Pere Alberch (1954–1998)
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Pere Alberch Vié (1954–1998) was an experimental embryologist, theoretical biologist, and evolutionary biologist of Catalan origins who studied and developed part of his career in the USA. With a focus on herpetology, his empirical studies combined conceptual research, theoretical models, and experiments in order to integrate development and evolution. The 1980s were the most productive and innovative period of his career, when he was assistant professor and curator at the Museum of Comparative Zoology, Harvard University. In the 1990s, he continued his work as Director of the Museum of Natural History in Madrid, Spain. His contributions on topics such as heterochrony, developmental constraints, evolvability, possible variation, construction rules, the morphospace, or the “logic of monsters” have largely been conducive to shape the core concepts of evo-devo.
KeywordsEvo-devo Form Monsters Developmental constraints Morphospace Morphogenetic process Evolvability
Pere Alberch Vié (1954–1998) was born in Badalona, Spain, on the 2nd of November 1954. He manifested an interest in natural history very early on and wrote his first two scientific papers on amphibians when he was only 19 years old.
In 1973 Alberch entered the University of Kansas, where 3 years later he completed a bachelor’s degree with a double major in philosophy, and systematics and ecology. In 1976 he joined the University of California at Berkeley, to write his PhD in Zoology under the co-supervision of David Wake, an evolutionary biologist specialized in salamanders, and George Oster, a mathematical biologist and dynamical systems theorist.
In 1980 Alberch was hired by Harvard University as assistant professor and assistant curator of the Museum of Comparative Zoology. He published his better-known papers on evolution and development in this period and supervised the work of several students who were to become prominent researchers in evo-devo, such as Neil Shubin, Cliff Tabin, Ann Burke, and Chris Rose.
In 1989 Harvard declined to give him tenure, and Alberch moved to Madrid where he was hired as Research Professor at the Spanish Research Council (CSIC) and as Director of the National Museum of Natural Sciences. There he carried on with theoretical and empirical research, while he was also committed to a thorough renovation and modernization of the Museum. In 1995 Alberch fell seriously ill and had to quit the directorship of the Museum and slow down his research activities. After 3 years, he accepted a research position at the Institute Cavanilles for Biodiversity and Evolutionary Biology, Valencia. While still in Madrid, he died at the age of 43 on the 13th of March 1998.
Most of Alberch’s papers, published in the major academic journals of the field, are tightly integrated in a distinctive research framework for the study of how the ontogenetic generation of morphological variation influences evolution. His most relevant contributions were written between 1980 and 1989, but some significant ones appeared also in the 1990s and comprised various topics such as the relationship between science and art or museum curatorship and management (see papers collected in Rasskin-Gutman and De Renzi 2009; De Renzi 1999, 2009; Moya and Peretó 2010; Reiss et al. 2009; Wake 1998). Alberch’s work is unique owing to his inventiveness in persuasively formulating and pursuing compelling new research paths in evolutionary biology and to his ability to illustrate theoretical claims, such as the role of developmental constraints, through original and audacious experiments. Within the framework of the integrative biology cultivated by David Wake’s group, Alberch’s contributions focused on the mechanistic approach to the generation of form, which he considered to have been largely neglected by the evolutionary biology of his time.
A Theory of Form
The role that morphology played in Alberch’s work gives credence to the thesis that morphology had a central position in the origination of evo-devo as a discipline (Love and Raff 2003; Love 2003). Alberch conceived of the study of morphological variation, of its generation and of its evolutionary significance largely arose within Jacobian interplay of what is possible and what is actual, at the intersection between the forms enabled by morphogenetic processes and those extant ones adapted to the local contingencies of the environment (Alberch 1982).
In contrast with the linear genotype-phenotype map of random and continuous variation assumed by neo-Darwinian models of evolution, Alberch argued that the patterns of phenotypic variation are clustered around major “themes corresponding to taxa or classes of teratologies” (see the chapter on the “Genotype-Phenotype Map”). When new morphological themes arise, the transitions among them are not random (Oster and Alberch 1982, p. 444), as morphological variants cluster in discrete groups of patterns. Of special interest are cases where patterns of variation cannot be explained by natural selection, such as the same recurrent phenotypic variants in widely unrelated species (Alberch 1983), or how, despite being strongly selected against, teratologies are generated in a regular way, following what he called a “logic of monsters” (Alberch 1989). He also explored how functional constraints related to the integration of the parts of the organism influence the appearance of similar convergent structures. Thus, after an invasion of new habitats in a highly diverse genus of salamanders (Bolitoglossa), convergent structures can be a response to adaptive requirements, but also a result of ontogenetic developmental correlations among parts (Alberch 1981; see the chapter on “Convergence”), the recurrence of the same phenotypic variants in widely unrelated species, and the “logic of monsters”, insofar as, despite being strongly selected against, teratologies are generated in a recurrent way.
