Saltational evolution: hopeful monsters are here to stay

Since 150 years it is hypothesized now that evolution always proceeds in a countless number of very small steps (Darwin in On the origin of species by means of natural selection or the preservation of favoured races in the struggle of life, Murray, London, 1859), a view termed “gradualism”. Few contemporary biologists will doubt that gradualism reflects the most frequent mode of evolution, but whether it is the only one remains controversial. It has been suggested that in some cases profound (“saltational”) changes may have occurred within one or a few generations of organisms. Organisms with a profound mutant phenotype that have the potential to establish a new evolutionary lineage have been termed “hopeful monsters”. Recently I have reviewed the concept of hopeful monsters in this journal mainly from a historical perspective, and provided some evidence for their past and present existence. Here I provide a brief update on data and discussions supporting the view that hopeful monsters and saltational evolution are valuable biological concepts. I suggest that far from being mutually exclusive scenarios, both gradual and saltational evolution are required to explain the complexity and diversity of life on earth. In my view, gradual changes represent the usual mode of evolution, but are unlikely to be able to explain all key innovations and changes in body plans. Saltational changes involving hopeful monsters are probably very exceptional events, but since they have the potential to establish profound novelties sometimes facilitating adaptive radiations, they are of quite some importance, even if they would occur in any evolutionary lineage less than once in a million years. From that point of view saltational changes are not more bizarre scenarios of evolutionary change than whole genome duplications, endosymbiosis or impacts of meteorites. In conclusion I argue that the complete dismissal of saltational evolution is a major historical error of evolutionary biology tracing back to Darwin that needs to be rectified.     

The roots of phylogeny: how did Haeckel build his trees?

Haeckel created much of our current vocabulary in evolutionary biology, such as the term phylogeny, which is currently used to designate trees. Assuming that Haeckel gave the same meaning to this term, one often reproduces Haeckel's trees as the first illustrations of phylogenetic trees. A detailed analysis of Haeckel's own evolutionary vocabulary and theory revealed that Haeckel's trees were genealogical trees and that Haeckel's phylogeny was a morphological concept. However, phylogeny was actually the core of Haeckel's tree reconstruction, and understanding the exact meaning Haeckel gave to phylogeny is crucial to understanding the information Haeckel wanted to convey in his famous trees. Haeckel's phylogeny was a linear series of main morphological stages along the line of descent of a given species. The phylogeny of a single species would provide a trunk around which lateral branches were added as mere ornament; the phylogeny selected for drawing a tree of a given group was considered the most complete line of progress from lower to higher forms of this group, such as the phylogeny of Man for the genealogical tree of Vertebrates. Haeckel's phylogeny was mainly inspired by the idea of the scala naturae, or scale of being. Therefore, Haeckel's genealogical trees, which were only branched on the surface, mainly represented the old idea of scale of being. Even though Haeckel decided to draw genealogical trees after reading On the Origin of Species and was called the German Darwin, he did not draw Darwinian branching diagrams. Although Haeckel always saw Lamarck, Goethe, and Darwin as the three fathers of the theory of evolution, he was mainly influenced by Lamarck and Goethe in his approach to tree reconstruction.     

The Impact of Lamarck’s Theory of Evolution Before Darwin’s Theory

This paper analyzes the impact that Lamarckian evolutionary theory had in the scientific community during the period between the advent of Zoological Philosophy and the publication Origin of Species. During these 50 years Lamarck’s model was a well known theory and it was discussed by the scientific community as a hypothesis to explain the changing nature of the fossil record throughout the history of Earth. Lamarck’s transmutation theory established the foundation of an evolutionary model introducing a new way to research in nature. Darwin’s selectionist theory was proposed in 1859 to explain the origin of species within this epistemological process. In this context, Charles Lyell’s Principles of Geology and Auguste Comte’s Cours de Philosophie Positive appear as two major works for the dissemination of Lamarck’s evolutionary ideology after the death of the French naturalist in 1829.     

