Natural selection is an important force that shapes the evolution of all living things by determining which individuals contribute the most descendents to future generations. The biological unit upon which selection acts has been the subject of serious debate, with reasonable arguments made on behalf of populations, individuals, individual phenotypic characters and, finally, individual genes themselves. In this essay, I argue that the usual unit of selection is the gene. There are powerful logical arguments in favor of this conclusion, as (...) well as many real-world examples. I also explore the possibility that epigenetic differences between individuals may be heritable between generations. Although few such examples exist, epigenetic differences provide an exciting source of potentially heritable variation that may allow rapid evolutionary change to occur, perhaps in response to environmental influences. (shrink)

This chapter contains sections titled: Introduction Natural Selection Operates within Genomes without Regard for Phenotypic Effect Selective Forces, Heritable Variation, and the Definition of Function Natural Selection Can, and Does, Act on the Products of Individual Genes Natural Selection Can Act Directly on Genes Themselves What Are the Limitations on the Unit of Selection Being “the Gene”? The “Complexity” Argument: Do Complex Phenotypes Require Complex Explanations? Do “Epigenes/Epialleles” Provide a “Non‐genetic” Source of Heritable Variation (...) Upon Which Natural Selection May Act? Summary Points: The Usual Unit of Selection Is the Gene Postscript: Counterpoint Note References. (shrink)

Natural selection is an important force that shapes the evolution of all living things by determining which individuals contribute the most descendents to future generations. The biological unit upon which selection acts has been the subject of serious debate, with reasonable arguments made on behalf of populations, individuals, individual phenotypic characters and, finally, individual genes themselves. In this essay, I argue that the usual unit of selection is the gene. There are powerful logical arguments in favor of this conclusion, as (...) well as many real-world examples. I also explore the possibility that epigenetic differences between individuals may be heritable between generations. Although few such examples exist, epigenetic differences provide an exciting source of potentially heritable variation that may allow rapid evolutionary change to occur, perhaps in response to environmental influences. (shrink)

By linking the concepts of homology and morphological organization to evolvability, this paper attempts to (1) bridge the gap between developmental and phylogenetic approaches to homology and to (2) show that developmental constraints and natural selection are compatible and in fact complementary. I conceive of a homologue as a unit of morphological evolvability, i.e., as a part of an organism that can exhibit heritable phenotypic variation independently of the organism’s other homologues. An account of homology therefore consists in (...) explaining how an organism’s developmental constitution results in different homologues/characters as units that can evolve independently of each other. The explanans of an account of homology is developmental, yet the very explanandum is an evolutionary phenomenon: evolvability in a character-by-character fashion, which manifests itself in phylogenetic patterns as recognized by phylogenetic approaches to homology. While developmental constraints and selection have often been viewed as antagonistic forces, I argue that both are complementary as they concern different parts of the evolutionary process. Developmental constraints, conceived of as the presence of the same set of homologues across phenotypic change, pertain to how heritable variation can be generated in the first place (evolvability), while natural selection operates subsequently on the produced variation. (shrink)

According to the standard formulation, natural selection requires variation, differential fitness, and heritability. I argue that this formulation is inadequate because it fails to distinguish natural selection from artificial selection, intelligent design, forward-looking orthogenetic selection, and adaptation via the selection of nonrandom variation. I suggest adding a _no teleology_ condition. The no teleology condition says that the evolutionary process is not guided toward an endpoint represented in the mind of an agent, variation is produced randomly with respect (...) to adaptation, and selection pressures are not forward looking. (shrink)

The view that complexity increases in evolution is uncontroversial, yet little is known about the possible causes of such a trend. One hypothesis, the Zero Force Evolutionary Law (ZFEL), predicts a strong drive toward complexity, although such a tendency can be overwhelmed by selection and constraints. In the absence of strong opposition, heritable variation accumulates and complexity increases. In order to investigate this claim, we evaluate the gross morphological complexity of laboratory mutants in Drosophila melanogaster, which represent organisms (...) that arise in a context where selective forces are greatly reduced. Complexity was measured with respect to part types, shape, and color over two independent focal levels. Compared to the wild type, we find that D. melanogaster mutants are significantly more complex. When the parts of mutants are categorized by degree of constraint, we find that weakly constrained parts are significantly more complex than more constrained parts. These results support the ZFEL hypothesis. They also represent a first step in establishing the domain of application of the ZFEL and show one way in which a larger empirical investigation of the principle might proceed. (shrink)

Neo-Darwinian theories of cultural evolution are apt to be criticized on the grounds that they merely borrow from the theory of natural selection concepts that are then metaphorically applied to conventional historical narratives to which they add no more, if anything, than an implicit presupposition of progress from one predetermined stage to the next. Such criticisms, of which a particularly forceful example is a recent article in this journal by Fracchia and Lewontin, can however be shown to be seriously misconceived. (...) The fundamental process of heritable variation and competitive selection of information affecting phenotype underlies both biological and cultural evolution despite the obvious differences between the mechanisms of information transfer by genetic inheritance and by exosomatic imitation and learning. Information transfer is in neither case a metaphor standing for any other thing, and in neither case does change over time proceed in accordance with developmental laws from which the future evolution of either species or cultures could be predicted in advance. For all the unresolved questions that remain, neo-Darwinian evolutionary theory has demonstrated the mutual compatibility of idiographic and nomothetic explanation in the study of species and of cultures alike. (shrink)