All these phenomena underpin Alberch’s view of the morphospace as a discrete cluster of forms. Influenced by David Raup’s theoretical morphology, Alberch observed that possible forms are not ubiquitous, because they do not fully occupy the space of conceivable forms. Unlike the standard view of population genetics, which assumes that phenotypic variations are generated by random small mutations later fixed by natural selection, the properties of the morphospace demand that morphological evolution be studied from a developmental perspective (Alberch 1980). Accordingly, evolutionary biology should explain not only the fixation of variant morphs in different populations, but also the developmental processes in which these novel variants generate or originate (Oster and Alberch 1982, p. 455). Patterns of variation appear as an order of forms inherently arising in development, their properties largely resulting from epigenetic interactions at the cellular level (Alberch 1983, 1985b).
Thus, Alberch vindicates an internalist approach to evolution and development: “I focus on the internal rules that control the appearance of morphological variation, on the mechanistic basis of such rules and on the evolutionary consequences of this internally determined order” (Alberch 1989, p. 28). The internalist program was a rather striking position and an extremely underdeveloped research line in the evolutionary biology of the time, opposed to the externalist or adaptationist program focused on natural selection (see the chapter on “Internalism”).
Related to internalism is Alberch’s vindication of a “theory of form” based on the global properties of the network of developmental interactions, independent of the adaptive role played by the resulting forms (Alberch 1989, p. 39). According to Alberch, neither genes nor environment specify form, generated by internal rules, but both mutations and environmental perturbations can change the outcome of development. Developmental systems are stable, endowed with the intrinsic, regulatory, and pattern-generating properties characteristic of complex dynamical systems.
Importantly, from this perspective, the notion of “biological function” is not restricted to the contribution of a character to adaptation by natural selection. Rather, in the internalist approach, functional constraints are those “imposed by functional interactions among different parts of the organism” (Alberch 1981, p. 84). Thus, new morphologies need to be integrated with the rest of the organism interacting with the environment (Alberch 1982b; see the chapter on “Internal Selection”).
From Heterochrony to Morphogenetic Processes
Chronologically, Alberch’s view on the relation between ontogeny and phylogeny progressed from a focus on the role of heterochrony, i.e., “the role of change in the relative timing of developmental events” (Hanken 2015), towards a mechanistic explanation of how developmental processes generate possible variations for evolution (the chapter on “Heterochrony”).
Since the late 1970s, heterochrony has experienced a renaissance in evolutionary biology (Gould 1977; see Hanken 2015 and Wake 2015), and at the beginning of his career, Alberch himself considered that studies on heterochrony were crucial for the emergence of the field of evo-devo (Alberch 1995, p. 230). His interest in how ontogenesis influences morphological diversification started with a paper that elaborated Gould’s clock model for describing how heterochronic changes in ontogeny are related to phyletic trends (Alberch et al. 1979; see also the chapter on “Stephen Jay Gould (1941–2002)”). There Alberch and collaborators offered a dynamic and quantitative version of the clock model, characterizing the modifications in development that produce relative changes in size and shape and defining heterochrony in terms of shifts in developmental processes (onset, cessation, or rate of growth) rather than of end results. This approach was applied in subsequent empirical work on heterochrony in the salamander Bolitoglossa occidentalis (Alberch and Alberch 1981).
However, already as early as 1985 Alberch challenged what he then characterized as a “static,” descriptive approach underlying heterochrony models to pursue a more dynamical, causal approach to development (Alberch 1985a; Oster et al. 1988; Alberch and Blanco 1996; see Nuño de la Rosa and Etxeberria 2012, p. 267). According to him, the static framework, inheritor of the traditional recapitulationist approach in comparative morphology, conceived of development as a sequence of discontinuous morphological stages conserved in evolution. In contrast, in the new dynamical approach, the changes between two related morphologies should “be searched for in the developmental rules of interaction or initial conditions” (Alberch 1985a, p. 51), instead of looking at the intermediate ontogenetic stages.
