et al. Reference O’Brien, Valverde, Duran-Nebreda, Vidiella and Bentley2024; Douglas et al. Reference Douglas, Bouckaert, Harris, Carter and Wills2025). And, in this respect, PE also helped forge important connections between paleontology and neontology, as many neontologists set out to test aspects of the theory (Eldredge et al. Reference Eldredge, Thompson, Brakefield, Gavrilets, Jablonski, Jackson, Lenski, Lieberman, McPeek and Miller2005), including developing novel approaches to characterize molecular and phylogenetic changes (Pagel et al. Reference Pagel, Venditti and Meade2006; Bokma Reference Bokma2008; Wada et al. Reference Wada, Kameda and Chiba2013; Garcia-Erill et al. Reference Garcia-Erill, Wang, Rasmussen, Quinn, Khan, Bertola and Santander2024). Clearly, PE sparked a fundamental reimagining of the causal levels and processes of evolution, and it continues to inspire a new generation of evolutionary biologists more than 50 years after its inception.

Historical Perspectives on PE

Fortunately for all, Niles Eldredge was able to attend and speak at the 50th anniversary celebration of PE, and thus very appropriately, Eldredge (Reference Eldredge2025) leads off this commemorative issue by briefly recounting how he and Stephen Jay Gould came to coauthor their 1972 paper, while providing an exegesis of the principles and data underlying PE. As related earlier, key elements of PE are that speciation primarily occurs in the allopatric mode and that most species display stasis throughout much of their histories—elements derived from data in both extant biota and the fossil record. Notably, Darwin’s early views on the origin and evolution of species were well aligned with the prevalence of both allopatry and stasis, although Darwin later came to embrace a view that emphasized sympatric divergence and gradual change. Eldredge (Reference Eldredge2025) discusses the important role that geographic factors play in mediating PE, both via the phenomena that trigger allopatry and the mechanism of stasis that involves the geographic structure of populations within species, such that each population experiences somewhat different selection regimes (see also Kelley et al. [Reference Kelley, Dietl and Handley2025] in this issue). Another important aspect is how PE is fundamentally connected to the hierarchical structure of nature, where changes at one level, the genome, do not necessarily directly extrapolate to changes at higher levels like the organism or the species. PE especially helped validate the view that many trends arose from the differential birth and death of species. Eldredge (Reference Eldredge2025) further describes the important extension of PE whereby not only single individual species lineages show stasis and change, but many species that occur together within a broader region have largely concurrent patterns of speciation and extinction, bracketing long intervening periods of stasis (see also Brett et al. [Reference Brett, Ivany, Zambito, Welych-Flanagan and Baird2025] in this issue on this concept of “coordinated stasis”)—which points out the need to understand the broader environmental and ecological context of change that affects groups of species.

Dresow (Reference Dresow2025) argues that Gould, at an early stage of his career, suggested that organisms were continually improving through time and held a commitment to the “adaptationist programme”—both concepts that Gould later spent much time criticizing. Dresow suggests that, over the decades after the publication of the 1972 PE paper, a number of factors led Gould to abandon his earlier positions and reject beliefs he held earlier in his career. Although Gould (Reference Gould2002) claimed that PE was the “coordinating centerpiece” of his life’s work, Dresow posits that Gould’s conceptual framework changed over the years (Lieberman and Vrba [Reference Lieberman and Vrba2005] and Lieberman and Eldredge [Reference Lieberman and Eldredge2024] provided examples of this in other instances as well).

Yacobucci et al. (Reference Yacobucci, Schopf and Goldsmith2025) conducted an exploratory survey to understand perceptions of PE by paleontologists and evolutionary biologists, including students and professionals. Although those who responded to the survey recognized the importance of PE, instructors typically spent a week or less on the topic in their courses, and misconceptions and inconsistencies in their understanding were noted (e.g., that anagenesis is common). Yacobucci et al. (Reference Yacobucci, Schopf and Goldsmith2025) recommend pedagogical strategies for improving students’ understanding of PE and ways to expand on this pilot survey.

The State of the Evidence Regarding PE

Stanley (Reference Stanley2025) discusses various tests of data sets, some of which he had previously used to test the validity of the PE model. These include the “test of adaptive radiations,” which shows that species in several families (his examples include mammals and bivalves) show stasis for millions of years, far too long for phyletic gradualism to explain their duration. Stanley also recaps his “test of living fossils,” which predicts that long-lived clades that experienced very little branching and diversification would be expected to show extreme stasis and look much like their ancestors—as they do. Stanley reviews a number of recent multivariate morphometric studies of fossils through long time intervals, most of which show stasis and almost no gradualism. Finally, he points to some recent examples documenting rapid speciation in modern organisms, which was predicted when the allopatric speciation model was the original basis for the PE model in 1972.