The publication of Darwin's On the Origin of Species in 1859 ignited a public storm he neither wanted nor enjoyed. Having offered his book as a contribution to science, Darwin discovered to his dismay that it was received as an affront by many scientists and as a sacrilege by clergy and Christian citizens. To answer the criticism that his theory was a theory only, and a wild one at that, he published two volumes in 1868 to demonstrate that evolution was (...) obvious to anyone who cared to look at a bull in a pasture or a dog on a hearth. In response to those who insisted that species were distinct since creation, Darwin pointed to breeders of pigs and pigeons. In reply to those who protested that human intervention is one thing and natural selection another, he argued, "If organic beings had not possessed an inherent tendency to vary, man could have done nothing." To counter those who scorned his descriptions of species in exotic places, he submitted local evidence of cabbages and cauliflower. Based on a wide array of sources, from ancient pictographs to Polish roosters, from skins and from skeletons, from scientific journals and breeding manuals, Darwin assembled a mass of proof--and a hypothesis about species reversion that risked his reputation anew. The Variation of Animals and Plants under Domestication is a two-volume compilation of his thorough and intensive research and the revolutionary conclusions that resulted. The first portion of his work is dedicated to a meticulous analysis of various aspects of plant and animal life, including an inventory of varieties and their physical and behavioral characteristics, investigation of the impact of a species' surrounding environment and the role that both natural and forced changes in this environment have had. Darwin then turns to a richly detailed discussion of the roles of inheritance and crossing in the development of species. A wealth of illustrations further support and enhance his findings. This fascinating, invaluable, and courageous undertaking eventually formed the foundation for our current understanding of evolution. "In science as in politics the victors tend to write the history books. As a result, the record of the past is edited, intentionally or unintentionally, so that it focuses mainly on the precursors of contemporary orthodoxy. Such a focus may accurately represent the genealogy of modern ideas, but it almost inevitably misrepresents the historical experience of their progenitors... Even the powerful, persuasive, and ultimately triumphant theory of evolution by natural selection required not only defense, but repeated buttressing and revision. Variation showed Darwin hard at work on this rearguard action, using the materials he had at hand... His information was gleaned from the observations of fanciers, breeders, and amateur naturalists, as well as from the treatises of those on the cutting edge of zoology and botany. As hindsight narrows the historical spotlight, it imposes its own sense of hierarchy on the preoccupations of the past. But Darwin was interested in all of these topics, valued all of these sources, and belonged, to a greater or lesser extent, to all of these communities."--from the Introduction. (shrink)

There are two ways evolution by natural selection is conceptualized in the literature. One provides a ‘recipe’ for ENS incorporating three ingredients: variation, differences in fitness, and heritability. The other provides formal equations of evolutionary change and partitions out selection from other causes of evolutionary changes such as transmission biases or drift. When comparing the two approaches there seems to be a tension around the concept of heritability. A recent claim has been made that the recipe approach is flawed (...) and should be abandoned. In this article, I show that the tension is only a superficial one. If one uses a concept of heritability strictly in line with the formal equations of evolutionary change, the recipe approach keeps its validity and generality. To demonstrate that the intuitive concept of heritability is not a general one, I use one formulation of the Price equation formulated by Okasha, show that the concept of heritability in his formulation incorporates both the intuitive notion of heritability as a measure of similarity between parent and offspring characters, and a measure of persistence. I advocate for persistence to be incorporated in the concept of heritability used in recipes for ENS in the same way heredity is, show that this is readily attainable and thereby dissolve any point of tension concerning heritability between the recipe and the analytical approach to ENS. 1 Introduction2 Heritability in the Price Equation2.1 Different approaches to heredity and heritability2.2 Heritability: From recipes to the Price equation3 A Problem for the Recipes?4 Reinterpreting Heritability for Recipes5 Conclusion Appendix. (shrink)

According to the received view of evolution, only genes are inherited. From this view it follows that only genetically-caused phenotypic variation is selectable and, thereby, that all selection is at bottom genetic selection. This paper argues that the received view is wrong. In many species, there are intergenerationally-stable phenotypic differences due to environmental differences. Natural selection can act on these nongenetically-caused phenotypic differences in the same way it acts on genetically-caused phenotypic differences. Some selection is at bottom nongenetic selection. (...) The argument against the received view involves a reformulation of the concepts of inheritance and heritability. Inherited factors are all those developmental factors responsible for parent–offspring similarity; some inherited factors are genetic and some are not. Heritable variation is intergenerationally-stable phenotypic variation; some such variation is genetically-caused and some is not. The received view and its critics The possibility of nongenetic selection (the lucky butterfly) The reality of nongenetic selection 3.1 Imprinting mechanisms 3.2 Other learning mechanisms 3.3 Other nongenetic mechanisms Genetic and nongenetic inheritance mechanisms Genetic and nongenetic inherited factors Genetic and nongenetic heritability Conclusions + Current address: Dr Matteo Mameli, Research Fellow, King's College, Cambridge, CB2 1ST, United Kingdom, matteo.mameli{at}kings.cam.ac.uk' + u + '@' + d + ''//-->. (shrink)

Human cooperation is highly unusual. We live in large groups composed mostly of non-relatives. Evolutionists have proposed a number of explanations for this pattern, including cultural group selection and extensions of more general processes such as reciprocity, kin selection, and multi-level selection acting on genes. Evolutionary processes are consilient; they affect several different empirical domains, such as patterns of behavior and the proximal drivers of that behavior. In this target article, we sketch the evidence from five domains that bear on (...) the explanatory adequacy of cultural group selection and competing hypotheses to explain human cooperation. Does cultural transmission constitute an inheritance system that can evolve in a Darwinian fashion? Are the norms that underpin institutions among the cultural traits so transmitted? Do we observe sufficient variation at the level of groups of considerable size for group selection to be a plausible process? Do human groups compete, and do success and failure in competition depend upon cultural variation? Do we observe adaptations for cooperation in humans that most plausibly arose by cultural group selection? If the answer to one of these questions is “no,” then we must look to other hypotheses. We present evidence, including quantitative evidence, that the answer to all of the questions is “yes” and argue that we must take the cultural group selection hypothesis seriously. If culturally transmitted systems of rules (institutions) that limit individual deviance organize cooperation in human societies, then it is not clear that any extant alternative to cultural group selection can be a complete explanation. (shrink)

In recent years philosophers have attempted to clarify the units of selection controversy in evolutionary biology by offering conceptual analyses of the term 'unit of selection'. A common feature of many of these analyses is an emphasis on the claim that units of selection are entities exhibiting heritable variation in fitness. In this paper I argue that the demand that units of selection be characterized in terms of heritability is unnecessary, as well as undesirable, on historical, theoretical, and (...) philosophical grounds. I propose a positive account of the proper referent of the term 'unit of selection', distinguishing between the processes of evolution and phenotypic selection. The main result of this analysis is greater clarity about the conceptual structure of evolutionary theory. (shrink)

