What can natural selection explain?

One approach to assess the explanatory power of natural selection is to ask what type of facts it can explain. The standard list of explananda includes facts like trait frequencies or the survival of particular organisms. Here, I argue that this list is incomplete: natural selection can also explain a specific kind of individual-level fact that involves traits. The ability of selection to explain this sort of fact (‘trait facts’) vindicates the explanatory commitments of empirical studies on microevolution. Trait facts must be distinguished from a closely related kind of fact, that is, the fact that a particular individual x has one trait rather than another. Whether or not selection can explain the latter type of fact is highly controversial. According to the so-called ‘Negative View’ it cannot be explained by selection. I defend the Negative View against Nanay’s (2005) objection.     

Complexity,adaptive complexity and the Creative View of natural selection

In this paper, I respond to arguments proposed by Brunnander in this journal issue concerning my position regarding the Creative View of natural selection (Razeto-Barry & Frick, 2011). Brunnander argues that (i) the Creative View we defend does not serve to answer William Paley’s question because (ii) Paley’s question is “why there are complex things rather than simple ones” and (iii) natural selection cannot answer this question. Brunnander’s arguments for (iii) defend a Non-creative View of natural selection (sensu Razeto-Barry & Frick, 2011). Here I claim that Brunnander’s arguments for (iii) are mistaken and I also argue that even accepting (iii) we do not have to accept (i), given that statement (ii) is historically and conceptually flawed. Thus here I analyze Paley’s question from a historical point of view and from a contemporary perspective in a quest for the potential conceptual relevance of Paley’s question today. In this vein I argue that from a contemporary point of view statement (iii) may be correct but for different reasons than those adduced by Brunnander.     

Evolution,altruism, and the prisoner's dilemma

I first argue against Peter Singer's exciting thesis that the Prisoner's Dilemma explains why there could be an evolutionary advantage in making reciprocal exchanges that are ultimately motivated by genuine altruism over making such exchanges on the basis of enlightened long-term self-interest. I then show that an alternative to Singer's thesis — one that is also meant to corroborate the view that natural selection favors genuine altruism, recently defended by Gregory Kavka, fails as well. Finally, I show that even granting Singer's and Kavka's claim about the selective advantage of altruism proper, it is doubtful whether that type of claim can be used in a particular sort of sociobiological argument against psychological egoism.     

Population Level Causation and a Unified Theory of Natural Selection

Sober (1984) presents an account of selection motivated by the view that one property can causally explain the occurrence of another only if the first plays a unique role in the causal production of the second. Sober holds that a causal property will play such a unique role if it is a population level cause of its effect, and on this basis argues that there is selection for a trait T only if T is a population level cause of survival and reproductive success. Sterelny and Kitcher (1988) claim against Sober that some traits directly subject to selection will not satisfy the probabilistic condition on population level causation. In this paper I show that Sober has the resources to resist the Sterelny-Kitcher complaint, but I argue that not all traits that satisfy the probabilistic condition play the required unique role in the production of their effects.     

Reasons to be fussy about cultural evolution

This discussion paper responds to two recent articles in Biology and Philosophy that raise similar objections to cultural attraction theory, a research trend in cultural evolution putting special emphasis on the fact that human minds create and transform their culture. Both papers are sympathetic to this idea, yet both also regret a lack of consilience with Boyd, Richerson and Henrich’s models of cultural evolution. I explain why cultural attraction theorists propose a different view on three points of concern for our critics. I start by detailing the claim that cultural transmission relies not chiefly on imitation or teaching, but on cognitive mechanisms like argumentation, ostensive communication, or selective trust, whose evolved or habitual function may not be the faithful reproduction of ideas or behaviours. Second, I explain why the distinction between context biases and content biases might not always be the best way to capture the interactions between culture and cognition. Lastly, I show that cultural attraction models cannot be reduced to a model of guided variation, which posits a clear separation between individual and social learning processes. With cultural attraction, the same cognitive mechanisms underlie both innovation and the preservation of traditions.     