Charles Darwin and the problem of evolutionary progress

According to Ch. Darwin's evolutionary theory, evolutionary progress (interpreted as morpho-physiological progress or arogenesis in recent terminology) is one of logical results of natural selection. At the same time, natural selection does not hold any factors especially promoting evolutionary progress. Darwin emphasized that the pattern of evolutionary changes depends on organism nature more than on the pattern of environment changes. Arogenesis specificity is determined by organization of rigorous biological systems - integral organisms. Onward progressive development is determined by fundamental features of living organisms: metabolism and homeostasis. The concept of social Darwinism differs fundamentally from Darwin's ideas about the most important role of social instincts in progress of mankind. Competition and selection play secondary role in socio-cultural progress of human society.     

PERSPECTIVE: DARWIN,PROGRESS, AND ECONOMIC PRINCIPLES

Darwin's concept of progress was an economic one, with the accumulation of adaptations having a strong analogy to technological innovations. This interpretation makes it easier to understand his differences with Lamarck and others whose views he considered metaphysically objectionable. It also clarifies his relationship to Malthus and such features of his theory as the episodic nature of evolutionary change. His position is consistent with modern theories of biotic invasions and long-term evolutionary trends. It also accords well with current efforts to produce a synthesis between biology and economics.     

Multiple Explanations in Darwinian Evolutionary Theory

Variational evolutionary theory as advocated by Darwin is not a single theory, but a bundle of related but independent theories, namely: (a) variational evolution; (b) gradualism rather than large leaps; (c) processes of phyletic evolution and of speciation; (d) causes for the formation of varying individuals in populations and for the action of selective agents; and (e) all organisms evolved from a common ancestor. The first four are nomological-deductive explanations and the fifth is historical-narrative. Therefore evolutionary theory must be divided into nomological and historical theories which are both testable against objective empirical observations. To be scientific, historical evolutionary theories must be based on well corroborated nomological theories, both evolutionary and functional. Nomological and general historical evolutionary theories are well tested and must be considered as strongly corroborated scientific theories. Opponents of evolutionary theory are concerned only with historical evolutionary theories, having little interest in nomological theory. Yet given a well corroborated nomological evolutionary theory, historical evolutionary theories follow automatically. If understood correctly, both forms of evolutionary theories stand on their own as corroborated scientific theories and should not be labeled as facts.     

Evolutionary concepts meet the neck of penguins (Aves: Sphenisciformes), towards a “survival strategy” for evo-devo

Evolutionary developmental biology (or evo-devo) is the scientific connectivity that allowed a more comprehensive and practical completeness in the contemporary conceptualisation of evolution. The links between genetics, developmental mechanics and evolution led to a better understanding of evolutionary mechanisms. An analysis of evolutionary concepts such as homology, homeoses, constraints, novelties, modularity, and selection is given through the recurring example of the variations identified in the modular repartition of the cervical vertebrae in extant and fossil penguins. The inclusion of this study about penguins in the evolutionary system also involves a reflection on the current state and the future of evo-devo. Three principles of assessment and method, applicable to many natural and conceptual scales, are introduced to define a ??survival strategy?? for evo-devo. The above-mentioned principles are intended to strengthen and continue the connectivity induced de facto. These current and future investigation challenges are discussed and connected to three main naturalist names related directly to the conceptualisation of evolution: Charles Darwin, étienne Geoffroy Saint-Hilaire, and Lamarck.     

Zum konzept der »inneren« selektion: Stellungnahme zu einer evolutionstheoretischen kontroverse

Recently the concept of natural selection in Darwin’s sense has been criticized by some authors. It has been argued that this concept does not explain certain phenomena of evolutionary change, especially in the reach of macroevolution. Some biologists, therefore, demanded for evolution a new model of selection which focuses internal factors in phytogeny. — This paper is a brief discussion of some aspects of “internal” selection and its meaning in contemporary evolutionary biology. The argument of the paper is that evolution can only be explained by a theory taking cognizance of interactions between external and internal selective agencies. Such a theory would be a systems theory of evolution.     