Firms’ choice of legal regime is not uniform. Despite Delaware’s significant advantages and success in attracting corporations, many firms still choose to incorporate in their home state, and some firms incorporate in a third state, most notably Nevada. Several factors - lawyers’ advice, political influence in the home state, and relative costs of out of state incorporation - were identified as contributing to these patterns. Yet none of these factors neither their combination, fully account for firms’ choices. This Article suggests (...) that unidentified heterogeneity, potentially in managers’ preferences for legal protection, might have contributed to, and could help in explaining, these patterns. Among other factors, this heterogeneity could result, for example, from variations in market forces and, in turn, private benefits that managers extract. Introducing heterogeneity in managers’ preferences, this Article suggests that managers that share a relatively strong preference for insider protection should be less inclined to incorporate in Delaware, and more inclined to incorporate in their home state where they have political clout, or in Nevada if their strong preference for protection is not satisfied in their home state. The analysis is too preliminary for normative implications to be derived, rather the Article suggests that more research into firms’ heterogeneity and their choice of law could prove valuable. (shrink)

The Continuous Parameter Estimation Model is applied to develop individual genomic-level heritabilities for the latent hierarchical structure and developmental dynamics of Life History (LH) strategy LH strategies relate to the allocations of bioenergetic resources into different domains of fitness. LH has moderate to high population-level heritability in humans, both at the level of the high-order Super-K Factor and the lower-order factors, the K-Factor, Covitality Factor, and General Factor of Personality (GFP). Several important questions remain unexplored. We developed measures of genome-level (...) heritabilities employing an American sample of 316 monozygotic (MZ) and 274 dizygotic (DZ) twin dyads and a Swedish sample of 863 MZ and 475 DZ twin dyads. This novel heritability index measures individual genetic transmissibility, therefore opening new avenues for analyzing complex interactions among heritable traits inaccessible to standard structural equations methods. For these samples: (1) moderate to high heritability of factor loadings of Super-K on its lower-order factors is demonstrated, evidencing biological preparedness, genetic accommodation, and the gene-culture coevolution of biased epigenetic rules of development; (2) moderate to high heritability of the magnitudes of the effect of the higher-order factors upon their loadings on their constituent factors, evidencing genetic constraints upon phenotypic plasticity; and (3) that heritability of the LH factors, of factor loadings, and of the magnitudes of the correlations among factors are weaker among those with slower LH speeds, demonstrating that inter-individual variation in transmissibility is a function of individual socioecological selection pressures. (shrink)

Many scholars have rejected cultural evolutionary theory on the grounds that cultural variation is directed and intentionally created, rather than incremental and blind with respect to function, as is the case for novel genetic variation in genetic evolution. Meanwhile, some cultural evolution researchers insist that cultural variation is blind and undirected, and the only directional force is selection of randomly-generated variants. Here I argue that neither of these positions are tenable. Cultural variation is directed in various (...) ways. While this does not invalidate cultural evolution, more attention should be paid to the different sources of non-randomness in culturally evolving systems. (shrink)

Given that natural selection is so powerful at optimizing complex adaptations, why does it seem unable to eliminate genes (susceptibility alleles) that predispose to common, harmful, heritable mental disorders, such as schizophrenia or bipolar disorder? We assess three leading explanations for this apparent paradox from evolutionary genetic theory: (1) ancestral neutrality (susceptibility alleles were not harmful among ancestors), (2) balancing selection (susceptibility alleles sometimes increased fitness), and (3) polygenic mutation-selection balance (mental disorders reflect the inevitable mutational load on the (...) thousands of genes underlying human behavior). The first two explanations are commonly assumed in psychiatric genetics and Darwinian psychiatry, while mutation-selection has often been discounted. All three models can explain persistent genetic variance in some traits under some conditions, but the first two have serious problems in explaining human mental disorders. Ancestral neutrality fails to explain low mental disorder frequencies and requires implausibly small selection coefficients against mental disorders given the data on the reproductive costs and impairment of mental disorders. Balancing selection (including spatio-temporal variation in selection, heterozygote advantage, antagonistic pleiotropy, and frequency-dependent selection) tends to favor environmentally contingent adaptations (which would show no heritability) or high-frequency alleles (which psychiatric genetics would have already found). Only polygenic mutation-selection balance seems consistent with the data on mental disorder prevalence rates, fitness costs, the likely rarity of susceptibility alleles, and the increased risks of mental disorders with brain trauma, inbreeding, and paternal age. This evolutionary genetic framework for mental disorders has wide-ranging implications for psychology, psychiatry, behavior genetics, molecular genetics, and evolutionary approaches to studying human behavior. (Published Online November 9 2006) Key Words: adaptation; behavior genetics; Darwinian psychiatry; evolution; evolutionary genetics; evolutionary psychology; mental disorders; mutation-selection balance; psychiatric genetics; quantitative trait loci (QTL). (shrink)

Darwinism is defined here as an evolving research tradition based upon the concepts of natural selection acting upon heritable variation articulated via background assumptions about systems dynamics. Darwin’s theory of evolution was developed within a context of the background assumptions of Newtonian systems dynamics. The Modern Evolutionary Synthesis, or neo-Darwinism, successfully joined Darwinian selection and Mendelian genetics by developing population genetics informed by background assumptions of Boltzmannian systems dynamics. Currently the Darwinian Research Tradition is changing as it incorporates (...) new information and ideas from molecular biology, paleontology, developmental biology, and systems ecology. This putative expanded and extended synthesis is most perspicuously deployed using background assumptions from complex systems dynamics. Such attempts seek to not only broaden the range of phenomena encompassed by the Darwinian Research Tradition, such as neutral molecular evolution, punctuated equilibrium, as well as developmental biology, and systems ecology more generally, but to also address issues of the emergence of evolutionary novelties as well as of life itself. (shrink)

This article examines eight “gaps” in order to clarify why the quantitative genetics methods of partitioning variation of a trait into heritability and other components has very limited power to show anything clear and useful about genetic and environmental influences, especially for human behaviors and other traits. The first two gaps should be kept open; the others should be bridged or the difficulty of doing so should be acknowledged: 1. Key terms have multiple meanings that are distinct; 2. Statistical (...) patterns are distinct from measurable underlying factors; 3. Translation from statistical analyses to hypotheses about measurable factors is difficult; 4. Predictions based on extrapolations from existing patterns of variation may not match outcomes; 5. The partitioning of variation in human studies does not reliably estimate the intended quantities; 6. Translation from statistical analyses to hypotheses about the measurable factors is even more difficult in light of the possible heterogeneity of underlying genetic or environmental factors; 7. Many steps lie between the analysis of observed traits and interventions based on well-founded claims about the causal influence of genetic or environmental factors; 8. Explanation of variation within groups does not translate to explanation of differences among groups. At the start, I engage readers’ attention with three puzzles that have not been resolved by past debates. The puzzles concern generational increases in IQ test scores, the possibility of underlying heterogeneity, and the translation of methods from selective breeding into human genetics. After discussing the gaps, I present each puzzle in a new light and point to several new puzzles that invite attention from analysts of variation in quantitative genetics and in social science more generally. The article’s critical perspectives on agricultural, laboratory, and human heritability studies are intended to elicit further contributions from readers across the fields of history, philosophy, sociology, and politics of biology and in the sciences. (shrink)