Competency and Risk-relativity

In this paper I discuss the view that the appropriate concept of competence is a decision-relative one: that a person may be competent to make one decision but not another. The argument that I present is that neither of the two competing theories supporting the decision-relative approach, internalism and externalism, can provide a coherent explanation of why a person's competence should be thought to be relative to a particular decision. On the one hand, internalism, which regards competence as exhaustively a matter of the person's understanding, fails to identify the specific skills or content that would warrant linking a specific decision with competence, and thus cannot provide an account of decision-relative that parallels task-relative. On the other hand, externalism, which regards competence as a matter of the person's understanding in relation to external elements such as risk, cannot adequately defend why a person's competence to make a decision should 'track' the level of probable harm that results from the decision.     

Natural selection and the limitations of environmental resources

In this paper, I am clarifying and defending my argument (Nanay 2005) in favor of the claim that cumulative selection can explain adaptation provided that the environmental resources are limited. Further, elaborate on what this limitation of environmental resources means and why it is relevant for the explanatory power of natural selection.     

The negative view of natural selection

An influential argument due to Elliott Sober, subsequently strengthened by Denis Walsh and Joel Pust, moves from plausible premises to the bold conclusion that natural selection cannot explain the traits of individual organisms. If the argument were sound, the explanatory scope of selection would depend, surprisingly, on metaphysical considerations concerning origin essentialism. I show that the Sober-Walsh-Pust argument rests on a flawed counterfactual criterion for explanatory relevance. I further show that a more defensible criterion for explanatory relevance recently proposed by Michael Strevens lends support to the view that natural selection can be relevant to the explanation of individual traits.     

Acting to Let Someone Die

This paper examines the recent prominent view in medical ethics that withdrawing life‐sustaining treatment (LST) is an act of killing. I trace this view to the rejection of the traditional claim that withdrawing LST is an omission rather than an act. Although that traditional claim is not as problematic as this recent prominent view suggests, my main claim is that even if we accepted that withdrawing LST should be classified as an act rather than as an omission, it could still be classified as letting die rather than killing. Even though omissions are contrasted with acts, letting die need not be, for one can let die by means of acts. The remainder of the paper is devoted to establishing this claim and addresses certain objections to it.     

Autonomy,Natality and Freedom: A Liberal Re‐examination of Habermas in the Enhancement Debate

Jurgen Habermas has argued that carrying out pre‐natal germline enhancements would be inimical to the future child's autonomy. In this article, I suggest that many of the objections that have been made against Habermas' arguments by liberals in the enhancement debate misconstrue his claims. To explain why, I begin by explaining how Habermas' view of personal autonomy confers particular importance to the agent's embodiment and social environment. In view of this, I explain that it is possible to draw two arguments against germline enhancements from Habermas' thought. I call these arguments ‘the argument from negative freedom’ and ‘the argument from natality’. Although I argue that many of the common liberal objections to Habermas are not applicable when his arguments are properly understood, I go on to suggest ways in which supporters of enhancement might appropriately respond to Habermas' arguments.     

Why a convincing argument for causalism cannot entirely eschew population-level properties: discussion of Otsuka

Causalism is the thesis that natural selection can cause evolution. A standard argument for causalism involves showing that a hypothetical intervention on some population-level property that is identified with natural selection (e.g., variation in fitness) will result in evolution. In a pair of articles, one of which recently appeared in the pages of this journal, Jun Otsuka has put forward a quite different argument for causalism. Otsuka attempts to show that natural selection can cause evolution by considering a hypothetical intervention on an individual-level property. Specifically, Otsuka identifies natural selection with the causal relationship between a trait and fitness, claims an intervention on the strength of this relationship can cause evolution, then concludes that natural selection can cause evolution. Below I describe why Otsuka’s argument for causalism is unconvincing. Central to my criticism is that Otsuka’s argument works only if one adopts an indefensible account of natural selection, according to which natural selection can occur in the absence of trait or fitness variation. I go on to explain why any attempt to demonstrate the truth of causalism via a hypothetical intervention on an individual-level property would appear to require one to adopt an account of natural selection that is inadequate for the same reason. This in turn means the plausibility of causalism does indeed depend on the plausibility of the claim that population-level properties, which supervene on the properties of the individuals in the population, can be causally efficacious.     

Can biology make ethics objective?