Following the tradition of experimental embryology, Alberch favored a “mechanistic” method in biology to explain how forms generate dynamically in development, as opposed to the standard view in evolutionary biology in which the origin of variation is taken for granted. This addressed the generation of morphological variation in ontogeny at the level of cellular dynamics, following an approach that had already come out in the early interactions with David Wake and George Oster. The aim was to capture how developmental “construction rules” emerge as dynamical systems mechanisms which remain stable during long periods of time, with a certain range of variation due to the alteration of developmental parameters. Construction rules arise from interactions among different “resources” at different organizational levels, from molecules up to tissues. Alberch thought that these rules “allow us to determine the relationships among different phenotypes, since the set of possible phenotypic transformations will be constrained by the generative potentialities of the morphogenetic rules involved in the process” (Alberch 1982, p. 321). In order to investigate these morphogenetic rules, he carried out experiments to determine their material, physico-chemical properties and studied their formal properties through dynamical systems theory models.
A Cyclical View of Development
In an important conceptual contribution, Alberch argued that development is not the result of gene expression, but of entangled feedback processes going back from tissues into the genome itself. In his own words: “This depiction of genes and development as independent levels is incorrect in the sense that genes do not specify development, or even form, because gene action itself is intimately linked to developmental interactions” (Alberch 1991, p. 5). The distinction between a “hierarchical” and a “cyclical” scheme of development underlies Alberch’s approach to evolution (Alberch 1991). The former portraits an extreme version of the neo-Darwinist view of genes as directly prescribing developmental processes that, in turn, specify morphology. This view reduces development and evolution to purely genetic problems, demoting development to a sequence of gene expression and evolution to a change in gene frequencies.
In several papers, Alberch emphasized the shortfalls associated to the concept of causality underlying this hierarchical scheme of development. First, such an open loop system would be extremely unstable against the random genetic and environmental perturbations of normal development. Second, the relation between genes and phenotypic traits is not a one-to-one correspondence (Alberch 1983). Rather, the effect of genes on morphology is mostly indirect: genes code for molecules which either regulate the expression of other genes or confer properties on cells (e.g., cell division rates, apoptosis, differentiation timing, or cytoskeletal properties), which then construct organs and structures in accordance with physico-chemical laws. Developmental interactions have properties that emerge from the dynamics of the system; they are not encoded in the genome (Alberch 1987, 1991; Oster and Alberch 1982; Oster et al. 1988). Moreover, due to the highly context-sensitive character of gene expression, similar genetic changes may yield different morphological effects, and the other way around. Therefore, in Alberch’s view, phenotypic diversity is not so much the product of new genes as of permutations in context (i.e., the timing and location of expression) of existing genes. The evolutionary consequences of this asymmetry are obvious: there are qualitative differences between modes of evolution at the genetic and at the epigenetic levels, and therefore there is often no direct correspondence between genetic and morphological divergence (Alberch 1983).
In the alternative “cyclical” scheme of development embraced by Alberch, “gene expression is both the cause and the effect of a morphogenetic process” (Alberch 1991, p. 6). Developmental processes are divided in three interacting levels, including gene interactions, proteins and enzymes generating cell properties involved in morphogenesis, and tissue interactions (Alberch 1982a, p. 320). Following Waddington’s ideas, Alberch considered that these regulatory interactions specify the epigenetics according to which phenotypes are well-buffered systems with respect to both genetic and environmental perturbations during ontogeny (Alberch 1980; see also the chapters on “Robustness” and “Conrad Hal Waddington (1905–1975)”).
A Dynamical Systems Theory of Developmental Evolution
Formally, Alberch appealed to the conceptual and mathematical tools of dynamical systems theory to study developmental processes “where a small set of simple rules of cellular and physico-chemical interaction can interact to generate a complex morphology” (Oster and Alberch 1982, p. 455). Construction rules are formally captured as developmental parameters, whereas genetic or environmental alterations of development are mathematically abstracted as parameter perturbations (Alberch 1982, p. 323). Thus, “morphological diversity is generated by perturbations in parameter values (such as rates of diffusion, mitotic rate, cell adhesion, etc.) while the structure of the interactions among the components remains constant” (Alberch 1989, p. 27).