Many contemporary biologists and philosophers of biology admit that selection occurs at any level of the biological hierarchy at which entities showing heritable variation in fitness are found, while insisting that fitness at any level entails differential reproduction, not differential persistence. Those who allow that persistence can be selected doubt that selection on nonreproducing entities can be reiterated, to produce “complex adaptations.” We present here a verbal model of subclones evolving in a simple idealized chemostat that calls into (...) question these suppositions and is usefully explanatory when taken as an analogy to selection for persistence of clades. (shrink)

Natural selection is commonly seen not just as an explanation for adaptive evolution, but as the inevitable consequence of “heritable variation in fitness among individuals”. Although it remains embedded in biological concepts, such a formalisation makes it tempting to explore whether this precondition may be met not only in life as we know it, but also in other physical systems. This would imply that these systems are subject to natural selection and may perhaps be investigated in a biological (...) framework, where properties are typically examined in light of their putative functions. Here we relate the major questions that were debated during a three-day workshop devoted to discussing whether natural selection may take place in non-living physical systems. We start this report with a brief overview of research fields dealing with “life-like” or “proto-biotic” systems, where mimicking evolution by natural selection in test tubes stands as a major objective. We contend the challenge may be as much conceptual as technical. Taking the problem from a physical angle, we then discuss the framework of dissipative structures. Although life is viewed in this context as a particular case within a larger ensemble of physical phenomena, this approach does not provide general principles from which natural selection can be derived. Turning back to evolutionary biology, we ask to what extent the most general formulations of the necessary conditions or signatures of natural selection may be applicable beyond biology. In our view, such a cross-disciplinary jump is impeded by reliance on individuality as a central yet implicit and loosely defined concept. Overall, these discussions thus lead us to conjecture that understanding, in physico-chemical terms, how individuality emerges and how it can be recognised, will be essential in the search for instances of evolution by natural selection outside of living systems. (shrink)

Many evolutionary models assume that behaviors are caused directly by genes. An implication is that behavioral uniformity should be found only in groups that are genetically uniform. Yet, the members of human social groups often behave in a uniform fashion, despite the fact that they are genetically diverse. Behavioral uniformity can occur through a variety of psychological mechanisms and social processes, such as imitation, consensus decision making, or the imposition of social norms. We present a series of models in which (...) genes code for social transmission rules, which in turn govern the behaviors that are adopted. Transmission rules can evolve in randomly formed groups that concentrate phenotypic variation at the between-group level, favoring the evolution of altruistic behaviors and other group-advantageous traits. In addition, a direct bias toward adopting altruistic behaviors can evolve. Our models begin to show how group selection can be a strong force in human evolution, despite the absence of extreme genetic variation among groups. (shrink)

Stanovich & West (S&W) appear to overlook the adaptivity of variation. Behavioral variability, both between and within individuals, is an absolute necessity for phylogenetic and ontological adaptation. As with all heritable characteristics, inter-individual behavioral variation is the foundation for natural selection. Similarly, intra-individual variation allows a broad exploration of potential solutions. Variation increases the likelihood that more optimal behaviors are available for selection. Four examples of the adaptivity of variation are discussed: (a) Genetic (...) class='Hi'>variation as it pertains to behavior and natural selection; (b) behavioral and cognitive aspects of mate selection which may facilitate genetic diversity; (c) variation as a strategy for optimizing learning through greater exploration; and (d) behavioral variation coupled with communication as a means to propagate individually discovered behavioral success. (shrink)

Variation or rearrangement of regulatory genes is responsible for cellular malignant change. These types of chromosomal variations also produce heterochrony or paedomorphic evolution at the organismal level. Analogously, neoplasia represents a cellular macroevolutionary event, and a tumour can be said to be an evolved population of cells. To understand this cellular evolution to malignancy, it may be necessary to go beyond a clonal selection (adaptationist) explanation of neoplastic alteration. In the pericellular environment natural selection consists of the organizational restraints (...) of surrounding cells as well as the host's immunological surveillance and non-specific monocyte-macrophage systems. Indirect evidence suggests that success for the neoplasm depends not upon clonal selection, but solely upon a genetic methodology—the function of which is to elude selection.The author has coined the term cellular heterochrony to illustrate analogic similarities in the molecular modes of speciation between anaplastic cancer cells and the heterochronic evolution of organisms. By reverting to a juvenile (embryonic) repertoire of cellular behaviour a tumour secures its own tenure or niche by usurping the host's armamentarium of selection forces, employing many of the same or similar methods by which implanting and invading tissues of the mammalian embryo forestall maternal detection and rejection. A number of ways by which the tumour blocks, subverts or evades selection are discussed. (shrink)