Lieberman and Strotz (Reference Lieberman and Strotz2025) examine studies of evolutionary change explicitly designed to test the hypothesis of PE published between 2008 and 2023. Papers reviewed include new analytical methodologies and techniques applied both to the fossil record and extant organisms. They find that the preponderance of work indeed documents stasis and cladogenesis, thus supporting the ubiquity of PE and the relative frequency argument of Gould. Authors who concluded that their studies did not support stasis employed a different, narrower definition of stasis, treating nondirectional morphological fluctuations as at odds with stasis, rather than the definition used by Eldredge and Gould (and many others) that considered stasis as little to no net change over the duration of a species.

Hunt et al. (Reference Hunt, Voje and Liow2025) analyze the state of the evidence for and against PE since the 1972 paper that started it all. They point out that a rigorous statistical method is needed to truly determine whether fossil lineages show stasis, gradual directional change, a random walk, or something else. Using statistical methods already developed by Hunt, they find that there were very few cases of gradual directional change among the dozens of studies that have been published over the last 50 years, whereas stasis is by far the most common pattern, along with random walks. However, these authors argue that more data are needed to show that morphological change is concentrated in pulses associated with speciation.

Anderson and Allmon (Reference Anderson and Allmon2025) argue that PE continues to be misunderstood, and in cases dismissed, by some evolutionary biologists, in part due to confusion about what is meant by “mode” of evolution, that is, cladogenesis and anagenesis. If cladogenesis is the dominant mode by which species originate, as proposed by PE, ancestors should persist along with their descendants. They test this hypothesis by examining recently published phylogenies for 30 clades (invertebrates, vertebrates, and radiolarians). Importantly, they find cladogenesis to be the most prevalent mode of species origination based on the “persistence of ancestor” criterion, thus supporting the dominance of PE (see Strotz and Allen [Reference Strotz and Allen2013] for a detailed demonstration that this is often the case in forams as well).

Documenting long-term stasis in fossil species requires accurate estimates of their stratigraphic duration and, therefore, that morphology can be tied to specimens with explicit relative stratigraphic position and age. Hendricks and Lieberman (Reference Hendricks and Lieberman2025) highlight how important it is to document species durations using fossil occurrences that are supported by museum voucher specimens. They illustrate the problem by showing how various online and published databases give different first and last occurrences for the trilobite Eldredgeops rana (Green, Reference Green1832) and its family, Phacopidae, which leads to varying estimates of the duration of stasis in this species (although all imply durations of many millions of years). Hendricks and Lieberman (Reference Hendricks and Lieberman2025) encourage the paleontological community to identify stratigraphic voucher specimens for fossil occurrences in museum collections and place photographs of these specimens in publicly accessible websites. This will facilitate a dramatic increase in the number of studies that can test for stasis and thereby assess PE.

The end of the Pleistocene ice age resulted in major changes to North American environments, which might be predicted to result in anatomical changes in vertebrate species, including decreased body size and changes in morphology. Syverson and Prothero (Reference Syverson and Prothero2025) use recalibrated dates and a new bootstrap method for estimating ages in a time series, in combination with morphological data, to document morphological stasis in multiple bird and mammal species from the famous Rancho La Brea tar pits in California. They uncovered stasis in the overwhelming majority of species. Moreover, the very few significant morphological changes recognized did not correspond to times of climate change, highlighting the complex relationship between late Quaternary climate and evolutionary responses.

Wagner and Wright (Reference Wagner and Wright2025) test PE using phylogenies. They point out that existing models of trait change assume a constant distribution of rates over time, whereas PE explicitly calls for elevated rates of change in association with cladogenesis and low rates thereafter. They therefore model character change as dependent upon the number of branching events rather than elapsed time and show that, for strophomenoid brachiopods during the Ordovician radiation, punctuational models are strongly favored over continuous ones. As rates of morphological change are tied to branching, their model is also better able to capture the high disparity often seen early in a clade’s history.

Mechanisms of Stasis and PE

The prevalence of evolutionary stasis within species lineages is one of the key aspects of PE. Kelley et al. (Reference Kelley, Dietl and Handley2025) argue that one way stasis can arise is through the variable evolutionary responses triggered by ecological interactions of predators and their prey and hosts and their parasites in geographically structured populations. In a process termed “coevolutionary alternation,” local populations experience different coevolutionary dynamics (e.g., involving changes in predator preferences and prey defenses), as well as different physical environments. This geographic mosaic of biotic interactions, selection pressures, and the resultant spatial variation in coevolutionary trajectories can produce stasis on the scale of entire species. Kelley et al. (Reference Kelley, Dietl and Handley2025) discuss the difficulty of modeling coevolutionary alternation and testing it as a potential cause for stasis. They argue that coupled stochastic differential equations will represent a useful way of modeling time series to better understand the effects of predator–prey and host–parasite interactions on selection pressures and morphological stasis and change, and they present a framework for how such modeling might be accomplished.

