Hierarchy Theory and the Extended Synthesis Debate
Hierarchy Theory and the Extended Synthesis Debate
Abstract and Keywords
In the closing remarks we propose a theoretical watermark that connects all the chapters of this volume. After a short summary about Eldredge’s twin hierarchies as a possible unifying frame for evolutionary biology, with different levels of organization above and below the organismic level, we stress the point the each level should be considered autonomous, and at the same time interdependent with the others. Hierarchy theory is a non-reductionist Darwinian research programme, with a heuristic and a theoretical relevance. In the first sense, Hierarchy Theory is a useful framework in order to understand the rapid advancements in specific evolutionary disciplines, such as paleo-anthropology. In the second sense, Hierarchy Theory aims at enlarging the standard neo-Darwinian theory of evolution through a more ecological and multilevel framework if compared to the so-called Extended Evolutionary Synthesis. The theory of evolution is still evolving, and this is what a healthy research programme should do.
Summing Up: Hierarchy Theory in a Nutshell
As several of the chapters in this volume show, the so-called hierarchy theory of evolution is an ecological and multilevel approach to potentially any evolutionary phenomena. According to this view, the micro-and the macrodimensions of the evolutionary patterns and processes are conceptually separated but empirically intertwined levels. The former pertain to any change inside the biological populations, the latter pertain to the birth, death, persistence, and branching of species and higher taxa (Eldredge and Lieberman 2014). The vertical dimension of evolution (i.e. changes due to the continuous and differential gene transmission) is strictly interrelated to the horizontal sphere of climatic and geographical factors, able to produce episodic ecological changes that, in turn, affect genetic and genealogical relations among organisms, populations, and species.
According to Eldredge’s “sloshing bucket” model, environmental events may have very different magnitudes—namely, localized or subregional ecosystem disturbances, regional disturbances, and global environmental disruptions (the case study proposed in this volume by Roopnarine and Angielczyk explains how a paleocommunity responded, in ecological time, after particular types and magnitudes of disturbances). The greater the magnitude, the greater ecosystems change (from rapid recoveries to turnover pulses of species to mass extinctions at the maximum, with subsequent (p.352) new radiations of larger groups); the greater the loss of higher taxa, the more different will be the newly evolved taxa. We may thus consider evolutionary units as belonging to a multilevel hierarchy: the microevolutionary levels of genes and their organismic environments; the intermediary, basic level of organisms; and the macroevolutionary levels of groups and species (as for the plurality of definitions of “levels of organization,” see Umerez in this volume).
At the highest level, ecosystems (containing abiotic elements) show typical hierarchical and organizational features (see Cooper et al. in this volume). On the opposite side, there are hierarchical layers also below the organismic level: according to Gregory et al. in this volume, an evolutionary analysis of the transposable elements in the human genome demonstrates the necessity of a multilevel approach to explain genome features and their variation all across the phylogenetic tree of life. Transposable elements are biological entities that reproduce, compete, and evolve at their focal level, with cross-level effects. Caianiello’s chapter in this volume outlines a tentative hierarchy theory at the organismic and suborganismic level, elucidating the genetic and epigenetic causality of phenotypic determination at the organismic level.
Each level should be considered autonomous (with its own patterns and focal “individual” entities—about the emergence of novel “individuals” in a hierarchical perspective, see Pavličev et al. in this volume) and at the same time interdependent with the others (cross-level patterns and processes). An event produced at the highest level can have repercussions downward at lower levels, or vice versa, a microevolutionary transformation can be propagated upward to higher levels. Among the downward causations, according to the quite original view proposed by Daniel W. McShea in this volume, even an apparent teleology is a byproduct of an inherent hierarchical organization of living systems.
The two “walls” of the bucket metaphorically represent the two different hierarchies described in this volume. Each hierarchy goes from micro-evolutionary to macroevolutionary levels and vice versa. The genealogical hierarchy, concerning reproduction or replication, involves genetically based information systems: the microevolutionary level of genes is part of the upper level of organisms, which are nested into local breeding populations and species. The ecological hierarchy is about matter-energy transfer systems: organisms are parts of local conspecific populations seen in this case as “economic” entities acting for physical survival in their ecological niches. Local ecosystems are parts of regional ecosystems, up to embrace the whole biosphere.