In evolutionary biology, niche construction is sometimes described as a genuine evolutionary process whereby organisms, through their activities and regulatory mechanisms, modify their environment such as to steer their own evolutionary trajectory, and that of other species. There is ongoing debate, however, on the extent to which niche construction ought to be considered a bona fide evolutionary force, on a par with natural selection. Recent formulations of the variational free-energy principle as applied to the life sciences describe the properties of (...) living systems, and their selection in evolution, in terms of variational inference. We argue that niche construction can be described using a variational approach. We propose new arguments to support the niche construction perspective, and to extend the variational approach to niche construction to current perspectives in various scientific fields. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:The Journal of Aesthetic Education 38.2 (2004) 53-66 [Access article in PDF] Art Selection, or the Preservation of Artworks in the Struggle for Art Christopher Perricone The argument of George C. Williams's book Adaptation and Natural Selection is against what biologists call the group selectionist view — that individuals will act on behalf of their species, or at least on behalf of the group to which they belong.1 Williams (...) showed, and now most biologists agree, "no creature could ever evolve the ability to help its species at the expense of itself. Only when the two interests coincided would it act selflessly."2 According to Matt Ridley, Williams is one of the towering figures in contemporary biology. Williams helps us to understand better what natural selection is and consequently helps us to understand better our place in the world. Williams claims that when we are dealing with life, "we are forced to invoke natural selection to achieve a complete explanation of an observed system."3 Physics and chemistry alone are not sufficient to explain life. Williams gives the example of a falling apple. If one has to explain the trajectory of the apple, the principles of mechanics are sufficient for providing an adequate explanation of the phenomenon. If, however, we were asked how the apple acquired its various properties, and why it has these properties instead of others, we would need the theory of natural selection, at least by implication. Only thus could we explain why the apple has a waterproof wax on the outside...why it contains dormant embryos....We would find that an impressive list of structural details and processes of the apple can be understood as elements of a design for an efficient role in the propagation of the tree from which it came."4Of course, a little over a hundred years before Williams published his book, Charles Darwin published Origin of Species, in which natural selection is the linchpin of his theory of evolution. Natural selection for Darwin not only helped to explain the competitive climate of the living world but its luscious fecundity as well. Natural selection works like this: Let's say you [End Page 53] have a population of large beaked and small beaked birds. The environment in which the birds live provides them with an abundant and nutritious supply of both large and small seeds. Typically the small beaked birds eat the small seeds. The large beaked birds are capable of eating both small and large seeds. However, a blight occurs killing off all the small seed producing trees causing the food supply of the small beaked birds to diminish to virtually nil. Given that the small beaked birds are unable to feed on the continued supply of large seeds, they are selected out or eliminated from that environment: either they must seek out another environment, or they die. The large beaked birds, however, are in this case more adaptable to the environment because they can eat large seeds, survive, and pass on their largebeaked traits to the next generation. Darwin characterizes this process as "the struggle for life." He says: Owing to this struggle, variations, however slight and from what ever cause proceeding, if they be in any way profitable to the individuals of a species, in their infinitely complex relation to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection, in order to mark its relation to man's power of selection. But the expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient.5Darwin sums up his discussion of natural selection by saying that the idea explains... (shrink)

Darwinian evolution can be simply stated: natural selection of inherited variations increasing differential reproduction. However, formulated thus, links with biochemistry, cell biology, ecology, and population dynamics remain unclear. To understand interactive contributions of chance and selection, higher levels of biological organization (e.g., endosymbiosis), complexities of competing selection forces, and emerging biological novelties (such as eukaryotes or meiotic sex), we must analyze actual examples. Focusing on mitochondria, I will illuminate how biology makes sense of life's evolution, and the concepts involved. (...) First, looking at the bacterium – mitochondrion transition: merging with an archaeon, it lost its independence, but played a decisive role in eukaryogenesis, as an extremely efficient aerobic ATP generator and internal ROS source. Second, surveying later mitochondrion adaptations and diversifications illustrates concepts such as constructive neutral evolution, dynamic interactions between endosymbionts and hosts, the contingency of life histories, and metabolic reprogramming. Without oxygen, mitochondria disappear; with (intermittent) oxygen diversification occurs in highly complex ways, especially upon (temporary) phototrophic substrate supply. These expositions show the Darwinian model to be a highly fruitful paradigm. (shrink)

Estimates of a trait’s heritability can be used to predict the advance through selective breeding in agriculture and the laboratory where researchers can replicate varieties and locations. These conditions do not apply to human populations, yet considerable attention is still given to high heritability and to small effects of family members growing up together relative to differences within families. This article shows that the conventional partitioning of a trait’s variation produces components that cannot be associated reliably with average (...) differences among varieties and locations , let alone underwrite hypotheses about measurable genetic and environmental influences. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Working on the Clinton Administration's Health Care Reform Task ForceNancy Neveloff Dubler (bio)This narrative is based on my understanding of the elements of the Health Security Act that may have ethical implications. I have reconstructed these elements from my experience on the Health Care Reform Task Force and they are part of the health care plan that the President presented to Congress. (At the time this article went to (...) press, the actual legislation, which should answer my remaining specific questions more clearly, had not yet been released.) I perceive the most important features of the plan to be:• The legislation will create a "right" to health care for all citizens—access to care will be universal;• The plan will involve some form of managed competition in which groups of providers (called Approved or Accredited Health Plans (AHPs)) will offer comparable benefit packages at slightly (or widely) different costs;• The AHPs will negotiate with the Health Alliance (HA) to determine the price that they will charge their consumer-patients;• There will be open enrollment with no barriers to coverage, such as exclusions for pre-existing conditions;• Community rating will be in effect in different localities and enrolled persons in any AHP will pay the same premium;• The coverage for any enrolled person will be portable;• The overall structure will be a cooperative federal/state enterprise in which the states will have quite extensive abilities to fashion variations of the basic plan to meet their particular needs;• Primary care will be encouraged and incentives for physicians to train in primary care specialties provided;• Non-physician providers will be central to the functioning of the health care plans;• Efforts will be made to develop a "person power" pool so that people with an abstract "right" to health care will have real access to providers, in their localities, who can fulfill the promise that the right creates; [End Page 421]• The goal of cost containment will be central to the organization and regulation of the system;• Administrative simplicity and quality protections will be central.Co-chairing the Bioethics Working Group (BWG) of the White House Task Force on Health Care Reform was a unique experience. Approximately 40 Task Force working groups were established by Hillary Rodham Clinton, the President, and the senior Task Force staff. Various members of the President's office and other knowledgeable members of the Executive Branch selected the participants. In our group, appointments were made largely on the recommendations of the co-chairs. The BWG, among the last of the groups formed, carried the official name "The Working Group on the Ethical Foundations of the New System" and was jointly led by me and Marion Gray Secundy, a Professor of Bioethics at Howard University. The fact that the leadership of the Task Force saw fit to establish such a group, thereby confirming that bioethics is a player in the arena of health care policy, was in itself significant and was made no less so by the lack of clarity regarding the group's aims and goals.The working groups of the Task Force were organized into clusters, determined by the relatedness of the somewhat distinct, but always overlapping, content of their investigations. Separate clusters and working groups, for example, looked at mental health, long term care, and the benefit package and coverage for low-income and non-working families. On this last issue there was also a working group that explored health policy initiatives for underserved populations. This multiple focus on related issues proved to be an excellent method for ensuring that the complexity of these topics was sufficiently explored. There was a published schedule for when these groups met together, and members of other groups who had similar or overlapping concerns were invited to attend and to participate.Every week or so, all of the working groups reported to the Task Force leadership, under the direction of Ira Magaziner, in a process called a "tollgate." Prior to the tollgate, the working groups were required to submit formal written analyses of the problems they had most recently been considering. Copies of these documents were placed in a special room so that those who were interested could review... (shrink)