The Omo-Turkana Basin fossil hominins and their contribution to our understanding of human evolution in Africa

The Omo-Turkana Basin, including the hominin fossil sites around Lake Turkana and the sites along the lower reaches of the Omo River, has made and continues to make an important contribution to improving our murky understanding of human evolution. This review highlights the various ways the Omo-Turkana Basin fossil record has contributed to, and continues to challenge, interpretations of human evolution. Despite many diagrams that look suspiciously like comprehensive hypotheses about human evolutionary history, any sensible paleoanthropologist knows that the early hominin fossil record is too meager to do anything other than offer very provisional statements about hominin taxonomy and phylogeny. If history tells us anything, it is that we still have much to learn about the hominin clade. Thus, we summarize the current state of knowledge of the hominin species represented at the Omo-Turkana Basin sites. We then focus on three specific topics for which the fossil evidence is especially relevant: the origin and nature of Paranthropus; the origin and nature of early Homo; and the ongoing debate about whether the pattern of human evolution is more consistent with speciation by cladogenesis, with greater taxonomic diversity or with speciation by anagenetic transformation, resulting in less taxonomic diversity and a more linear interpretation of human evolutionary history.     

On some historical and theoretical foundations of the concept of chordates

The concept of chordates arose from the alliance between embryology and evolution in the second half of the nineteenth century, as a result of a theoretical elaboration on Kowalevsky’s discoveries about some fundamental similarities between the ontogeny of the lancelet, a putative primitive fish, and that of ascidians, then classified as molluscs. Carrying out his embryological studies in the light of Darwin’s theory and von Baer’s account of the germ layers, Kowalevsky was influenced by the German tradition of idealistic morphology that was concerned with transformations driven by laws of form, rather than with a gradual evolution occurring by means of variation, selection and adaptation. In agreement with this tradition, Kowalevsky interpreted the vertebrate-like structures of the ascidian larva according to von Kölliker’s model of heterogeneous generation. Then, he asserted the homology of the germ layers and their derivatives in different types of animals and suggested a common descent of annelids and vertebrates, in agreement with Saint-Hilaire’s hypothesis of the unity of composition of body plans, but in contrast with Haeckel’s idea of the Chordonia (chordates). In The Descent of Man Darwin quoted Kowalevsky’s discoveries, but accepted Haeckel’s interpretation of the ascidian embryology within the frame of a monophyletic tree of life that was produced by the fundamental biogenetic law. Joining embryology to evolution in the light of idealistic morphology, the biogenetic law turned out to be instrumental in bringing forth different evolutionary hypotheses: it was used by Haeckel and Darwin to link vertebrates to invertebrates by means of the concept of chordates, and by Kowalevsky to corroborate the annelid theory of the origin of vertebrates. Yet, there was still another interpretation of Kowalevsky’s discoveries. As an adherent to empiricism and to Cuvier’s theory of types, von Baer asserted that these discoveries did not prove convincingly a dorsal position of the nervous system in the ascidian tadpole larva; hence, they could not support a homology between different animal types suggesting a kinship between ascidians and vertebrates.     

DNA structural variability as a factor in gene expression and evolution

Redundant DNA can buffer sequence dependent structural deviations from an ideal double helix. Buffering serves a mechanistic function by reducing extraneous conformational effects which could interfere with readout or which would impose energetic constraints on evolution. It also serves an evolutionary function by allowing for gradual variations in conformation-dependent regulation of gene expression. Such gradualism is critical for the rate of evolution. The buffer structure concept provides a new interpretation for repetitive DNA and for exons and introns.     

