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Darwinism Evolving examines the Darwinian research tradition in evolutionary biology from its Systems Dynamics and the Genealogy of Natural Selection.
Table of contents
- Evolution and systems
- Crossing the Complexity Barrier
- Darwinism Evolving. System Dynamics and the Genealogy of Natural Selection
- The Never-Ending Problems In Evolutionary Biology
There is, however, a way of formulating the theory in its modern guise which maintains an essentially Darwinian character. Since there are a number of confirmed ways in which natural populations can evolve in the absence of natural selection, and since balancing selection, i. That is, it is a way of establishing that a population either is or is not in equilibrium, and it provides sophisticated tools for measuring rates of change in a population across generations. Moreover, like the kinematics of any physical theory, if it establishes cross-generational change, it also tells us that there are causes to be found—the detailed contours of those measures may even provide suggestions as to where to look for those causes.
What it cannot do on its own is provide knowledge of the forces at work. To use language introduced by Elliott Sober, fitness, unlike natural selection, is causally inert. For further information, see the entry on population genetics. That means that, as valuable as population genetics is, it should not be equated with the theory of natural selection. Too often in both biological presentations of the theory and philosophical discussions of it, this is forgotten. For example:. Natural selection, if it is to resemble the Darwinian concept that bears that name, must be reserved for reference to an interaction between a variable, heritable feature of an organic system and the environment of that system.
Evolution and systems
That interaction may or may not change the proportions of those features across generations, and those proportions may change for reasons other than those interactions. But a plausible natural selection hypothesis must posit some such interaction. On this issue I will give the last word to Stephen Jay Gould:. If we suppose that for Darwin natural selection was almost exclusively thought of as an interaction between individual organisms and their organic and inorganic environments, then we can see two challenges to Darwinism today with respect to levels of selection.
There are those, such as G. Williams and Richard Dawkins, who argue that selection is always and only of genes. Here is a clear statement:. Throughout that book selection is always said to be of individual alleles, regardless of the role environments at various levels may play in the process. This view has been extensively challenged by philosophers of biology on both methodological and conceptual grounds, though there are, among philosophers, enthusiastic supporters cf.
Dennett In all the give and take, it is seldom noticed that defenders of this view claim to be carrying the Darwinian flag Gayon and Gould are exceptions. Yet it is certainly not a position that Darwin would recognize--and not merely because he lacked a coherent theory of the units of inheritance. It is not a Darwinian view because for Darwin it was differences in the abilities of organisms at various stages of development to respond to the challenges of life that had causal primacy in the explanation of evolutionary change.
Darwinism also has challenges from the opposite direction. A very different result emerges if one assumes that groups of organisms such as demes, kin-groups, or species, though not individuals, are nevertheless subject to selection.
Crossing the Complexity Barrier
Others define group selection primarily in terms of group level effects. For further discussion, see Sterelny and Griffiths , —; Hull , 49—90; and see the entry on: levels and units of selection. One might say this was the central promise of Darwinism—to account for both phylogenic continuity and adaptive differentiation by means of the same principles; or as Darwin puts it, to integrate in one theory the supposed opposition between Unity of Type and Conditions of Existence.
Moreover, the evidence from the study of variation in domestic and natural populations put the lie to any claim that God directs all or most variation along beneficial lines. Darwinian selection theory is a two-step process—the production of variation unrelated to the adaptive requirements of the organism, and differential perpetuation of those variations that serve adaptive needs. Again, a theory of evolution that could not be so described would not be a Darwinian theory.
Here I want to focus on only one important question—to what extent is the teleological appearance of such explanations simply that, an appearance masking a causal process in which goals play no role? The appearance of teleology is certainly present in Darwinian explanations, and has been since Darwin spoke of natural selection working solely for the good of each being. The appearance of teleology stems from the ease with which both evolutionary biology and common sense take it for granted that animals and plants have the adaptations they do because of some benefit or advantage to the organism provided by those adaptations.
This is a hotly contested question, and I will here simply sketch a case that selective explanations of adaptations are robustly teleological. The interested reader may want to refer to the literature on this question referred to in the discussion and listed in the list of readings provided at the end of this entry. Etymologically, they come to the same thing; and the philosophical arguments given in favor of the change all rest on an historically doubtful assumption—that philosophical defenses of teleology have always been either theistic or vitalistic.