The encounter between the Darwinian theory of evolution and Mendelism could be resolved only when reductionist tools could be applied to the analysis of complex systems. The instrumental reductionist interpretation of the hereditary basis of continuously varying traits provided mathematical tools which eventually allowed the construction of the Modern Synthesis of the theory of evolution.When genotypic as well as environmental variance allow the isolation of parts of the system, it is possible to apply Mendelian reductionism, that is , to treat (...) the phenotypic trait as if ti causally determined by discrete genes for the trait. howeverm such a beanbag genetics approach obscures the system's eye-view. The concept of heritability, defined as the proportion of the total phenotypic variance due to (additive) hereditary variation, asserts that genetic elements have discrete effects; but by relating to the genotypic variance, it avoids the trap of reffering to genes for characters. (shrink)

Bernard Norton's friends in the history of science have had many reasons for commemorating, with admiration and affection, not only his research and teaching but no less his conversation and his company. One of his most estimable traits was his refusal to beat about the bush in raising the questions he thought worthwhile pursuing. I still remember discoursing at Pittsburgh on Darwin's route to his theory of natural selection, and being asked at the end by Bernard what were Darwin's views (...) on heredity. I answered with the conventional waffle to the effect that the theory concerned the populational fate rather than the individual production and transmission of heritable variation, so that whatever views Darwin had on heredity had only a subsidiary place in his theorizing. Bernard was not fooled. ‘I would have thought’, he said, ‘that in order to understand anyone's theorising about evolution it would be necessary to look at his views on heredity’. (shrink)

The Yangtze River Economic Zone is a major corridor of national science and innovation culture, an innovation-driven region that fosters new drivers of growth and leads transformation and development, and plays an important strategic support and exemplary leading role in the overall pattern of regional development. This paper analyzes the spatiotemporal differentiation characteristics of innovation output of 110 cities of YREZ from 2008 to 2018 by using Gini coefficient, coefficient of variation, geographical weighted regression, and other methods. The factors (...) affecting innovation output are selected from the perspective of innovation ecosystem. The results show the following. Innovation output showed an increasing trend, and the high-value concentration cities in downstream areas gradually became prominent, the geographical concentration degree fluctuated and declined, and the distribution of innovation output gradually became balanced. The global Moran’s I index of innovation output shows a fluctuation pattern of “M” shape and an overall upward trend. The analysis of local spatial correlation indicates that spatial distribution pattern of innovation output has not changed significantly. There is obvious regional heterogeneity under different impacts of factors of innovation ecosystem on innovation output. Enterprises have the greatest impact, followed by financial resources and infrastructure environment. (shrink)

Recently, several philosophers have challenged the view that evolutionary theory is usefully understood by way of an analogy with Newtonian mechanics. Instead, they argue that evolutionary theory is merely a statistical theory. According to this alternate approach, natural selection and random genetic drift are not even causes, much less forces. I argue that, properly understood, the Newtonian analogy is unproblematic and illuminating. I defend the view that selection and drift are causes in part by attending to a pair of (...) important distinctions—that between process and product and that between natural selection and fitness. (shrink)

DARWIN’S FUNDAMENTAL INSIGHT is that evolution consists in “ descent with heritable variations that are sifted by natural selection to retain the adaptive changes.” Contemporary Darwinian biology tends to be restricted to an exclusively twofold focus: first, the gene, which is conceived as the basic element of biological reality, and hence of life, to the extent that it represents the fundamental unit of heredity; and second, selection, which is conceived as the sole source of order in biological organisms, to (...) the extent that inherited genetic variations leading to a better adaptation of an organism to its environment are selected to survive within this environment. This focus is fundamentally reductionist and functional. The integrity of the organism is. (shrink)

Adaptive evolution is usually assumed to be directed by selective processes, development by instructive processes; evolution involves random genetic changes, development involves induced epigenetic changes. However, these distinctions are no longer unequivocal. Selection of genetic changes is a normal part of development in some organisms, and through the epigenetic system external factors can induce selectable heritable variations. Incorporating the effects of instructive processes into evolutionary thinking alters ideas about the way environmental changes lead to evolutionary change, and about (...) the interplay between genetic and epigenetic systems. (shrink)

The necessary but not sufficient conditions for biological informational concepts like signs, symbols, memories, instructions, and messages are (1) an object or referent that the information is about, (2) a physical embodiment or vehicle that stands for what the information is about (the object), and (3) an interpreter or agent that separates the referent information from the vehicle’s material structure, and that establishes the stands-for relation. This separation is named the epistemic cut, and explaining clearly how the stands-for relation is (...) realized is named the symbol-matter problem. (4) A necessary physical condition is that all informational vehicles are material boundary conditions or constraints acting on the lawful dynamics of local systems. It is useful to define a dependency hierarchy of information types: (1) syntactic information (i.e., communication theory), (2) heritable information acquired by variation and natural selection, (3) non-heritable learned or creative information, and (4) measured physical information in the context of natural laws. High information storage capacity is most reliably implemented by discrete linear sequences of non-dynamic vehicles, while the execution of information for control and construction is a non-holonomic dynamic process. The first epistemic cut occurs in self-replication. The first interpretation of base sequence information is by protein folding; the last interpretation of base sequence information is by natural selection. Evolution has evolved senses and nervous systems that acquire non-heritable information, and only very recently after billions of years, the competence for human language. Genetic and human languages are the only known complete general purpose languages. They have fundamental properties in common, but are entirely different in their acquisition, storage and interpretation. (shrink)

