Life Sciences

Philosophy of the Life Sciences

by David L Hull

 

Introduction

Philosophers of science are just one of several groups of scholars who take science as their subject matter. Philosophers tend to concentrate on the general structure of science, its laws and theories, problems surrounding testability, the relation between theories, etc. Traditional philosophy of science does not differ all that much from epistemology – the study of highly abstract problems concerned with the acquisition of knowledge in its most general sense. But in recent decades philosophers of science have tended to delve more deeply into the content of science. As part of this change in emphasis, philosophy of the life sciences has become increasingly prominent. If science really matters to philosophers of science, then it just may be the case that the same general analysis of science, derived mainly from physics, will not do for all the sciences. Philosophers of the life sciences concentrate on biology, in particular those areas of biology that are uniquely biological. However, the primary target of philosophical analyses of biology has been evolutionary theory. It provides a host of issues that philosophers find intriguing.

Darwinian Evolution

Both Charles Darwin and Alfred Russel Wallace set out theories about the mechanisms that result in ancestral species evolving into descendant species to produce one huge tree of life. The main mechanism that drives this process is natural selection. More organisms are produced every generation than can possibly survive and reproduce. Which organisms survive to reproduce is a function of how successfully they cope with their environments. Darwin also acknowledged that Lamarckian mechanisms might have some minor influence on evolution. According to Lamarck, organisms can adjust to their environments and pass on these adaptations to their offspring by means of the hereditary material. As a result evolution could proceed at a more rapid pace. In spite of Darwin's acknowledging a possible though minor role for Lamarckian inheritance in evolution, subsequent authors set the two in opposition. Either you are a Darwinian or a Lamarckian; nothing in between.

Noticeably absent from Darwin's theory were well-formulated theories of heredity and development. Darwin himself did set out his provisional theory of pangenesis, but it was only a sketch, and very few of his contemporaries adopted it. Although the study of development was an active area of research in Darwin's day, it was mainly descriptive. In addition, because theories of development at the time were so permeated with a static world-view, biologists could not see how this work could be integrated into evolutionary theory. The main exception was the biogenetic law – the view that ontogeny recapitulates phylogeny. Because developmental sequences (ontogeny) arise via selection as ancestral species produce descendent species (phylogeny), one should be able to infer the course of phylogeny from these developmental sequences. Instead of attempting to integrate theories of heredity and development into a Darwinian theory of evolution, later workers in the late nineteenth century busied themselves in reconstructing phylogenies, transforming the archetypes of ideal morphologists into ancestors.

Darwin and his followers needed a theory of heredity to supplement their theory of evolution. Little did they know that an obscure monk, Gregor Mendel, had already formulated just such a theory in 1865. Thirty-five years later, at the turn of the century, a very similar theory was proposed and quickly adopted. Unfortunately, these early geneticists thought that their theory of genetics – Mendelian genetics – was incompatible with Darwinian versions of evolutionary theory. The chief issue was how ‘particulate’ genes and their phenotypic effects are. Because early Mendelian geneticists tended to study those traits that appear in two discrete forms (e.g. wrinkled versus smooth seed coat), they argued that evolution could not be as ‘continuous’ as the Darwinians at the time thought.

Thus, in the early decades of the twentieth century, evolutionary biologists were confronted with numerous problems. Many biologists, especially palaeontologists, accepted Lamarckian inheritance as the main mode of heredity. Much of palaeontology at the time was still ruled surreptitiously by archetypes that had not been successfully transformed into ancestors, and Mendelian genetics seemed at the time incompatible with Darwinian theory. In addition, developmental biologists had yet to come up with a theory of development that could be wedded to evolutionary theory. The need for a synthesis was obvious.