(p.353) Evolution is a process occurring at different levels, from genes to ecosystems. At all scales, changes in ecological dynamics affect the information stored in the genealogical hierarchy and vice versa. Lower level phenomena can penetrate the higher levels, but higher-level events set the stage for the agency of lower-level processes. In other words, hierarchy theory is a research program aiming at the unification of micro-and macroevolution: a possible solution for a very old problem in evolutionary debates since Darwin.
Hierarchy theory is still a Darwinian research program for two reasons: (1) because the original structure of Darwinian evolutionary theory was much more ecologically centered and pluralistic as for the causes and patterns of evolutionary change than the gene-centered neo-Darwinian reloads in the twentieth century and (2) because natural selection (among individuals) stands at the center of the sloshing bucket. Organisms are simultaneously systems and lineages (see Caponi in this volume) and simultaneously part of the two different interacting hierarchies: as reproducing “packages” of genetic information (replicators), they are part of the genealogical hierarchy; as matter-energy transfer systems (interactors), they are part of the ecological hierarchy, and their business is to survive. Thus the process of natural selection at the level of organisms incorporates the two dimensions (ecological pressures and genetic variations), and it is the bottom of the “bucket” in Eldredge’s metaphor. Genetic variations and ecological pressures are two sides of the same coin. Hierarchy theory is a nonreductionist Darwinian research program.
As Niles Eldredge pointed out in the historical and introductory chapter of this volume, the first pillar of the approach is the idea that “biological nature is complexly organized into economic and genealogical hierarchies, and [we] have come to ask how those twin hierarchies interact to produce evolutionary history.” The second pillar is that in the history of life, “large-scale environmental disruptions produce mass extinctions, with proportionally great concomitant evolutionary reactions—and with smaller environmental perturbations producing correspondingly lesser effects.”
A Practical Application to a Specific Disciplinary Field: Paleoanthropology
Hierarchy theory is a unifying frame in which life emerges from a complex architecture of intersecting hierarchies of levels: the genealogical hierarchy (p.354) of reproduction, the economic hierarchy of survival and finding resources, and even the outer hierarchy of physical structures of the Earth’s crust. The evolution of organisms that reproduce, the evolution of ecosystems, and the evolution of the planet are inextricably intertwined and interdependent: the fluctuations of one reverberate proportionally on the other. They form an integrated functional network.
Yet it must be said that the mainstream of evolutionary biologists still retains an antihierarchical attitude, even among the supporters of the “extended evolutionary synthesis.” Probably all of them associate hierarchy theory to another version of the rather inconclusive theories of complexity or to vaguely holistic approaches. As a theoretical attempt, hierarchy theory seems too large and too abstract, mixing together everything under the sky of evolution. However, as many chapters in this volume (written by scientists working in the field) explain, such a multilevel approach illustrates the need for a global and extended coherence in a rapidly expanding field. It has a heuristic and a theoretical relevance.
Starting with the first—heuristic and methodology—hierarchy theory could be a useful framework in order to understand the rapid advancements in specific evolutionary disciplines. We take human evolution as a practical example. An amount of recent findings in paleoanthropology are stressing climate instability and ecological disturbances as key factors affecting the highly branching hominin phylogeny, from early hominins to the appearance of cognitively modern humans (for a recent review, see Parravicini and Pievani 2016). Allopatric speciations due to geographic displacements, turnover pulses of species, adaptive radiations, mosaic evolution of traits in several coeval species, bursts of behavioral innovations, and serial processes of dispersal out of Africa are some of the macroevolutionary patterns emerging from the field. In this enlarged picture, human evolution is seen as an integration of different levels of evolutionary change, from local adaptations in limited ecological niches to dispersal phenotypes able to colonize huge, unprecedented ranges of ecosystems.
Human evolution has been the outcome of global ecological changes that transformed first our African cradle and then the Old World and the New World. The interactions among different levels and between the biotic and abiotic factors shaped the main features of human evolution. Genus Homo is the descendant of such a complex intertwining of different levels of evolutionary change. It is the consequence of an explosion of punctuated equilibria and turnover pulses in early Pleistocene, which are in turn a side effect of complex environmental changes caused by the (p.355) effects of the ice ages in Africa. The same environmental conditions might have promoted our attitude to dispersal until the final wave of cognitively modern humans. We may be the effect of a sequence of large climatic and ecological disturbances that moved the water in the African “sloshing bucket” of evolution.