The serious philosophical issue can be put simply and directly: in selection explanations of adaptations, are the functions served by adaptations a central and irreducible feature of the explanans in such explanations? If the answer is yes, the explanations are teleological.
A good place to begin is with a simple, yet realistic, example. In research carried out over many years and combining painstaking field work and laboratory experimentation, John Endler was able to demonstrate that the color patterns of males in the guppy populations he was studying in rivers feeding into the southern Caribbean were a consequence of a balance between mate selection and predator selection. To take one startling example, he was able to test and confirm a hypothesis that a group of males, with a color pattern that matched that of the pebbles on the bottoms of the streams and ponds they populated except for bright red spots, have that pattern because a common predator in those populations, a prawn, is color blind for red.
Red spots did not put their possessors at a selective disadvantage, and were attractors for mates Endler , — We may refer to this pattern of coloration as a complex adaptation that serves the functions of predator avoidance and mate attraction. But what role do those functions play in explaining why it is that the males in this population have the coloration they do? This color pattern is an adaptation, as that term is used in Darwinism, only if it is a production of natural selection Williams ; Brandon ; Burian Which factors are critical, then, in producing differential survival and reproduction of guppies with this particular pattern?
The answer would seem to be the value-consequences this pattern has compared to others available in promoting viability and reproduction. In popular parlance and the parlance favored by Darwin , this color pattern is good for the male guppies that have it, and for their male offspring, and that is why they have it Binswanger ; Brandon ; Lennox The reason for one among a number of color patterns having a higher fitness value has to do with the value of that pattern relative to the survival and reproductive success of its possessors.
In listing the topics I would discuss under the heading of neo-Darwinism, I distinguished the question of the ontological status of species from the epistemological status of the species concept. Though they are closely related questions, it is important to keep them distinct. As will become clear as we proceed, this distinction is rarely honored. Moreover, it is equally important to distinguish the species concept from the categories of features that belong in a definition of species Rheins Advances in our theoretical understanding may lead us to reconsider the sorts of attributes that are most important for determining whether a group of organisms is a species, and thus whether it deserves to be assigned a name at that taxonomic level.
It should not be assumed that such changes constitute a change in the species concept, though at least some such changes may lead us to restrict or expand the range of taxa that are designated as species. Dobzhansky in gave what he claimed to be a definition of species, but which seems, as Mayr noted Mayr , much more a definition of speciation :. Simpson and others built even more historicity into the concept. The test for species membership is the capacity to interbreed; the test distinguishing two species is incapacity to interbreed.
Dobzhansky makes the importance of this test transparent—the transition from a single interbreeding population to two reproductively isolated ones is the process of speciation. Now in each of these definitions, little attention is paid to the actual methods used by taxonomists and systematists in differentiating between varieties of a species and distinct species, something to which Darwin gave a great deal of attention. But nominalism typically combines a view about the ontology of species with one about the epistemological status of the species concept.
On the first question, the nominalist insists that there are no species—there are more or less similar individuals.
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On the second question, the nominalist typically insists that the species concept is, at best, a useful or convenient grouping of similar individuals or, at worst, an arbitrary grouping of similar individuals. An interesting alternative account of the species concept based on a sophisticated, multidimensional theory of similarity has recently been defended in Rheins In his work, Mayr relates different approaches to the species concept to the philosophical distinction between essentialism and nominalism.
Darwinism Evolving. System Dynamics and the Genealogy of Natural Selection
He associates essentialism with the view that a species concept refers to a universal or type. Lennox, ; repr. At the opposite extreme is nominalism, which combines the view that only individuals exist in nature and that species are concepts invented for the purpose of grouping these individuals collectively.
Mayr claims that his Biological Species Concept BSC is an advance on both; individual species members are objectively related to one another not by a shared relation to a type but by causal and historical relationships to one another. He can thus be understood as arguing for a new, objective way of understanding the epistemological grounds for grouping individuals into species. This new way of grouping stresses historical, genetic and various ecological relationships among the individuals as the grounds for determining species membership.
His claim is that this is more reliable and objective than similarities of phenotypic characteristics. This makes sense of the importance he eventually places on the fact the BSC defines species relationally:. Mayr has in mind that brothers may or may not look alike; the question of whether two people are brothers is determined by their historical and genetic ties to a common ancestry. That is, it is a defense of a sort of essentialism. A critical issue in this debate over the account of the species concept most appropriate for Darwinism is the extent to which the process of biological classification—taxonomy—should be informed by advances in biological theory.