The Modern Synthesis

The architects of the modern synthesis fall neatly into two triumvirates – the mathematically minded R. A. Fisher, J. B. S. Haldane and Sewall Wright and the more empirically minded Theodosius Dobzhansky, G. G. Simpson and Ernst Mayr. Fisher, Haldane and Wright showed that many of the intuitive ideas about the evolutionary process popular at the time were mistaken. For example, Mendelian genetics was not in the least incompatible with a Darwinian theory of evolution. Several discrete genes working in consort could produce smoothly gradated effects (e.g. skin colour in humans). Fisher argued, contrary to a common belief at the time, that very small selection pressures could have significant effects. He also showed how dominance might evolve, how selection could maintain two or more alleles in a population, and what the direct connection was between genetic variance in fitness in a population and the rate of increase in fitness – his famous fundamental theorem of natural selection.

In his work Haldane not only emphasized the role of chromosomes in evolution but also supported Fisher's conclusions about the influence of slight advantages. He also raised the issue of ‘altruism’. How could a behaviour or trait in an organism evolve if it benefited another organism at its own expense? In principle, such behaviours or traits should not evolve, but they seemed to. Haldane's solution was that altruism could evolve if the recipient of the benefit is closely related to the benefactor – kin selection. The genes for altruistic traits are not passed down directly from parent to offspring but indirectly through one's collateral relatives. Haldane was supposed to have remarked, half seriously, that one should be prepared to give one's life for more than two siblings, four half-siblings, eight first cousins, etc.

In order to make their mathematical models tractable, these early workers had to introduce simplifying assumptions. One of the most frequent was that populations in nature are large enough to be treated as if they were infinite. Fisher's paradigm of evolution was the mass selection of slight variations. Wright took just the opposite tack. According to Wright, a much more common state of affairs in nature is the subdivision of species into small, partially isolated groups or ‘demes’, as Wright termed them. For example, mice living in different haystacks might count as living in separate demes. Within a deme a new mutant might have a chance of becoming established even if it were not all that adaptive – a process that Wright termed ‘drift’. According to Wright's shifting-balance theory, an allele becomes established in a deme by drift followed by selection among demes. Because none of the mathematical tools available at the time could handle such phenomena, Wright had to develop his own mathematics, a technique termed ‘path-analysis’.

As the preceding discussions indicate, Fisher, Haldane and Wright produced mathematical versions of evolution theory that made it look more like the theories in physics. In physics, observational consequences can be generated from basic principles and tested. The problem in evolutionary biology was how to test these mathematical models of the evolutionary process. Dobzhansky's great achievement in his seminal Genetics and the Origin of Species (1937) was to show in detail that Mendelian genetics could be integrated flawlessly into a Darwinian version of evolutionary theory. Of equal importance, he showed how the basic principles of evolutionary theory could be tested, albeit indirectly, and inspired later generations of evolutionary biologists to do the same. In this effort he was aided by Wright, who supplied the mathematical ability that Dobzhansky lacked.

Simpson in turn showed how the emerging synthetic theory of evolution was compatible with the principles of palaeontology, once these principles were shorn of their saltative, idealistic, Lamarckian elements. Simpson argued that, in the main, evolution is gradual. New species do not emerge from their ancestors saltatively in the space of one, two or even several generations but gradually over numerous generations. It was the idealist mind-set that treated biological species as if they were akin to sharply distinguishable geometric figures that made biological evolution look so saltative. Perhaps a three-sided closed figure cannot evolve gradually into a four-sided closed figure, but one species can evolve gradually into another.

Mayr did for systematics what Simpson did for palaeontology. He showed that the relationships recognized by systematists supported the synthetic theory. In particular, the chief (possibly only) mechanism for new species of sexual organisms to arise is via reproductive isolation. According to Mayr species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups. But Mayr's most fundamental contribution was a revision of the notion of a population. Both mathematicians and evolutionary biologists use the term ‘population’ but in significantly different senses. Biological populations are finite, usually quite small, and exhibit significant population structure. A common way for new species to arise is for a few organisms at the periphery of a species to become geographically isolated from the larger species. Because these early populations are so small, chance events can have a significant effect. Most such peripheral isolates go extinct, but the few that succeed in surviving can produce new species quite rapidly.