This shift in the general perspective about human phylogeny has been due to dramatic technological advances. New tools of integrated analysis and the extensive use of new dating techniques are available. The pale-ontological and archaeological record has been largely expanded. The “mosaic” anatomy of the earliest hominins (with unique combinations of retained and derived traits) suggests the idea of a series of adaptive postural “experiments” right around the putative time, when hominin lineage branched from chimpanzee lineage (five to six million years ago). Climate and environmental changes transformed the eastern and southern African environment from a relatively flat, homogeneous region covered with tropical mixed forest to a heterogeneous region, with high mountains and a mosaic of habitats ranging from cloud forests and closed woodlands to grasslands and deserts. In such a macroevolutionary process, we see the matching between a mosaic of unstable environments (the ecological hierarchy) and a mosaic of transitional forms with different adaptations in the bipedalism trend (the genealogical hierarchy).
The impressive radiation of hominin forms in Africa between three and two million years ago (australopithecines, genus Paranthropus, genus Homo) and the concomitant climatic instability and habitat fragmentation strongly suggest a high incidence of geographic speciations and a punctuational pattern of change. This branching picture of human phylogeny is very far from the anagenetic view held by the fathers of modern synthesis’s phyletic gradualism and much more similar to the evolutionary history observed in other coeval mammalian taxa, revealing its higher parsimony.
Other subexamples show the same ecological pattern. When genus Homo emerged around 2.8 million years ago, bipedalism became first prevalent and then obligate. This evolutionary transition was carried out by a few species morphologically instable or by a plurality of separated species, each one with a specific set of traits. It follows that also the transition from a smaller hominin, more adapted to an arboreal lifestyle, to an obligate bipedal reveals the pattern of a “mosaic” evolution. Climate instability and habitat fragmentation were the macroevolutionary challenges. A plurality of adaptive strategies, sharing the same flexibility, was the answer.
(p.356) Global ecological phenomena triggered severe ice ages, with intense and continuous fluctuations between glacial and interglacial phases. As a consequence of these global changes, East African climate became even drier because of the long-term aridity trend, while in the short term it became more instable with the alternation of high and low climate variability and cycles of moisture and aridity. Due to habitat fragmentation in African environments, many peripheral populations became isolated, providing the best conditions for the expected occurrence of extinction events, allopatric speciations, and adaptive radiations in a punctuational evolutionary mode. Climate instability, turnover pulses, and a plurality of genera feature the still unclear birth of genus Homo (still less clear after the discovery of Homo naledi, another mosaic of traits, in South Africa).
The still cryptic interspecific trend in genus Homo encephalization could be an example of the macroevolutionary pattern that Allmon in this volume defines as the result of lineage splitting, not only of within-lineage transformation. New, interesting ecological hypotheses have been proposed in order to account for the repeated hominin dispersals out of Africa as well. Again, microevolutionary factors (adaptations to local selective pressures) match with macroevolutionary patterns and processes. During the Pleistocene and its ice ages, the territories were plenty of mobile physical barriers. Geographical expansion through highly fragmented and diversified habitats produced a predictable fragmentation of populations. Vicariance and dispersal of human populations out of Africa, thus, may have triggered pulses of allopatric speciations, now in Eurasia too. This could be the reason for the remarkable morphological variation showed by the fossil record assigned to genus Homo and found in different parts of the Old World after 1.8 million years ago until very recent times with the cohabitations of Homo sapiens, Homo neanderthalensis, Denisovans, and Homo floresiensis.
Also the speciation of Homo sapiens in Africa, around two hundred years ago, has been affected by climate instability and dry phases. The birth of cognitively modern humans, however, seems to be more recent, as if anatomy and behavior were temporally disjointed. Only around sixty to eighty thousand years ago, robust findings point to an unprecedented appearance of symbolic expressions throughout the areas of distribution of Homo sapiens. Biogeography and migrations become crucial also in this case. Recent evidence corroborates the idea that the bursts of behavioral and cognitive innovation in South Africa (seventy thousand to eighty thousand years ago) are related to a growing population of Homo sapiens that was expanding out of Africa, carrying the symbolic and linguistic (p.357) capacities showed by the members of this group later in Europe and Far East—again, small adaptive changes in fragmented populations and regional ecological disturbances due to global changes. In a pluralistic approach based on the twin hierarchies, gradualistic changes and punctuational patterns, genetic and epigenetic variation, adaptive transformations and plasticity, and biological evolution and cultural evolution can be seen as no longer in contrast. Old dichotomies crumble.