Besides those already discussed, the moderate pluralism associated with Robert Brandon and Brent Mischler or the more radical pluralism defended by Philip Kitcher, argues that different explanatory aims within the biological sciences will require different criteria for determining whether a group constitutes a species.
Cladists, on the other hand, employ strictly defined phylogenetic tests to determine species rank see Rheins In a recent collection of papers defending most of the alternatives currently being advanced Ereshefsky , my suspicion is that virtually every author in that collection would identify himself as Darwinian. Though there are an abundance of web sites on Darwinism, the three most useful sites meeting the highest of academic standards are listed below. The first is the official site for the publication of material in the extensive Darwin Archives at Cambridge University, but has grown to become the default site for Darwin texts and related literature as well.
Introduction 2. Darwin and Darwinism 2. Introduction Scientific theories are historical entities. As Jean Gayon has put it: The Darwin-Darwinism relation is in certain respects a causal relation, in the sense that Darwin influenced the debates that followed him. Gayon , Darwinism identifies a core set of concepts, principles and methodological maxims that were first articulated and defended by Charles Darwin and which continue to be identified with a certain approach to evolutionary questions. This work, and Sir J.
No one or a dozen other books influenced me nearly so much as these two. His principal tasks are to develop an accurate and comprehensive record of those changes, to encapsulate that knowledge in general laws, and to search for their causes. All the evidence supports the view that species variability is limited, and that one species cannot be transformed into another.
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Barrett et al. Species are comprised of individuals that vary ever so slightly from each other with respect to their many traits. Species have a tendency to increase in numbers over generations at a geometric rate. Some individuals will have variations that give them a slight advantage in this struggle, variations that allow more efficient or better access to resources, greater resistance to disease, greater success at avoiding predation, and so on. These individuals will tend to survive better and leave more offspring. Offspring tend to inherit the variations of their parents.
Over time, especially in a slowly changing environment, this process will cause the character of species to change. Given a long enough period of time, the descendant populations of an ancestor species will differ enough both from it and each other to be classified as different species, a process capable of indefinite iteration. There are, in addition, forces that encourage divergence among descendant populations, and the elimination of intermediate varieties.
To illustrate it, look carefully at the first question that Charles Lyell wishes to address in the second volume of the Principles of Geology : …first, whether species have a real and permanent existence in nature; or whether they are capable, as some naturalists pretend, of being indefinitely modified in the course of a long series of generations.
And I look at varieties which are in any degree more distinct and permanent, as steps leading to more strongly marked and more permanent varieties; and at these latter, as leading to sub-species, and to species. Darwin , 52 Permanence, as applied to species, is for Darwin a relative concept, and there are no fixed limits to variability within a species.
Moreover, he concludes the Origin with very strong words on this topic, words bound to alarm his philosophical readers: Systematists will be able to pursue their labours as at present; but they will not be incessantly haunted by the shadowy doubt whether this or that form be in essence a species. This may not be a cheering prospect; but we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species. Darwin , 49 From a Darwinian perspective, this is a predictable consequence of the fact that the organisms we today wish to classify as species are merely the most recent stage of a slow, gradual evolutionary process.
Let us begin with the language Darwin uses when he first sketches his theory at the beginning of the fourth chapter of the Origin : Can it, then, be thought improbable , seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt remembering that many more individuals are born than can possibly survive that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind?
Note one clear statement of the Principle of Natural Selection from the philosophical literature: If a is better adapted than b to their mutual environment E , then probably a will have greater reproductive success than b in E. Brandon, There was no question as to which was more important at a particular stage. But now that we have the concept of random drift taking over where random variation leaves off, we are faced with just such a question. That is, given chance variations, are further changes in the frequencies of those variations more a matter of chance or more a matter of natural selection?
With respect to the generation of variation, chapter 5 of On the Origin of Species opens with the following apology: I have hitherto sometimes spoken as if the variations—so common and multiform in organic beings under domestication, and in a lesser degree in those in a state of nature—had been due to chance. Goodwin, B. Gould, S.
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The Never-Ending Problems In Evolutionary Biology
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