A familiar position regarding the evolution of ethics is that biology can explain the origin of morals but that in doing so it removes the possibility of their having objective justification. This position is set fourth in detail in the writings of Michael Ruse (1986, 1987, 1989, 1990a, 1990b) but it is also taken by many others, notably, Jeffrie Murphy (1982), Andrew Oldenquist (1990), and Allan Gibbard (1990), I argue the contrary view that biology provides a justification of the existence of morals which is objective in the sense of being independent of people's moral views and their particular desires and preferences. Ironically, my argument builds on the very premises which are supposed to undermine the objectivity of morals. But my argument stops short of claiming that biology can give us a basis for justifying some particular system of morals. Drawing on an analogy with social contract theory, I offer a general reason why this more ambitious project cannot be expected to succeed if the argument is pursued along the same lines. Finally, I give reasons why the possibility of objective justification for a particular morality cannot be ruled out in general on evolutionary grounds.     

Changing conceptions of species

Species are thought by many to be important units of evolution. In this paper, I argue against that view. My argument is based on an examination of the role of species in the synthetic theory of evolution. I argue that if one adopts a gradualist view of evolution, one cannot make sense of the claim that species are units in the minimal sense needed to claim that they are units of evolution, namely, that they exist as discrete entities over time. My second argument is directed against an appeal to Eldredge and Gould's theory of punctuated equilibria to support the claim that species are units of evolution. If one adopts their view, it may be possible to identify discrete temporal entities that can plausibly be termed species, but there is no reason to claim that those entities are units of evolution. Thus, on two plausible interpretations of the role of natural selection in the process of evolution, species are of no special importance. I then consider some of the reasons why species have been thought to be important evolutionary units by many contemporary evolutionary biologists. Finally, I discuss briefly the implications of this conclusion for evolutionary biology.     

PERSPECTIVE: A CRITIQUE OF SEWALL WRIGHT'S SHIFTING BALANCE THEORY OF EVOLUTION

We evaluate Sewall Wright's three-phase “shifting balance” theory of evolution, examining both the theoretical issues and the relevant data from nature and the laboratory. We conclude that while phases I and II of Wright's theory (the movement of populations from one “adaptive peak” to another via drift and selection) can occur under some conditions, genetic drift is often unnecessary for movement between peaks. Phase III of the shifting balance, in which adaptations spread from particular populations to the entire species, faces two major theoretical obstacles: (1) unlike adaptations favored by simple directional selection, adaptations whose fixation requires some genetic drift are often prevented from spreading by barriers to gene flow; and (2) it is difficult to assemble complex adaptations whose constituent parts arise via peak shifts in different demes. Our review of the data from nature shows that although there is some evidence for individual phases of the shifting balance process, there are few empirical observations explained better by Wright's three-phase mechanism than by simple mass selection. Similarly, artificial selection experiments fail to show that selection in subdivided populations produces greater response than does mass selection in large populations. The complexity of the shifting balance process and the difficulty of establishing that adaptive valleys have been crossed by genetic drift make it impossible to test Wright's claim that adaptations commonly originate by this process. In view of these problems, it seems unreasonable to consider the shifting balance process as an important explanation for the evolution of adaptations.     

EXPERIMENTAL STUDIES OF COMMUNITY EVOLUTION I: THE RESPONSE TO SELECTION AT THE COMMUNITY LEVEL

Coevolution generally refers to the process of two or more organisms adapting to each other as a result of individual selection. Another possibility, however, is that coevolution may result from selection acting directly at the community level. Certain types of multispecies associations, such as lichens, which are a symbiotic association between an alga and a fungus, are examples of simple two species communities that may be units of selection. The study presented here uses two species communities of Tribolium castaneum and T. confusum in an investigation of selection acting at the community level. Selection at the community level is performed on one trait measured in one species and correlated responses in other traits measured both within species and among species are monitored. I demonstrate that community selection, defined as the differential survival and or reproduction of communities, can result in significant changes in the phenotype of a community. The observed changes in the phenotype of a community as a result of community selection included changes in the trait under selection (direct effects of selection), as well as changes in traits that are not under selection (correlated responses to selection). Furthermore, two types of correlated responses to selection were observed. The first, within-species correlated responses to selection, are changes in a trait measured in one species as a result of community selection acting on another trait measured in the same species. The second, between-species correlated responses to selection, are changes in a trait measured in one species as a result of community selection acting on a trait measured in another species. Between species correlated responses to selection are of particular interest because they cannot be mediated by pathways of gene action that are internal to an individual, rather they can be mediated only through ecological pathways. In other words, between-species correlated responses to selection suggest that genetically based interactions among individuals are contributing to the response to community selection. These among species ecological pathways of gene action cannot contribute to a response to selection at a lower level; thus community selection may be able to bring about a response to selection that is qualitatively different from the response selection that would occur as a result of selection acting at a lower level.     