In the framework of dynamical systems theory, the possible pathways of transformation among phenotypes are visualized using transformational diagrams (Alberch 1991). Each species or trait has a unique transformation diagram dependent on its position in the parameter space, and smooth perturbations of the parameters (resulting from genetic mutation or experimental manipulation) can result in a limited set of phenotypes. Alberch illustrated this idea with cases of teratologies, showing that even nonadaptive variations are discrete and constrained by developmental transformation rules generating the space of possible forms. In other words, even “monsters” have a logic. Thus, Alberch aimed at formalizing the stabilities and bifurcations of Waddington’s epigenetic landscape with the language and mathematical tools of dynamical systems theory. In this framework, phenotypic stabilities are seen as emerging from dynamical attractors, regions in the parameter space where small perturbations do not disrupt the basic organization of development, whereas bifurcations correspond to developmental thresholds (e.g., critical cell number or inductive relationships) so that modifications that go beyond them may cause nonlinear effects. Thus, continuous changes of developmental parameters can result in phenotypic discontinuities. Both stability and the direction of variation depend on the formal properties of the developmental system. Transformational diagrams show the potential evolutionary transformations of phenotypes, predicting the most probable ones in the absence of external forces. They can be used as a “null hypothesis” of evolutionary transformations, because selection can only drive phenotypes along the internally specified directions. Thus, in Alberch’s view, the dynamical properties of developmental systems limit possible variation in phenotypic space, but at the same time, provide potential directions to evolutionary change.
Unlike the neo-Darwinian view of evolution based on chance and contingency, Alberch believed that the study of variation was partly deterministic and predictable, since an understanding of developmental mechanisms “allows for predictions of what patterns of variation should be expected” (Alberch 1983, p. 915; see also Alberch 1982, p. 314; Etxeberria and Nuño de la Rosa 2009). “In evolution” – he argued – “selection may decide the winner of a given game but development non-randomly selects the players” (Alberch 1980, p. 665). Nonetheless, contrary to other internalist approaches such as Goodwin’s process structuralism, Alberch did not conceive of evolution as an absolutely deterministic process, but as a relative one (Alberch 1981; on this debate, see the chapter on “Inherency”). In his view, the morphogenetic level emerges as a realm of determinism between two sources of uncertainty: the irreducible stochasticity of cell dynamics coming from development (Oster and Alberch 1982, p. 444) and the historical contingency resulting from the interaction between development and selection, given that the most probable forms from the point of view of development might or might not be those favoured by selection. As a result, randomness, determinism, and contingency coexist in biological processes, leading to a “world of opportunity within constraint” (Alberch 1986, p. 8).
An Experimental Approach to Development and Evolution
In addition to his innovative work, Alberch proved to be a bright experimental embryologist, determined to capture the mechanical and chemical aspects of morphogenesis (Oster et al. 1988). In a series of papers written together with Emily Gale, the influence of perturbations of developmental parameters on the generation of new forms was studied experimentally (Alberch and Gale 1983, 1985, 1986; Alberch 1986; Alberch et al. 1986). Alberch and Gale compared the results of treating the limb buds of a frog and a salamander with colchicine, a mitotic inhibitor. This treatment results in various abnormal morphologies such as limbs of smaller size and with some skeletal elements missing. However, these malformations exhibited a high degree of order, leading the authors to conclude that most of the patterns of diversity of digital morphology in amphibians could be explained as a reflection of developmental properties (Alberch and Gale 1985). In particular, Alberch used these experimental results to test the hypothesis that the digital pattern is affected by reduction in the number of mesenchymal cells in the limb bud. Changes in pattern formation took place when the size and the number of cells of the limb bud were reduced under a critical value, a result consistent with the mathematical models studied with Oster (Oster et al. 1988).
In Alberch’s work, the formal and the experimental approaches were always seen as complementary. The theoretical consequences of the experimental manipulation of development for our understanding of evolution were, according to him, twofold. First, the experimentally generated patterns of variation can be compared with the patterns of natural variation, thus facilitating phylogenetic inferences and tracing possible evolutionary pathways. For example, salamanders develop their limbs in a very different way from other tetrapods because the sequence of digit formation appears inverted (Alberch and Gale 1983). Digit reduction is a phenomenon that has taken place several times independently in amphibian evolution (see the chapter on “Parallelism”). Frogs usually lose their most internal digit (preaxial), whereas salamanders lose the most external one (postaxial). The result is a parallelism between experimentally generated patterns and the evolutionary trends towards digit reduction observed in the wild (Alberch and Gale 1985). Second, the variation generated in the laboratory is bounded, revealing that natural morphologies are limited by the system’s morphogenetic properties. Only the interaction parameters, rather than the basic morphogenetic rules, seem to have changed during the evolutionary history of vertebrates (Shubin and Alberch 1986). Quantitative variations of these parameters may produce qualitative alterations such as changes in the branching and segmentation sequences, but since the rules of interaction remain the same, we are only able to explore their potentialities, mostly reiterating forms that have already been realized in evolution (Alberch 1991).