As we see in this example, the theory of evolution has a highly heterogeneous empirical basis, ranging from molecules to fossils, from changes occurring in a few minutes in bacteria to changes lasting for geological eras. Hierarchy theory predicts the growing role of interdisciplinary, integrated models. The recent advancements in paleoanthropology integrate archaeological, paleoclimatological, molecular, and demographic data in order to explain the biological and cultural evolution of cognitively modern humans. The consilience of very different data—biogeography, molecular biology, paleoclimatology, geophysics, paleoecology, and archaeology—highlights the most important patterns in hominin phylogeny today.
The multilevel approach to evolution provides a useful framework for bringing together and unifying the complex interplays observed among patterns and processes that belong to different evolutionary hierarchies (ecological and genealogical) structured along different levels. Micrevolutionary explanations that appeal to processes occurring within local populations—that is, adaptation through natural selection and genetic drift—are fundamental, but they have to be integrated with macroevolutionary patterns in a pluralistic perspective, giving due relevance to the ecological and geophysical patterns and processes in human evolution. The physical and ecological world is not a mere backdrop of evolution: it changes, and such changes had profound effects on human evolution as well.
Extended Syntheses: A Matter of Focus
We anticipated that hierarchy theory could have also a theoretical relevance. In the last two decades, a lot of accumulating discoveries show that not all the evolutionary game is genetic and selective. Phenotypic and developmental plasticity (morphologies and behaviors that vary under changing environmental circumstances without genetic modifications) is a powerful and widespread adaptive strategy that can even cause the diversification of species. Furthermore, the material on which the selective processes act is not only given by small, continuous, and spontaneous (p.358) genetic mutations. Rather, selection has to find trade-offs with internal constraints and channelings of development that do not always have a negative and limiting power but can positively orient evolution, generating crucial innovations.
According to a growing number of scholars, these findings should lead to an extension of the explanatory structure of the standard neo-Darwinian theory of evolution based on genetic variation and natural selection (Pigliucci and Müller 2010). The promoters of the extended evolutionary synthesis stress that the processes by which organisms grow and develop are active causes of evolution and speciation. While the views of the opposing side (the so-called standard theory) are polemically branded as “gene-centered” and too narrow (Laland et al. 2014), it is interesting to note that so far, the extended synthesis is strongly focused on organism as the fulcrum of evolutionary change. Development (so, the organismic level) is the influential internal “environment” of any variation. Due to epigenetic changes and inheritance and through the influence of developmental constraints biasing the emergence of novel phenotypic traits, the external environment can interact with organisms’ traits in a constructive way. However, even in the extended synthesis, the larger ecological scenario gains its relevance only through the mediation of lower causal levels (the organism in this case).
Considering what is happening in the field, the extended synthesis is already there, we need just to recognize it. Nevertheless, the so-called extended evolutionary synthesis does not have a coherent structure yet: it still seems a summation of observed processes, very frequent and deserving great attention, even though sometimes dramatized, without a common theoretical thread. The only glue that apparently emerges is the opposition to genetic reductionism, but the fight against the straw man of gene-centrism and the focus on organisms seem not enough to launch a post-Darwinian coherent theory of evolution.
For reasons of focusing, the extended synthesis is not yet a multilevel explanation of the evolutionary phenomena because it lacks a theoretical frame for the interconnections between different levels of change (ecological and genealogical). Let us take the example of another pillar of the extended synthesis: niche construction. The point is that organisms (the main focus) are not passive entities, malleable at will by selection. The metabolic and behavioral activities of biological populations (to build a termite mound, to erect a dam on the river, or to pursue a cultural and technological advancement) change the ecological niches, thus influencing the environmental resources and the selective pressures that in turn (p.359) retroact on organisms themselves. Such a process produces a crucial feedback and recursive phenomenon, called “niche construction” (Odling-Smee et al. 2003). Organisms actively change their environment, and the environments selectively change organisms.
Then the organism is an active player who codirects its own evolution, systematically changing the environments and so influencing the frame of selective pressures. Any biological population inherits from the previous generation not only a package of genes but also an amended ecological niche. If we see the process through the lens of hierarchy theory, niche construction becomes another example of water sloshing in Eldredge’s bucket. Selective pressures come from the ecological hierarchy, affecting populations of organisms in their differential survival. But organisms can actively transform their environments for adaptive reasons and so construct new ecological niches that will be the frame of selective pressures for the next generations. The feedback and recursive processes occur at different levels: the niche construction of a species affects the ecological niche of other species; a limited niche construction process can be extended, influencing a larger ecological surrounding.