Evolutionary theory and systematics: relationships between process and patterns

The relevance of the Modern Evolutionary Synthesis to the foundations of taxonomy (the construction of groups, both taxa and phyla) is reexamined. The nondimensional biological species concept, and not the multidimensional, taxonomic, species notion which is based on it, represents a culmination of an evolutionary understanding. It demonstrates how established evolutionary mechanisms acting on populations of sexually reproducing organisms provide the testable ontological basis of the species category. We question the ontology and epistemology of the phylogenetic or evolutionary species concept, and find it to be a fundamentally untenable one. We argue that at best, the phylogenetic species is a taxonomic species notion which is not a theoretical concept, and therefore should not serve as foundation for taxonomic theory in general, phylogenetics, and macroevolutionary reconstruction in particular. Although both evolutionary systematists and cladists are phylogeneticists, the reconstruction of the history of life is fundamentally different in these two approaches. We maintain that all method, including taxonomic ones, must fall out of well corroborated theory. In the case of taxonomic methodology the theoretical base must be evolutionary. The axiomatic assumptions that all phena, living and fossil, must be holophyletic taxa (species, and above), resulting from splitting events, and subsequently that evaluation of evolutionary change must be based on a taxic perspective codified by the Hennig ian taxonomic species notion, are not testable premises. We discuss the relationship between some biologically, and therefore taxonomically, significant patterns in nature, and the process dependence of these patterns. Process-free establishment of deductively tested “genealogies” is a contradiction in terms; it is impossible to “recover” phylogenetic patterns without the investment of causal and processual explanations of characters to establish well tested taxonomic properties of these (such as homologies, apomorphies, synapomorphies, or transformation series). Phylogenies of either characters or of taxa are historical-narrative explanations (H-N Es), based on both inductively formulated hypotheses and tested against objective, empirical evidence. We further discuss why construction of a “genealogy”, the alleged framework for “evolutionary reconstruction”, based on a taxic, cladistic outgroup comparison and a posteriori weighting of characters is circular. We define how the procedure called null-group comparison leads to the noncircular testing of the taxonomic properties of characters against which the group phylogenies must be tested. This is the only valid rooting procedure for either character or taxon evolution. While the Hennig -principle is obviously a sound deduction from the theory of descent, cladistic reconstruction of evolutionary history itself lacks a valid methodology for testing transformation hypotheses of both characters and species. We discuss why the paleontological method is part of comparative biology with a critical time dimension ana why we believe that an “ontogenetic method” is not valid. In our view, a merger of exclusive (causal and interactive, but best described as levels of organization) and inclusive (classificatory) hierarchies has not been accomplished by a taxic scheme of evolution advocated by some. Transformational change by its very nature is not classifiable in an inclusive hierarchy, and therefore no classification can fully reflect the causal and interactive chains of events constituting phylogeny, without ignoring and contradicting large areas of corroborated evolutionary theory. Attempts to equate progressive evolutionary change with taxic schemes by Haeckel were fundamentally flawed. His ideas found 19th century expression in a taxic perception of the evolutionary process (“phylogenesis”), a merger of typology, hierarchic and taxic notions of progress, all rooted in an ontogenetic view of phylogeny. The modern schemes of genealogical hierarchies, based on punctuation and a notion of “species” individuality, have yet to demonstrate that they hold promise beyond the Haeckel ian view of progressive evolution.     

“And then a miracle occurs” — weak links in the chain of argument from punctuation to hierarchy

Weak links, in the form of inadequacies in both reasoning and supporting evidence, exist at several critical steps in the derivation of an hierarchical concept of evolution from punctuated equilibria. Punctuation itself is predicated on a distorted reading of phyletic change as phyletic gradualism, and of allopatric speciation as the instantaneous formation of unchanging typological taxa. The concept of punctuation is further confounded by the indescriminate employment of the same term to denote both a causal explanation for evolutionary change and an outcome of substantiated evolutionary processes. Even when the intended usage for the term is specified, each denotation of punctuation entails respective drawbacks. As a causal explanation, punctuation clearly belongs to the class of quantum theories with all their attendant impedimenta, including special salsatory non-adaptive mechanisms of evolutionary change. Redefinition of punctuation as a pattern of morphologic change reduces it to one possible outcome of known microevolutioanry processes, thus obviating any need for an hierarchical explanation of macroevolution. While vacillation between usages has preserved the term in the literature, the end result of this obfuscation has been a circle of faulty reasoning in which the pattern of punctuation is invoked as its own proof. Widespread confusion concerning what constitutes an adequate test of punctuation is directly attributable to imprecision in both the original and revised formulations of the concept.The argument for species-level selection is based on the typological and philosphically flawed premise of species as individuals, and further requires the hypothesis of heritable emergent properties, for which empirical evidence is lacking.Extrapolation of hierarchy to higher taxonomic levels depends on the unproven assumption that mass extinction constitutes a causal mechanism for macroevolution which is qualitatively distinct from, and not reducible to, the causes of microevolution.Because key elements of hierarchical theory depend upon misrepresentations of the synthetic theory, semantic circumvention, and questionable reasoning, and because its central assertions remain unsubstantiated by empirical evidence, the derivation of an hierarchical theory of evolution from punctuation constitutes an exercise an futility and should be abandoned.     