Darwin's species category realism

Ever since Charles Darwin's On the Origin of Species was published, the received view has been that Darwin literally thought of species as not extra-mentally real. In 1969 Michael Ghiselin upset the received view by interpreting Darwin to mean that species taxa are indeed real but not the species category. In 1985 John Beatty took Ghiselin's thesis a step further by providing a strategy theory to explain why Darwin would say one thing (his repeated nominalistic definition of species) and do another (hold that species taxa are real). In the present paper I attempt to take this line of interpretation to a new level. Guided by the principle of charity, I provide and analyze a considerable amount of evidence from Darwin's mature writings (both private and published) to show that (contra Ghiselin and Beatty) Darwin did not simply accept the species delimitations of his fellow naturalists but actually employed, repeatedly and consistently, a species concept in a thoroughly modern sense, albeit with an implicit definition, a concept uniquely his own and fully in accord with his theory of evolution by natural selection. This implicit concept and definition is carefully reconstructed in the present paper. A new strategy theory is then provided to account for why Darwin would define species (both taxa and category) nominalistically on the one hand but delimit species realistically on the other.     

Are Random Drift and Natural Selection Conceptually Distinct?

The latter half of thetwentieth century has been marked by debates inevolutionary biology over the relativesignificance of natural selection and randomdrift: the so-called ``neutralist/selectionist'debates. Yet John Beatty has argued that it isdifficult, if not impossible, to distinguishthe concept of random drift from the concept ofnatural selection, a claim that has beenaccepted by many philosophers of biology. Ifthis claim is correct, then theneutralist/selectionist debates seem at bestfutile, and at worst, meaningless. I reexaminethe issues that Beatty raises, and argue thatrandom drift and natural selection, conceivedas processes, can be distinguished from one another.     

Natural selection affects multiple aspects of genetic variation at putatively neutral sites across the human genome

A major question in evolutionary biology is how natural selection has shaped patterns of genetic variation across the human genome. Previous work has documented a reduction in genetic diversity in regions of the genome with low recombination rates. However, it is unclear whether other summaries of genetic variation, like allele frequencies, are also correlated with recombination rate and whether these correlations can be explained solely by negative selection against deleterious mutations or whether positive selection acting on favorable alleles is also required. Here we attempt to address these questions by analyzing three different genome-wide resequencing datasets from European individuals. We document several significant correlations between different genomic features. In particular, we find that average minor allele frequency and diversity are reduced in regions of low recombination and that human diversity, human-chimp divergence, and average minor allele frequency are reduced near genes. Population genetic simulations show that either positive natural selection acting on favorable mutations or negative natural selection acting against deleterious mutations can explain these correlations. However, models with strong positive selection on nonsynonymous mutations and little negative selection predict a stronger negative correlation between neutral diversity and nonsynonymous divergence than observed in the actual data, supporting the importance of negative, rather than positive, selection throughout the genome. Further, we show that the widespread presence of weakly deleterious alleles, rather than a small number of strongly positively selected mutations, is responsible for the correlation between neutral genetic diversity and recombination rate. This work suggests that natural selection has affected multiple aspects of linked neutral variation throughout the human genome and that positive selection is not required to explain these observations.     

Directional selection in temporally replicated studies is remarkably consistent

Temporal variation in selection is a fundamental determinant of evolutionary outcomes. A recent paper presented a synthetic analysis of temporal variation in selection in natural populations. The authors concluded that there is substantial variation in the strength and direction of selection over time, but acknowledged that sampling error would result in estimates of selection that were more variable than the true values. We reanalyze their dataset using techniques that account for the necessary effect of sampling error to inflate apparent levels of variation and show that directional selection is remarkably constant over time, both in magnitude and direction. Thus we cannot claim that the available data support the existence of substantial temporal heterogeneity in selection. Nonetheless, we conject that temporal variation in selection could be important, but that there are good reasons why it may not appear in the available data. These new analyses highlight the importance of applying techniques that estimate parameters of the distribution of selection, rather than parameters of the distribution of estimated selection (which will reflect both sampling error and "real" variation in selection); indeed, despite availability of methods for the former, focus on the latter has been common in synthetic reviews of the aspects of selection in nature, and can lead to serious misinterpretations.     

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.