Analysis of grip force signals tailored to hand and finger movement evolution and changes in grip force control during task execution provide unprecedented functional insight into somatosensory cognition. Somatosensory cognition is a basis of our ability to manipulate, move, and transform objects of the physical world around us, to recognize them on the basis of touch alone, and to grasp them with the right amount of force for lifting and manipulating them. Recent technology has permitted the wireless monitoring of grip (...) force signals recorded from biosensors in the palm of the human hand to track and trace human grip forces deployed in cognitive tasks executed under conditions of variable sensory (visual, auditory) input. Non-invasive multi-finger grip force sensor technology can be exploited to explore functional interactions between somatosensory brain mechanisms and motor control, in particular during learning a cognitive task where the planning and strategic execution of hand movements is essential. Sensorial and cognitive processes underlying manual skills and/or hand-specific (dominant versus non-dominant hand) behaviors can be studied in a variety of contexts by probing selected measurement loci in the fingers and palm of the human hand. Thousands of sensor data recorded from multiple spatial locations can be approached statistically to breathe functional sense into the forces measured under specific task constraints. Grip force patterns in individual performance profiling may reveal the evolution of grip force control as a direct result of cognitive changes during task learning. Grip forces can be functionally mapped to from-global-to-local coding principles in brain networks governing somatosensory processes for motor control in cognitive tasks leading to a specific task expertise or skill. Under the light of a comprehensive overview of recent discoveries into the functional significance of human grip force variations, perspectives for future studies in cognition, in particular the cognitive control of strategic and task relevant hand movements in complex real-world precision task, are pointed out. (shrink)

Once described as hermaphrodites and later as intersex people, individuals born with intersex variations are routinely subject to so-called “normalizing” medical interventions, often in childhood. Opposition to such practices has been met by attempts to discredit critics and reasserted clinical authority over the bodies of women and men with “disorders of sex development.” However, claims of clinical consensus have been selectively constructed and applied and lack evidence. Limited transparency and lack of access to justice have helped to perpetuate forced interventions. (...) At the same time, associated with the diffusion of distinct concepts of sex and gender, intersex has been constructed as a third legal sex classification, accompanied by pious hopes and unwarranted expectations of consequences. The existence of intersex has also been instrumentalized for the benefit of other, intersecting, populations. The creation of gender categories associated with intersex bodies has created profound risks: a paradoxically narrowed and normative gender binary, maintenance of medical authority over the bodies of “disordered” females and males, and claims that transgressions of social roles ascribed to a third gender are deceptive. Claims that medicalization saves intersex people from “othering,” or that legal othering saves intersex people from medicalization, are contradictory and empty rhetoric. In practice, intersex bodies remain “normalized” or eliminated by medicine, while society and the law “others” intersex identities. That is, medicine constructs intersex bodies as either female or male, while law and society construct intersex identities as neither female nor male. Australian attempts at reforms to recognize the rights of intersex people have either failed to adequately comprehend the population affected or lacked implementation. An emerging human rights consensus demands an end to social prejudice, stigma, and forced medical interventions, focusing on the right to bodily integrity and principles of self-determination. (shrink)

Elliott Sober and his defenders think of selection, drift, mutation, and migration as distinct evolutionary forces. This paper exposes an ambiguity in Sober's account of the force of selection: sometimes he appears to equate the force of selection with variation in fitness, sometimes with ‘selection for properties’. Sober's own account of fitness as a property analogous to life-expectancy shows how the two conceptions come apart. Cases where there is selection against variance in offspring number also show that selection (...) and drift cannot be distinguished in the way Sober hopes for. These issues have significance beyond the parochial matter of the coherence of Sober's system. There is no good principled answer to the question of which features of a population should count among the contributors to fitness. This means there is no non-arbitrary account of the nature of selection. (shrink)

Evolutionary epistemology has experienced a continuous rise over the last decades. Important new theoretical considerations and novel empirical findings have been integrated into the existing framework. In this paper, I would like to suggest three lines of research that I believe will significantly contribute to further advance EE: ontogenetic considerations, key ideas from cognitive biology, and the framework of the Extended Evolutionary Synthesis. EE, in particular the program of the evolution of epistemological mechanisms, seeks to provide a phylogenetic account of (...) the generation of cognitive processes underlying knowledge creation. Traditionally, EE and EEM have been oriented towards an account of evolutionary theory that mainly drew from the tenets of the Modern Synthesis. The Modern Synthesis largely dismisses ontogenetic processes and considers them irrelevant for evolutionary explanations. If anything, the role of development in evolution is believed to be that of a constraint. There is, however, ample evidence for a tight intertwinement of developmental and evolutionary processes. Organisms employ their cognitive apparatus to interact with the environment in order to achieve a fully functioning perceptual and cognitive nervous system. Also, ontogeny provides generative potentials to enable variations that natural selection can act upon. EEM’s agenda may, therefore, strongly benefit from bringing together ontogenetic and phylogenetic approaches. To grapple with this challenge, an alternative vision of the evolutionary theory termed Extended Evolutionary Synthesis could be used. This extended evolutionary theory explores relationships between the processes of individual development and phenotypic change during evolution and can provide a more suitable framework for EEM to draw from. In recent years, cognitive biology has gained momentum as an independent research field. Cognitive biology builds on the concepts of EEM and understands knowledge as a biogenic phenomenon. Its main objective is also the formulation of substantiated interrelations between cognition and evolution but it focuses on cognitive functionality at all levels of biological organization. It thus employs a “vertical” approach that encompasses nested hierarchies which span from single molecules, cells, and tissues to the organismal level, communities, and societies. In contrast to cognitive biology, EEM is here understood to adopt a “horizontal” approach that focuses on phylogenetic explanations of cognition and knowledge acquisition. Linking EEM with the key ideas of cognitive biology could make EEM’s research program stronger as it can more easily accommodate phylogenetic and ontogenetic questions within a hierarchical, multilevel perspective. This is of particular importance for a more comprehensive account of cognition since living systems are subject to context-dependent causal influences from different organizational levels. In addition to EEM, there is a second program of EE. This program has been labeled evolutionary epistemology of theories and understands the increase in human knowledge, such as scientific theories, as naturalistic accounts of evolution. Both, EEM and EET initially drew from the core concepts of the Modern Synthesis. Several scholars have severely criticized the analogies made between EET and the Neo-Darwinian key processes of evolution. In particular processes of random mutation, the rate of variation, natural selection as the unique driving force, and the adaptationist agenda are believed to reveal disanalogies. In contrast to the Modern Synthesis, the Extended Evolutionary Synthesis not only recognizes developmental processes but also ecological interactions and systems dynamics as well as social and cultural evolutionary reciprocity as important evolutionary processes. Concepts of the Extended Evolutionary Synthesis are therefore expected to be more fruitful for re-conceptualizing parallels between scientific theorizing and biological evolution. (shrink)