Chance as an explanatory factor in evolutionary biology.

Darwinian evolutionary biology has often been criticized for appealing to the notion of 'chance' in its explanations. According to some critics, such appeals exhibit the explanatory poverty of evolutionary theory. In response, defenders of Darwinism sometimes downplay the importance of 'chance' in evolution. I believe that both of these approaches are mistaken. The main thesis of this paper is that the term 'chance' encompasses a number of distinct concepts, and that at least some of these concepts serve essential explanatory functions in evolutionary biology. This claim is defended by way of an historical survey of the major concepts of 'chance' in the history of evolutionary biology, especially the concepts used by Jean Baptiste Lamarck, Charles Darwin, and Sewall Wright. An examination of their biologies shows how the concepts of 'chance' used cohere with their major scientific objectives and methods. These concepts survive and continue to function as important explanatory factors in contemporary evolutionary biology. Examples of such usage are given, and the explanatory status of 'chance' assessed.     

Cranial capacity in hominid evolution

We present an analysis of cranial capacity of 118 hominid crania available from the literature. The crania belong to both the genusAustralopithecus andHomo and provide a clear outline of hominid cranial evolution starting at more than 3 million years ago. Beginning withA. afarensis there is a clear increase in both absolute and relative brain size with every successive time period.H.s. neandertal has an absolutely and relatively smaller brain size (1412cc, E.Q.=5.6) than fossil modernH.s. sapiens (1487cc, E.Q.=5.9). Three evolutionary models of hominid brain evolution were tested: gradualism, punctuated equilibrium, and a mixed model using both gradualism and punctuated equilibrium. Both parametric and non-parametric analyses show a clear trend toward increasing brain size withH. erectus and a possible relationship within archaicH. sapiens. An evolutionary stasis in cranial capacity could not be refuted for all other taxa. Consequently, the mixed model appears to more fully explain hominid cranial capacity evolution. However, taxonomic decisions could directly compromise the possibility of testing the evolutionary mechanisms hypothesized to be operating in hominid brain expansion.     

Charles Darwin,embryology, evolution and skeletal plasticity

Darwin provided us with the theory of evolutionary change through natural selection. Just as important to the science of biology was Darwin’s recognition that all organisms could be classified and were related to one another because they arose from a single common universal ancestor – what we know as the universal tree of life (UtoL). All the features of the skeletal biology of fish therefore can be explained, both in an evolutionary framework (ultimate causation) and in the framework of development, growth and physiology (proximate causation). Neither approach is complete without the other. I will outline the elements of Darwin’s theories on evolution and classification and, as importantly, discuss what was missing from Darwin’s theories. An important class of evidence for evolution used by Darwin came from embryology, both comparative embryology and the existence of vestiges and atavisms. After discussing this evidence I examine some fundamental features of skeletal development and evolution These include: the presence of four skeletal systems in all vertebrates; the existence of two skeletons, one based on cartilage, the other on bone and dentine; the modular nature of skeletal development and evolution; and the plasticity of the skeleton in response to either genetic or environmental changes.     

Ancon sheep: a now disproven example of macroevolution.