Developmental Constraints and Evolution
Alberch’s mechanistic view of development underlies his experimental and theoretical elaboration of the concept of “developmental constraint,” a notion that was being intensely discussed in the field in the 1980s. Alberch’s work arouse the interest in discussing phenomena associated with this concept, and it constituted his most well-known contribution to evo-devo, especially because of the experiments conducted in collaboration with Emily Gale which became crucial exemplars of developmental constraints (Alberch 1982b, 1985b, 1986, 1989; Alberch and Gale 1983; Maynard Smith et al. 1985; see the chapter on “Developmental Constraint”). While the notion of developmental constraint became very famous after Gould and Lewontin’s Spandrels paper (1979), it was imported to biology from fields close to classical mechanics, where a constraint is understood as some limitation of degrees of freedom which at the same time drives or canalizes the system within a path that enables some novelty. In fact, this sense of a limitation of variation at one level combined with the emergence of possibilities at a higher level had been already noticed in a very influential paper by François Jacob (1977), which was discussed in Wake’s lab (Wake 2015), and underlies Alberch’s own use of the term.
As several scholars have recognized, since then the notion of constraint has struggled between these two seemingly contradictory senses of limiting factors of the variation available to natural selection and generative factors for organizing form (Amundson 1994; Brigandt 2015; Gould 1989; Schwenk 1995). Alberch’s developmental constraints act on possible forms and not on natural selection (Amundson 1994). In his view, epigenetic interactions do not constrain natural selection, but how genetic mutations are expressed at the morphological level. As a consequence, constraints reflect the intrinsic abilities of developmental systems to generate “a biased subset of phenotypes upon which natural selection or population stochastic factors can operate” (Alberch and Gale 1985, pp.19–20). They do not only limit the universe of possible novelties in evolution, but also “impose directionality in morphological transformations through phylogeny” (Alberch 1980, p. 654). Therefore, constraints trim adaptationist optimality thinking, but most importantly, they inspire the evolutionist’s search of the source of innovations and possible directions of future evolutionary change.
Alberch’s views of development and evolution entailed a deep reformulation of the classical notion of homology (see the chapter on “Developmental Homology”). In his view, homologies should be established on the basis of “the developmental processes which created them, rather than on their final geometric form” (Oster et al. 1988, p. 877). For example, the skeletal structure of the vertebrate limb was explained by a mechanistic model of embryonic branching and segmentation in initial chondrogenesis. According to this model, the loss of a digit may result from a failure of a branching bifurcation, and then, it is not sensible to ask “which” digit was lost, since it is the basic sequence what has been altered in evolution. Thus, from a developmental perspective, the units of comparison for homologies are no longer the morphological elements, but the morphogenetic processes generating them (Oster et al. 1988).
The replacement of typological thinking by population thinking was seen by Ernst Mayr as Darwin’s greatest achievement, and every biological work suspicious of endorsing typology was censured as linked to essentialism and idealism, the big obstacles to evolutionism. In contrast, Alberch conceived of his own work as endorsing a form of typological thinking formulated in a purely mechanistic context, disconnected from a metaphysical commitment to immutable essences (Love 2003). In his own words: “The quest for a general set of principles of form is legitimate if we exchange the metaphysical concept of the Bauplan for a mechanistic one based on principles of morphogenesis and internal integration” (Shubin and Alberch 1986, p. 377). Thus, in Alberch’s work on tetrapod limbs, type is not seen as an ideal entity, but as the result of a historically conserved developmental process which determines the range of possible variation upon which selection can act.
Alberch’s understanding of developmental constraints as positive causal factors of evolution is particularly well illustrated in his work on evolvability (see the chapter on “Developmental Evolvability”). After Richard Dawkins (1989) coined the term, Alberch published an article on the differential capabilities that make developmental systems “better at evolving” (Alberch 1991, p. 9). From a developmental perspective, evolvability requires that developmental systems remain stable against perturbations, but not so much as to be immune to absorb change and variation. Alberch argued that the general properties of developmental systems define their evolvability and allow evolutionary biologists to think of a new level of selection, “one that … does not act on the phenotype nor on the genotype, but rather on the emergent properties of developmental systems” (Alberch 1991, p. 10). Selection among pattern-generating systems would favor those that “exhibit the adequate balance between stability and potentiality to generate sufficient phenotypic variability” (Ibid.).