Survival of the Fittest

From the start evolutionary theory gave rise to problems that looked very ‘philosophical’ to biologists and philosophers alike. Is evolution in any sense ‘progressive’? Does it at least have a ‘direction’? What role does ‘chance’ play in evolution? Does biology include laws and theories of the sort exhibited in physics? Can biology be reduced to molecular biology and from there to physics and chemistry? One virtue of this early literature is that philosophically sophisticated biologists played an important role in dealing with these problems aided and abetted by professional philosophers who were knowledgeable of biology. This commensalism has only increased through time.

One example of the contributions made by philosophers of biology to biology turns on the notion of fitness. In fact, fitness plays such a central role in philosophy of biology that some commentators have wryly termed this discipline the ‘philosophy of fitness’. Early in the development of evolutionary theory, Herbert Spencer suggested the phrase ‘survival of the fittest’ to characterize the action of natural selection. It did not take long for critics to complain that the principle of the survival of the fittest is a mere tautology. Whatever organisms survive to reproduce are by definition the fittest, and those that are the fittest automatically survive to reproduce.

Philosophers commonly divide declarative sentences into two kinds – analytic and synthetic. Analytic statements are very peculiar. They are true or false solely in virtue of the definitions of the terms that they contain. Some of these statements are necessarily true – tautologies. Others are necessarily false – contradictions. Synthetic claims require more than knowledge of definitions to know if they are true or false. To be sure, one must know what the various terms that they contain mean, but in addition one must know something about the empirical world. For example, do all rivers flow downstream? If ‘downstream’ is defined in terms of the direction that the water is flowing, then the answer is yes, but not because of any facts about the world. It is a tautology. If ‘downstream’ is defined in terms of gravity, the answer is still yes, but for empirical reasons.

The principle of the survival of the fittest looks on the surface to be a tautology if ‘fittest’ is defined solely in terms of survival. Of course, arguing that a statement is a tautology does not mean that it is entirely worthless. After all, tautologies have the virtue of being true. In addition, on the received view at the time, all of logic and mathematics consist in nothing but tautologies. Unfortunately, claims about the survival of the fittest are a crucial part of the causal mechanism of evolutionary change. One thing that tautologies are not is descriptions of causal mechanisms. If the tautology objection is accurate, then Darwinian theory is deprived of its primary mechanism.

One way to evaluate the tautology objection is to compare the principle of the survival of the fittest to comparable statements in physics. For example, one of the basic laws of physics is F = ma. To some, this law also looks like a mere tautology. Force is nothing but the relation of mass and acceleration. Anything that does not obey this law does not count as a genuine mass. If the principle of the survival of the fittest is a tautology, then so are numerous statements in physics that physicists treat as genuine laws of nature. A more fundamental response is that laws cannot be evaluated in isolation from the theoretical context in which they occur and are tested. Biologists have developed various engineering principles to help decide which traits are advantageous, which not, and under what circumstances. Being able to produce sugar by means of photosynthesis certainly looks as if it would increase the fitness of any organisms that possess such an ability. By and large they would produce more viable offspring than other, closely related organisms that lack this ability. However, if biologists were to discover that fewer organisms that can synthesize sugar via photosynthesis survive to reproduce than those that cannot, they would not automatically abandon either their engineering principles or the principle of the survival of the fittest. They would look more deeply into the facts of the matter, a practice that is peculiar if it is a tautology.