The Ancon breed of sheep provided, for decades, a critical support for the existence of major evolutionary changes or jumps called "sports." Putative examples of sports have been used as evidence of rapid macroevolution since Darwin first discussed the Ancon sheep mutation in 1859. Ancon sheep had very short legs that were considered an advantage for shepherds because the sheep were less likely to escape over fences. Many textbooks and articles implied that the breed was an example of how a major new trait could evolve in a single generation. The Ancon sheep example has been used both to prove gradual Darwinism and also to argue for rapid evolution as opposed to gradualism. It now is recognized that Ancon sheep resulted from genetic diseases, and that they usually suffered from achondroplasia. The so-called new breed had so many major health problems that the condition caused the breed's extinction decades ago, yet was mentioned in textbooks as evidence for evolutionary jumps as recently as 2005.     

Branches in the lines of descent: Charles Darwin and the evolution of the species concept

Charles Darwin introduced a novel idea into the concept of species, namely that species are branches in the lines of descent (segments of population lineages). In addition to this novel evolutionary component, Darwin's species concept also retained an older taxonomic component, namely the view that the species category is a taxonomic rank; moreover, he adopted amount of difference as a criterion for ranking lineages as species. Subsequent biologists retained both components of Darwin's species concept, although they replaced Darwin's ranking criterion with ranking criteria that either are more objectively defined or relate more directly to the biological bases of lineage separation and divergence. Numerous alternative ranking criteria were proposed, resulting in a proliferation of species definitions and a controversy concerning the concept of species. That controversy can be resolved by distinguishing more explicitly between the theoretical concept of species and the operational criteria that are used to apply the concept in practice. By viewing the various alternative ranking criteria as operational indicators of lineage separation rather than necessary properties of species, the conflicts among competing species concepts are eliminated, resulting in a unified concept of species. A brief examination of the history of biology reveals that an important shift related to the unified species concept has been emerging ever since Darwin reformulated the concept of species with an evolutionary basis. The species category is effectively being decoupled from the hierarchy of taxonomic ranks and transferred to the hierarchy of biological organization. Published 2011. This article is a US Government work and is in the public domain in the USA. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 19–35.     

TESTING MACROEVOLUTIONARY HYPOTHESES WITH CLADISTIC ANALYSIS: EVIDENCE AGAINST RECTANGULAR EVOLUTION

The properties of cladistic data sets from small monophyletic groups (6–12 species) are investigated using computer simulations of macroevolution. Two evolutionary models are simulated: gradualism and the punctuated-equilibrium hypothesis. Under the conditions of our simulations these two models of evolution make consistently different predictions about the distribution of autapomorphies among species. When strict stasis is enforced, the punctuated-equilibrium hypothesis predicts that the most expected number of autapomorphies per species will be zero, no matter how many characters are used in the analysis. As the number of characters used in the analysis increases, the distribution of the number of autapomorphies per species becomes bimodal. Under gradualism, the distribution of autapomorphies remains unimodal under all conditions, but the number of species without autapomorphies can fall to zero. A survey of real cladograms of extant monophyletic groups from a wide range of taxa indicates that the predictions of the punctuated-equilibrium hypothesis about autapomorphies do not hold. This constitutes strong evidence against the punctuated-equilibrium hypothesis.     

Towards a postmodern synthesis of evolutionary biology

In 2009, we are celebrating the 200th anniversary of Charles Darwin and the 150th jubilee of his masterpiece, the Origin of Species. Darwin constructed the first coherent and compelling narrative of biological evolution and thus founded evolutionary biology—and modern biology in general, remembering the famous dictum of Dobzhansky. It is, however, counter-productive, and ultimately, a disservice to Darwin’s legacy, to define modern evolutionary biology as neo-Darwinism. The current picture of evolution, informed by results of comparative genomics and systems biology, is by far more complex than that presented in the Origin of Species, so that Darwinian principles, including natural selection, are incorporated into the evolving new synthesis as important but certainly not all-embracing tenets. This expansion of evolutionary biology does not denigrate Darwin in the least but rather emphasizes the fertility of his ideas.