Such a “multilevel niche construction” is an excellent example of the recursive relationships between the twin hierarchies, with ecological and macroevolutionary patterns as crucial drivers of evolution. In this sense, hierarchy theory is another kind of “extended synthesis,” a multilevel extended synthesis, still strongly Darwinian in its core (the bottom of the bucket). The dynamics of growth and extension of the theory of evolution is based on processes of theoretical revision and empirical enlargement of an elastic set of explanations already supported but constantly needing adjustments and integrations (Pievani 2012). New problems and apparent exceptions can be solved and understood through integrative explanations, different modulations of the empirical domain of already established patterns, and new quantitative calculations of the relative frequency of a pattern with respect to another.
In the case of punctuated equilibria theory (Eldredge and Gould 1972), after decades of debates, the emerging consensus around the mechanisms of speciation is that we need a multiplicity of processes and modes of birth of new species (punctuated in some ecological circumstances and gradual in others), a multiplicity of possible rates of speciation, and a multiplicity of levels of change (from an ecological and a genealogical point of view) to be considered (Coyne and Orr 2004). Thus the main methodological stance today is a calculation of the relative frequencies of one speciational pattern (punctuationism) with respect to another (gradualism and trends), (p.360) using for instance meta-analyses on molecular and integrated phylogenies (like in Pagel et al. 2006). A radical alternative among incompatible patterns (stasis, slow phyletic gradualism, abrupt punctuations) is no longer the case (about the causes of evolutionary and ecological stasis, see Brett et al. in this volume).
This is what we know as a “pluralistic” approach to biological evolution, considering that punctuated equilibria (being a matter of both “tempo” and “mode” of evolution—see Allmon in this volume) has been the access door to hierarchy theory. We see here a passage from an exclusive pattern (previous phyletic gradualism) to a plurality of patterns about the rates of speciation, being species a fundamental unit in macroevolutionary patterns. Summing up, hierarchy theory is a different extended synthesis able to cover all the levels that make the evolutionary game so complex, from genes to organisms to species and to the largest ecological scenarios. Its theoretical potential lies here.
The strong methodological assumption that every macroevolutionary phenomenon should be reduced to the uniform accumulation of microevolutionary processes has been definitely challenged. Adopting hierarchy theory as a theoretical tool of assimilation and accommodation in different evolutionary fields, we could better appreciate the multiple patterns involved in the most promising lines of evolutionary research today. The fieldwork of Peter and Rosemary Grant on the Galápagos finches—mixing genes and developmental and ecological conditions and involving all the core patterns of evolution (mutation, natural selection, drift, migration, hybridization, speciation, biological and cultural evolution, regional ecological disturbances)—is an outstanding case study of this extended toolbox (Grant and Grant 2008).
As for another example, regarding transgenerational epigenetic inheritance, rather than relying on the unlikely return of Lamarckism, any Darwinian pluralist should agree with Robert J. Schmitz: “These results provide strong evidence that epialleles contribute to the heritability of complex traits and therefore provide a substrate on which Darwinian evolution may act” (Schmitz 2014). Genetic and epigenetic inheritable variability is the complex substrate for Darwinian evolution to work. In epistemological terms, we see a diversification of the variational materials on which the selective processes may act (and not a revolutionary (p.361) neo-Lamarckian overthrow). Here and in many other examples, the structure of the current evolutionary research program is continuously evolving toward a more pluralistic version (Ayala and Arp 2010), and hierarchy theory is a promising candidate for such unification.
Biological evolution has expressed some recurrent “lawlike” patterns, somehow ordered configurations, or repeated schemes of historical events, but each time its specific historical outputs are unpredictable and unique (for a reconciliation of nomothetic and idiographic sciences in evolutionary biology according to a hierarchical perspective, see Lieberman in this volume). In the dialectic between the arrow of history and the cycle of recurrent regularities, when a pattern is assumed from data, it selects the pertinent further data and influences the scientific questions (i.e. it has a heuristic agency). Thus as a tool of scientific discovery and explanation, the pattern has both an epistemological status (it is in our minds) and an ontological status (it raises from objective data out there; Eldredge 1999). The task facing evolutionists is seeking recurrent patterns within a multiplicity of interconnected evolutionary lines whose trajectories appear highly unforeseeable. The goal is a more realistic comprehension of the evolutionary transformations and processes. In this sense, hierarchy theory is a metapattern gathering all the main patterns of evolutionary change (in this volume, William Miller III sketches a “metatheory” of evolution—namely, a “macroevolutionary consonance theory”—including also the macroecological aspect of evolution).