Real objects in booming, buzzing nature do not obey biological laws perfectly. They approach this ideal only under highly restricted ideal circumstances. The first law recognized as a law in population genetics was the Hardy–Weinberg law. According to this law, if nothing interferes, the transmission of two alleles at a single locus, produces a binary distribution – the same as flipping a coin. Instead of heads and tails, the Hardy–Weinberg law concerns such things as the distribution of genes for blue eyes and brown eyes. Any departures from this distribution must be due to one or more of the boundary conditions being breached. These departures might be due to immigration, emigration, selection, biased mating, etc. However, this list of boundary conditions is limited, and new items are not added to it casually. Comparable reservations have been noted in the laws of physics, leading some philosophers to conclude that there are no laws of physics. If strict standards are used, then no laws exist anywhere, whether in physics or in biology. If more reasonable standards are employed, then lots of laws exist in both physics and biology.

Reduction

In the traditional hierarchical ordering of the sciences, biology can be found lodged between psychology and sociology above it and physics and chemistry below. Many biologists assume that eventually psychology and sociology will be reduced to biology. After all, mental events are nothing but activity in the central nervous system. However, they are less enthusiastic about biology being reduced to physics and chemistry, even if living creatures are nothing but complex systems of molecules. Arguments about reduction become highly emotional because they concern the relative worth of the various sciences.

One role that philosophers have played in this dispute is to tease apart various senses of ‘reduction’ and to present explicit characterizations of some of these senses. The most carefully analysed notion of reduction is theory reduction. If scientific theories are construed as deductively organized systems of laws, then derivations can take place within such theories. But the same can be said for the relations between different theories. If terms in one theory can be identified with terms in another theory, then the higher level theory can be derived from lower level theory; for example, thermodynamics can be derived from statistical mechanics. Similarly, Mendelian genetics (a higher level theory) can be derived from molecular biology (a lower level theory).

For example, the Mendelian ratio of 3:1 for dominance should be derivable from knowledge of what is going on at the molecular level. However, when philosophers tried to produce such derivations, all sorts of problems arose. For example, the same Mendelian ratio can be produced by more than one molecular mechanism, and conversely, the same molecular mechanism can produce more than one Mendelian ratio. Of course, the preceding claim turns on how one construes ‘Mendelian ratio’ and ‘molecular mechanism’. If these notions are gerrymandered to fit the needs of reduction, then the derivations can be brought off and reduction becomes nothing but an exercise in logic. However, these two notions are defined in the context of their respective theories. When they are, decisions with respect to reduction lean heavily on matters of fact. Thus far no one has yet to derive the basic principles of Mendelian genetics from molecular biology while doing justice to the actual state of these two theories. Once again using physics as the standard of comparison is misleading. As it turns out, the reduction of thermodynamics to statistical mechanics when taken seriously poses the same array of problems that philosophers have found in biology.

Levels of Selection

The biological world is organized into entities of increasing complexity – genes, chromosomes, organelles, cells, tissues, organs, organisms, kinship groups, demes and species. A recurrent debate in biology is the level or levels at which selection takes place. Gene selectionists insist that selection occurs only at the level of individual genes. Others are willing to acknowledge that selection can occur at some of the lower levels in the organizational hierarchy but draw the line long before the species level is reached. Species selection, so they argue, cannot take place. Still others insist that selection wanders up and down the organizational hierarchy.

The chief problem in this dispute is that ‘selection’ is being used in an ambiguous way. Selection consists in two processes – replication and environmental interaction. Certain entities are able to pass on their structure largely intact through successive replications. Certain entities also causally interact with their environments in such a way that replication is differential; for example, certain sparrows are better able to withstand freezing temperatures than others. Hence they survive to reproduce in greater numbers. In selection replication alternates with environmental interaction. Thus questions about levels of selection are transformed into two questions. At what level or levels does replication occur? The answer is primarily at the lower levels of organization, chiefly genes. At what level or levels does environmental interaction occur? The answer is at a wide variety of levels, from genes and organisms to kinship groups and possibly entire species. See also Philosophy of Selection (natural, Sexual and Drift)

Gene selectionists can now be interpreted to be making either a modest or an extravagant claim. If all that gene selectionists are arguing is that replication takes place primarily at the level of the genetic material, then this position is not very controversial. However, if they insist that all that is needed for selection to occur is replication, then their position seems clearly false because it leaves out all reference to environmental interaction, and changes in gene frequencies without environmental interaction are instances of drift, not selection (see Wright). However, gene selectionists do acknowledge the role of the environment in selection, but they insist that, in the long run, selection processes can be reduced to replication because the only effects of environmental interaction that matter are those that are registered in the genetic material. Besides, environmental interactions are extremely variable and contingent. Including such vagaries within the basic principles of evolutionary theory makes it look very different from the laws of physics and chemistry.