The classical approach to testing for PE entails the study of morphology over time within a lineage. Slattery et al. (Reference Slattery, Harries, Jarrett and Sandness2025) adopt this approach in their study of nuculid bivalves, but they do so explicitly in the context of environmental change to assess the potential role of the physical environment in generating stasis. Sheldon’s (Reference Sheldon1996) “Plus ça change” model predicts stasis in settings characterized by substantial environmental variability but anagenesis when the environment is more stable and adaptation can keep pace with any slow variation. They find significant morphological change during the stable warm Cretaceous and demonstrate stasis during the more variable “icehouse” conditions of the Neogene–Quaternary, supporting hypotheses that causally relate evolutionary mode to environment.

Polly (Reference Polly2025) also investigates the role of environment in setting the pace of morphological change within a species using a set of models to simulate sea-level change, population size and fragmentation, and morphological change in Bermuda land snails that approximate the Poecilozonites Boettger, Reference Boettger1884 used in Eldredge and Gould (Reference Eldredge, Gould and Schopf1972) (see also Gould Reference Gould1969). He shows that glacial/interglacial-scale changes in sea-level drive changes in morphology by fragmenting and isolating small populations during highstands and uniting populations across a broader area during lowstands. Genetic drift during highstands fosters rapid morphological change in small isolated populations, but lowstands enable the population to coalesce and grow, damping the potential for change. Further, he shows that the sedimentary record itself can influence the interpreted pattern of stasis and rapid change, because sea-level rise (in simulated Bermuda) is more likely to be characterized by expanded sedimentary sequences, while lowstands correlate with pedogenesis and a comparatively thin stratigraphic record. In this case, depositional patterns make it easier to document gradualism, because times of comparative stasis are characterized by thin stratigraphic sections, while times of genetic drift are recorded in thick sequences of aeolian sedimentation. That it was possible to document stasis despite this bias, argues Polly (Reference Polly2025), provides even greater support for the PE pattern.

Casanova (Reference Casanova2025) examines molecular mechanisms that could explain the key patterns of PE—long-term species stasis and the concentration of evolutionary change at speciation. Analyses of patterns of variation within developmental regulatory genes show that they also display long periods of little change, punctuated by relatively rapid changes associated with cladogenetic events. Casanova (Reference Casanova2025) uses principles from game theory, along with simulations, to illustrate how changes in developmental regulatory genes and their dosage provide a molecular mechanism to explain PE. She argues that long periods of stasis and relatively rapid periods of evolution emerge due to the existence of stable gene networks that are in turn subjected to ongoing neutral and adaptive molecular changes occurring all the time. These networks are moreover buffeted by occasional substantial perturbations to the physical environment that organisms variably experience, facilitating rapid change and cladogenesis.

Broader Theoretical Perspectives Building on PE

Tëmkin (Reference Tëmkin2025) discusses the relationship between PE and hierarchical approaches to evolutionary biology. Biological entities such as genes, organisms, and species are nested from smaller to larger levels and interact in complex ways. Patterns and processes at any lower level cannot necessarily be extrapolated to determine what happens at a higher level. PE validated the notion that species are important biological entities that persist for long periods of time and need to be better incorporated into evolutionary theory. Tëmkin (Reference Tëmkin2025) elaborates on the differences between entities in the ecological/economic hierarchy and the reproductive hierarchy and explains how these hierarchies interact. Tëmkin (Reference Tëmkin2025) further argues that the nature of the hierarchical system, along with interactions between entities and the physical environment, determines when entities persist and when they change.

Brett et al. (Reference Brett, Ivany, Zambito, Welych-Flanagan and Baird2025) reexamine the concept of coordinated stasis: the idea that groups of evolutionarily stable species also demonstrate a degree of ecological stability in their associations, with stable ecological evolutionary subunits (EESUs) separated by geologically brief periods of rapid change between EESUs. The study expands the scope of coordinated stasis beyond the classic example from the Middle Devonian Appalachian Basin and, with additional Late Ordovician examples, finds that EESUs and their boundaries were coincident over broad areas (provinces and even beyond). The authors attribute large-scale faunal turnovers to widespread habitat degradation leading to isolation of marginal populations that promote coordinated episodes of speciation, extinction, and migration—a process consistent with PE.