Adaptation by means of natural selection depends on the ability of populations to maintain variation in heritable traits. According to the Modern Synthesis this variation is sustained by mutations and genetic drift. Epigenetics, evodevo, niche construction and cultural factors have more recently been shown to contribute to heritable variation, however, leading an increasing number of biologists to call for an extended view of speciation and evolution. An additional common feature across the animal kingdom is learning, (...) defined as the ability to change behavior according to novel experiences or skills. Learning constitutes an additional source for phenotypic variation, and change in behavior may induce long lasting shifts in fitness, and hence favor evolutionary novelties. Based on published studies, I demonstrate how learning about food, mate choice and habitats has contributed substantially to speciation in the canonical story of Darwin’s finches on the Galapagos Islands. Learning cannot be reduced to genetics, because it demands decisions, which requires a subject. Evolutionary novelties may hence emerge both from shifts in allelic frequencies and from shifts in learned, subject driven behavior. The existence of two principally different sources of variation also prevents the Modern Synthesis from self-referring explanations. (shrink)

Fisher’s 1918 paper accomplished two distinct goals: unifying discrete Mendelian genetics with continuous biometric phenotypes and quantifying the variance components of variation in complex human characteristics. The former contributed to the foundation of modern quantitative genetics; the latter was adopted by social scientists interested in the pursuit of Galtonian nature-nurture questions about the biological and social origins of human behavior, especially human intelligence. This historical divergence has produced competing notions of the estimation of variance ratios referred to as heritability. (...) Jay Lush showed that they could be applied to selective breeding on the farm, while the early twin geneticists used them as a descriptive statistic to describe the degree of genetic determination in complex human traits. Here we trace the early history of the heritability coefficient now used by social scientists. (shrink)

‘Natural selection’ is, it seems, an ambiguous term. It is sometimes held to denote a consequence of variation, heredity, and environment, while at other times as denoting a force that creates adaptations. I argue that the latter, the force interpretation, is a redundant notion of natural selection. I will point to difficulties in making sense of this linguistic practise, and argue that it is frequently at odds with standard interpretations of evolutionary theory. I provide examples to show this; one (...) example involving the relation between adaptations and other traits, and a second involving the relation between selection and drift. (shrink)

Recent investigations surprisingly indicate that single RNA "stem-loops" operate solely by chemical laws that act without selective forces, and in contrast, self-ligated consortia of RNA stem-loops operate by biological selection. To understand consortial RNA selection, the concept of single quasi-species and its mutant spectra as drivers of RNA variation and evolution is rethought here. Instead, we evaluate the current RNA world scenario in which consortia of cooperating RNA stem-loops are the basic players. We thus redefine quasispecies as (...) RNA quasispecies consortia and argue that it has essential behavioral motifs that are relevant to the inherent variation, evolution and diversity in biology. We propose that qs-c is an especially innovative force. We apply qs-c thinking to RNA stem-loops and evaluate how it yields altered bulges and loops in the stem-loop regions, not as errors, but as a natural capability to generate diversity. This basic competence-not error-opens a variety of combinatorial possibilities which may alter and create new biological interactions, identities and newly emerged self identity functions. Thus RNA stem-loops typically operate as cooperative modules, like members of social groups. From such qs-c of stem-loop groups we can trace a variety of RNA secondary structures such as ribozymes, viroids, viruses, mobile genetic elements as abundant infection derived agents that provide the stem-loop societies of small and long non-coding RNAs. (shrink)

Since the emergence of our species at least, natural selection based on genetic variation has been replaced by culture as the major driving force in human evolution. It has made us what we are today, by ratcheting up cultural innovations, promoting new cognitive skills, rewiring brain networks, and even shifting gene distributions. Adopting an evolutionary perspective can therefore be highly informative for cognitive science in several ways: It encourages us to ask grand questions about the origins and ramifications of (...) our cognitive abilities; it equips us with the means to investigate, explain, and understand key dimensions of cognition; and it allows us to recognize the continued and ubiquitous workings of culture and evolution in everyday instances of cognitive behavior. Taking advantage of this reorientation presupposes a shift in focus, though, from human cognition as a general, homogenous phenomenon to the appreciation of cultural diversity in cognition as an invaluable source of data. (shrink)

In responding to three reviews of Evolution in Four Dimensions (Jablonka and Lamb, 2005, MIT Press), we briefly consider the historical background to the present genecentred view of evolution, especially the way in which Weismann’s theories have influenced it, and discuss the origins of the notion of epigenetic inheritance. We reaffirm our belief that all types of hereditary information—genetic, epigenetic, behavioural and cultural—have contributed to evolutionary change, and outline recent evidence, mainly from epigenetic studies, that suggests that non-DNA heritable (...) variations are not rare and can be quite stable. We describe ways in which such variations may have influenced evolution. The approach we take leads to broader definitions of terms such as ‘units of heredity’, ‘units of evolution’, and ‘units of selection’, and we maintain that ‘information’ can be a useful concept if it is defined in terms of its effects on the receiver. Although we agree that evolutionary theory is not undergoing a Kuhnian revolution, the incorporation of new data and ideas about hereditary variation, and about the role of development in generating it, is leading to a version of Darwinism that is very different from the gene-centred one that dominated evolutionary thinking in the second half of the twentieth century. (shrink)

Both language and genes evolve by transmission over generations with opportunity for differential replication of forms. The understanding that gene frequencies change at random by genetic drift, even in the absence of natural selection, was a seminal advance in evolutionary biology. Stochastic drift must also occur in language as a result of randomness in how linguistic forms are copied between speakers. Here we quantify the strength of selection relative to stochastic drift in language evolution. We use time series derived from (...) large corpora of annotated texts dating from the 12th to 21st centuries to analyse three well-known grammatical changes in English: the regularization of past-tense verbs, the introduction of the periphrastic ’do’, and variation in verbal negation. We reject stochastic drift in favour of selection in some cases but not in others. In particular, we infer selection towards the irregular forms of some past-tense verbs, which is likely driven by changing frequencies of rhyming patterns over time. We show that stochastic drift is stronger for rare words, which may explain why rare forms are more prone to replacement than common ones. This work provides a method for testing selective theories of language change against a null model and reveals an underappreciated role for stochasticity in language evolution. (shrink)