Equally involved controversies occur with respect to environmental interaction. Everyone agrees that a main focus of environmental interaction is organisms, but how about higher levels of organization? We have already discussed Wright's shifting-balance theory. If demes are considered ‘groups’, then it is a form of group selection, but the groups of greatest interest in this dispute are species themselves. Can they function as distinct entities in the selection process? Do species exhibit characteristics of their own that can count as adaptations, or can all such characteristics be reduced to the characteristics of organisms and their interrelations? For example, if speciation occurs commonly by the isolation of small peripheral isolates, then those species that have longer more convoluted peripheries are likely to speciate more often than those that do not. The question then becomes can the range of a species be reduced to the specification of each organism and its spatial relation to other organisms. A more recent and equally controversial form of group selection is of ‘groups’ defined in terms of the traits that they exhibit – trait-group selection.

Development

The preceding discussion of selection is couched in terms of traditional entities and processes. From Darwin's day to the present, biologists have recognized the importance of development in the living world but were unable to integrate their knowledge into a Darwinian theory of evolution. In recent years considerable headway has been made in our understanding of development. Yes, we have vast stores of empirical data about development, but more importantly general knowledge of developmental processes as such is emerging. Possibly now is the time finally to bring this area of biology within the purview of evolution.

One such attempt is developmental systems theory. As is usually the case, advocates of this new position take it to be incompatible with traditional Darwinian theory and spend most of their time denigrating what they take to be the weaknesses of this theory. One of the chief weaknesses of Darwinian theory, as seen from the developmentalist perspective, is the distinction between nature and nurture, a distinction that they take to be inherent in Darwinian selection (see earlier discussion of selection). Quite a bit of nonsense has been written about the nature/nurture issue, especially in the social sciences. Contrary to what one might think from reading this literature, ‘genetic’ is not equivalent to ‘fixed’. Genes do mutate. Nor does ‘genetic’ entail ‘universal’. Various alleles can be found at a wide variety of loci. Some are universally distributed within their species; others are not.

But even when one turns to sophisticated discussions of the nature/nurture issue, one thing is clear – it is extremely complicated. It is almost impossible to say anything cogent and meaningful about particular instances. The extremes are easy. Eye colour in humans is due to nature in the sense that in any environment in which humans can survive, changes in the environment have no influence on the colour of our eyes. Perhaps some environment exists that can influence eye colour in humans and we simply have not stumbled on it yet, but for now, eye colour in humans is as ‘genetic’ as a trait can get. Many characters are comparably ‘environmental’: lots of genes may be necessary for the organism to function at all, but genetic differences are not correlated with differences in this character. I say tomato; you say tomäto.

But in most cases both genetic differences and environmental differences play a role in the resulting trait, and these relations are extremely complicated and variable. The best characterization of these complex interrelations is the reaction norm – a diagram that results from exposing clones (organisms with the same genetic make-up) to a single environmental factor that is systematically varied. What effect does varying the amount of carbon dioxide in the environment have on these clones? Then move on to another environmental variable. The trouble with this procedure is that the resulting interrelations are so complicated, it is impossible to integrate them into a single conception.