Today cosmology is somehow an “evolutionary” science. In the last decades, geology, thanks to the theory of plate tectonics, has become an important “evolutionary” discipline, and it is alongside biology and ecology in the reconstruction of the natural history of life on Earth. Ecologists discover historical patterns of coevolution between species and their environmental niches. Some patterns of change and repeated schemes of historical events seem to emerge from extremely diversified fields of research, even in the context of cultural and technological evolution. Even historians of science sight in the collective adventure of human knowledge a few “evolutionary” regularities. In the varied scientific disciplines involved in the study of history, a common sensibility for explanatory patterns of an evolutionary type is dawning, along with the possibility to apply more generally (even though prudently) the twin hierarchy of ecological (contextual) factors and internal factors.
As another future direction—following the idea that human social systems reintegrate the genealogical and the ecological domains at a level above the individuals—we could explore the possibility that the interactive (p.362) distinction between the economic, material, physical, and ecological dimension of evolutionary processes (the history of “matter in motion” and energy transferring) and the gene-molecular dimension of the transmission of biological information from one generation to the next applies also to nonstrictly biological evolutionary processes (i.e., social, cultural, and technological evolution). As an example, McKinney in this volume applies the hierarchy approach to describe the multidimensional reduction of diversity in the biosphere during the Anthropocene, focusing on urbanization and urban scaling.
This is an exciting time for evolutionary biologists, as their discipline is facing a vivid debate on its epistemological status. The parties involved are not simply dicotomically lined up. A plurality of contributions is working for the enrichment of the evolutionary research program (Pievani 2015). Human evolution, again, with its sets of open problems still looking for proper evolutionary explanations (the interspecific encephalization trend, the evolution of language, the appearance of behavioral modernity, etc.), stands as an ideal arena on which novel models, theories, and hypotheses can be advanced and whose validity can be tested. Hierarchy theory is able to display a sound coupling between acknowledged sets of heterogeneous data coming from paleoanthropological studies and heuristic and theoretical models, stressing the relevance of a multilevel analysis. Moreover, human evolution has to be understood in light of the complex interplay between biological and cultural evolution. This could be another exciting line of future researches in hierarchy theory.
The most recent archeological findings (D’Errico and Stringer 2011; D’Errico and Banks 2013) are informing us that Homo sapiens was not the only Homo species capable of behaviors whose complexity might have played a still underestimated evolutionary role. The emergence and persistence of material cumulative cultures (the capacity for accumulating modifications over time) represent an outstanding case of niche construction activity being able to change the ecological and social environment, systematically biasing selective pressures. The role of ecological and biogeographical factors in the evolution of material cultures in genus Homo is an area waiting for a systematic study, especially focusing on the different regional trajectories and on the potential interplay among cultural adaptations, brain plasticity, environmental changes, and ecological and demographical factors. Such a feedback and recursive biocultural process needs to be appreciated in its multilevel deployment: effects and by-products are recognizable at the individual, group, and ecosystem levels but also at the (p.363) behavioral, cognitive, neural, and genetic levels. Gene-culture coevolutionary explanations are among the most promising research paths in this field (Laland et al. 2010), and, although this is still poorly appreciated, their explanatory structure is intrinsically a multilevel one.
Evolutionary biology itself is an evolving scientific discipline, demanding for pluralistic explanatory models. Key concepts, advanced by the extended synthesis supporters, such as reciprocal causation (Laland et al. 2015), or catching the constructive relationship between the ecological environment and organisms’ behavior and development, could perfectly match with the multilevel framework proposed by hierarchy theory. The fruitfulness of such an enterprise would be twofold: (1) providing the coherent and unified structure that the pluralistic debate on the need for an updating of the evolutionary research program is looking for, and (2) addressing a set of specific unsolved or open problems (such as the ones mentioned above) with a proper set of extended explanatory tools, being able to put them under a new light in order to increase the empirical content of the set of explanations in respect of new observations.
New discoveries pile up. Models are changing. New concepts gain attention. The theory of evolution is evolving. The wish is that hierarchy theory could be another brick in this field under construction. To the young generations of evolutionary biologists, the task is shaping the future landscape of this fascinating interdisciplinary enterprise.
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