While the conservation of sets of interaction rules within ontogenetic types constrains the range of creativity of developmental systems, the truly creative mechanism that can produce really new forms is the transformation of the generative space by changing or removing some of these rules (see the chapter on “Epigenetic Innovation”). Alberch saw the Cambrian explosion as one of the best examples of this form of creativity: “the invention of multicellularity, segmentation or the sequestration of the germ line appear … to have been key developmental events that have speeded up the evolutionary proliferation of lineages” (Alberch 1991, p. 9). He thought that the Cambrian was a period of experimentation in rules of cell-cell interaction, rules that exhibited different form-generating abilities as well as distinct stability properties (Alberch 1991). After this period–he concluded, no qualitatively new structural body plans seem to have appeared, and morphological variation looks as if reduced to variations within extant themes.
The main threads of current evo-devo have advanced after ideas and research projects in which Alberch, along with many others, was involved during the last quarter of the twentieth century. In particular, the Dahlem Conference of 1982 organized by John Tyler Bonner (Bonner 1982), and in which Pere Alberch took part along with his mentors David Wake and George Oster, is considered a landmark in the history of the discipline (Love 2015). Although his earlier work focused on models of heterochrony, and his collective paper on the clock model remains as his most cited article, Alberch’s name is especially associated with his efforts to clarify the notion of developmental constraints in evolution. It was at the Dahlem conference that he presented his views on constraints, which would become highly influential after the publication of that paper (Alberch 1982a) and particularly since the 1985 collective article headed by John Maynard Smith (Maynard Smith et al. 1985). Following Amundson’s distinction between the notions of constraint used by adaptationist and developmentalist evolutionary biologists (Amundson 1994), Alberch’s theoretical and experimental work on constraints has become the major illustration of the “constraints on form” versus the “constraints on adaptation.”
Moreover, Alberch also appears as one of the core exponents of the positive notion of constraint. Schwenk (1995) and Brigandt (2015) have shown that the notion of developmental constraint was central in the field of evolution and development until the 1990s, while other positive concepts such as evolutionary novelty and evolvability have become more prominent in contemporary evo-devo. As we saw in examining Alberch’s early contributions to evolutionary novelties and evolvability, he was a forerunner of this shift of trend. Today Alberch is widely regarded as one of the founders of the evolvability research agenda, and particularly as the precursor of the developmental approach to evolvability (Pigliucci 2008). His 1991 paper contained some of the core conceptual elements that would later become central in evo-devo approaches to evolvability, including the concept of evolvability as a property depending on the G-P map, or the combination of robustness and flexibility as a basic property of evolvable developmental systems (Nuño de la Rosa 2017).
After the discovery of shared developmental genes across animal phyla in the late 1970s, the developmental genetics approach to evolution became the main trend of research in “molecular evo-devo” (Rasskin-Gutman 2009). Nonetheless, it is distinctive of authors in Alberch’s tradition (including Gavin de Beer, Conrad H. Waddington, or Stephen J. Gould as precursors, and Gerd Müller, Stuart Newman, or Isaac Salazar-Ciudad thereafter) to focus on the morphogenetic level. In this sense, Alberch also emerges as one of the pioneers of a “morphological evo-devo” (Nuño de la Rosa and Etxeberria 2012; Olsson 2012; Rasskin-Gutman 2009), where the variational properties of developmental processes, rather than gene regulatory pathways, explain the bounded patterns of morphological variation (see the chapters on “Mechanisms of Pattern Formation in Evolution,” and “Inherency”).
Alberch was also highly influential in the rescue of types and homology in evo-devo. While the general philosophical disapproval of typology and essentialism in biology resulted in difficulties in understanding the nature of variation at the morphological level, Alberch’s insistence on the importance of studying the patterns of phenotypic variation was decisive for the evo-devo approaches interested in pattern formation. Moreover, Alberch’s views on homology have played a significant role in the theoretical and philosophical discussions on the notions of type and homology. By grounding homological relationships in developmental processes, Alberch can be considered as one of the founders of the biological homology concept (Wagner 1989). His work on the homology of vertebrate limbs turned out to be the most cited case-study to illustrate this view (see, e.g., Wagner and Laubichler 2001; Rieppel 2006), and partly inspired the philosophical reinterpretation of homologies and body plans as “homeostatic property clusters” kinds. In this view, the essence of a natural kind is no longer identified with the properties that characterize that kind, but with the causal, developmental processes that account for the similarity of its members (Wagner 1996; Rieppel 2005; see the chapter on “Typology and Natural Kinds in Evo-Devo”).
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