Advocates of developmental systems theory propose to replace traditional Darwinian theory and its extremely problematic nature/nurture concept, not with replicators, not with environmental interactions, but with life cycles. Development is basic. Must Darwinian evolution be junked? Is the increased knowledge that we are obtaining about development incompatible with Darwinian evolution or will it eventually be subsumed under this theory? The apparent conflict between Mendelian genetics and Darwinian theory turned out to be the result of misunderstandings. Will the same be the case for developmental systems theory or will biologists have to revise their understanding of the evolutionary process so fundamentally that it cannot in all good conscience be termed ‘Darwinian’? These questions cannot be answered until this new theory has been developed in greater detail. So far we have the promise of an alternative to traditional Darwinian theory but only a promise.

Sociobiology

From the beginning, everyone acknowledged that evolutionary theory applied to all species, including the human species. The fear of this extension was one of the strongest motivations for Darwin's contemporaries to reject his theory. Perhaps species of pigeons, fruitflies and sequoia trees evolve but not Homo sapiens. In biology this response is no longer acceptable. The human species evolves as do all species. However, the mechanisms involved in human evolution may be different from other species. As is the case with all species, the human species is unique. Sociobiology is the latest attempt to make good on the promises of applying evolutionary theory to the human species. The name ‘sociobiology’ is derived from E. O. Wilson's book Sociobiology: The New Synthesis (1975). Wilson intended to expand the modern synthesis to apply to the psychological and sociological characteristics of all organisms including people. However, in his seminal work, Wilson barely touched on human beings. In spite of the brouhaha that once again followed on an attempt to bring human beings into the purview of evolutionary theory, sociobiology had prospered. The intellectual sources of this latest attempt at synthesis can be found in the work of W. D. Hamilton and G. C. Williams in the 1960s. Hamilton showed that many of the phenomena commonly referred to as instances of ‘group selection’ can be explained by kin selection. Williams argued that the principle of parsimony dictates that phenomena be explained at the lowest level possible; for example, if a sort of behaviour can be explained at the level of individual organisms, no need exists for introducing explanations of phenomena at higher levels of organization.

Numerous objections have been raised to sociobiology, both in its tendency to assume gene selection and in its applications to human beings. Gene selection has already been discussed. Sociobiologists can strive to extend evolutionary theory to human beings without this theory being gene selectionist. The selection of entire organisms, demes and even trait groups can count as contributing to the sociobiological research programme. Most of the issues that present-day philosophers of biology address are fundamentally biological; e.g. gene selectionism. But some are very narrowly philosophical. The fundamental objection to sociobiology turns on an attempt to salvage free will, and if any issue counts as narrowly philosophical the contrast between free will and determinism does. The source of this problem is once again the contrast between nature and nurture.

The opponents of sociobiology repeatedly complain about the phrase ‘the gene for’. Is there really such a thing as ‘the gene for blue eyes’? A whole series of steps are necessary to produce brown pigment in the irises of human beings. At some point in human history, a mutation happened to occur near the end of this biosynthetic pathway, a mutation that interfered with the production of brown pigment. The effect is blue eyes. Defenders of this usage complain that reference to ‘the gene for blue eyes’ is harmless enough. Of course, lots of other genes play a role in this long biosynthetic pathway. The critics respond that this mode of expression not only makes genes and their effects seem more isolated and particulate than they actually are but also encourages a belief in genetic determinism. My genes made me do it. Opponents of genetic determinism emphasize the equally important role of the environment in human actions. Of course, environmental determinism is no better. In point of fact, the effects of genes and environment intertwine in very complicated ways. But the interplay between genes and environments may be more complicated than either of the other two forms of determinism, but it is no less deterministic. My genes did not make me do it. My environment did not make me do it. Instead, the complex interplay of my genes and my environment made me do it. Once one understands everything that biologists have to say about development, the problem of free will remains. What is free will? Is it a magic spark? If not, what? See also Gene Flow and Natural Selection

Glossary
Archetype
An idealized image of the most fundamental ‘ground plan’ upon which the structure of the group of animals or plants is based.
Drift
Differential survival and reproduction not due to environmental interaction.
Group selection
Selection in which an entire group as such is the target of environmental interaction.
Kin selection
Selection that results from close kin behaving in ways to favour each other resulting in increased inclusive fitness.
Recapitulation
The biogenetic law according to which the evolutionary history of a species (phylogeny) is repeated in a speeded-up form in the embryological development of the individual organism (ontogeny).
Saltation
The evolution of new species by sudden, discontinuous steps.
Selection
Differential survival and reproduction due to environmental interaction.

Further Reading

  • Ayala FJ and Dobzhansky T (1974) Studies in the Philosophy of Science. New York: Macmillan.
  • Brandon RN (1996) Concepts and Methods in Evolutionary Biology. Cambridge: Cambridge University Press.
  • Cartwright N (1983) How the Laws of Physics Lie. Oxford: Oxford University Press.
  • Dawkins R (1976) The Selfish Gene. Oxford: Oxford University Press.
  • Desmond A (1982) Archetypes and Ancestors: Palaeontology in Victorian London, 1850–1875. London: Blond & Briggs.
  • Dobzhansky T (1937) Genetics and the Origin of Species. New York: Columbia University Press.
  • EreshefskyM (ed.) (1992) The Units of Evolution: Essays on the Nature of Species. Cambridge, MA: MIT Press.
  • Fisher RA (1930) The Genetical Theory of Natural Selection. Oxford: Oxford University Press.
  • Gayon J (1998) Darwinism's Struggle for Survival. Cambridge: Cambridge University Press.
  • Haldane (1932) The Causes of Evolution. London: Harper.
  • Hamilton WD (1964) The genetical evolution of social behavior. Journal of Theoretical Biology 7: 152.
  • HullDL and RuseM (eds) (1998) The Philosophy of Biology. Oxford: Oxford University Press.
  • Kitcher P (1985) Vaulting Ambition: Sociobiology and the Quest for Human Nature. Cambridge, MA: MIT Press.
  • Mayr E (1942) Systematics and the Origin of Species. New York: Columbia University Press.
  • MayrE and ProvineWB (eds) (1980) The Evolutionary Synthesis: Perspectives on the Unification of Biology. Cambridge, MA: Harvard University Press.
  • Mitchell SD (2000) Dimensions of scientific law. Philosophy of Science 67: 242265.
  • Oyama S (1985) The Ontogeny of Information: Developmental Systems and Evolution. Cambridge: Cambridge University Press.
  • Ruse M (1979) The Darwinian Revolution: Science Red in Tooth and Claw. Chicago: University of Chicago Press.
  • Sarkar S (1999) Genetics and Reductionism. Cambridge: Cambridge University Press.
  • Schlichting CD and Pigliucci M (1998) Phenotypic Evolution: A Reaction Norm Perspective. Sunderland, MA: Sinauer.
  • Simpson GG (1944) Tempo and Mode in Evolution. New York: Columbia University Press.
  • Sober E (2000) Philosophy of Biology, 2nd edn. Boulder, CO: Westview Press.
  • Sober E and Wilson DS (1998) Unto Others: The Evolution and Psychology of Unselfish Behaviour. Cambridge, MA: Harvard University Press.
  • Sterelny K (2001) The Evolution of Agency and Other Essays. Cambridge, UK: Cambridge University Press.
  • Sterelny K and Griffiths PE (1999) Sex and Death: An Introduction to Philosophy of Biology. Chicago: University of Chicago Press.
  • Williams GC (1966) Adaptation and Natural Selection. Princeton, NJ: Princeton University Press.
  • Wilson DS and Sober E (1994) Re-introducing group selection in the human behavioral sciences. Behavioral and Brain Sciences 17: 585608.
  • Wilson EO (1975) Sociobiology: The New Synthesis. Cambridge, MA: Harvard University Press.
  • Wright S (1929) Fisher's theory of dominance. American Naturalist 63: 274279.
  • Wright S (1931) Evolution in Mendelian populations. Genetics 16: 97100, 155159.

 

 

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