"Evolution is a mechanism by means of which a fortuitous variability is converted by a dynamical process into a pattern of development which reads in one direction." (Norbert Wiener p.37) Wiener's awkward prose captures the key issues of modern evolutionary theory: It depends on a random process for the push for change (genetic mutations) Natural selection, along perhaps with more intrinsic properties of the process of evolution itself, act to convert random changes into an ordered pattern. And these changes are historical and irreversable. Thus, James Crick describes the results of evolution as"Frozen accidents." And, more poetically Jacques Monod describes evolution as "Chance caught on the wing." Francois Jacob describes evolution as "a tinkerer," and S.J. Gould claims that "There is no progress in evolution."
While evolution is now fundamentally associated with Darwin, with the development of new species, and with an element of chance, the term formerly had a very different meaning. In premodern biology, especially in prefomationist theories, the term evolution was understood as an unfolding of predermined events.
The modern view is that evolution is not an unfolding but a historically contingent wandering pathway through the space of possibilities. Whether this pathway is a "random walk" or a more structured, "robust" path is still open to question. But the distinguishing feature of a Darwinian theory of evolution is explaining evolutionary change by a theory of natural selection.
"...and then Darwinism... explained how by throwing stones one could build houses of a typical style." (Hans Driesch, p.137) -- a new "argument for design!
The "argument for design" i.e. for God and against Darwin, described in Richard Dawkins' The Blind Watchmaker, was an argument based on the unlikelihood that life would just have assembled from parts like a watch spontaneously assembling itself without a prescient watchmaker. Darwinian theory sees evolution as a "blind watchmaker", that is to say, there is no preordained design but a constant process of trial and error.
For Darwin, random hereditary variations among members of a species and natural selection over long periods of evolutionary time became the basis for explaining the adaptation of organisms to their habitats. Darwin showed how "organs of extreme perfection and complication, which justly excite our admiration" arise not from God's foresight by from the evolution of replicators over immense spans of time. In the "Neo-Darwinian Synthesis." random mutation at the genetic level is subject to natural selection and leads to branching phylogenies converting varieties within populations to variation between populations It is an important question for biology as to whether evolution occurs in continuous or discontinuous steps. Darwin shared the view that individuals varied continuously in all their common characteristics and formulated his theory of the accumulation of small intergenerational differences as the explanation of the origin of species (which define discontinuity) within that framework.
The modern approach to taxonomy is called cladistics. (The name is from the Greek term for "twig," klados.) Cladistics attempts to group taxa hierarchically, based on shared derived characters. (this is the hypothesis of homology.) Cladograms show us a logic of life that actually emerges out of the history of life. In The Problem of Knowledge, Cassirer criticized Darwinism for imposing upon historical thought "tasks that were quite foreign to its nature and which it was not competent to fulfill." (p.172) "Historical description was supposed to perform at once the whole duty of 'explanation':
(see population / typological)
"The modern theory of evolution, like all historical theories, is explanatory rather than predictive.... Evolution, like history, is not like coin tossing or a game of cards. It has another essential characteristic: irreversibility. All that will be is the descendent of what is, not from what might have been. Men are the children of reality, not of hypothetical situations.." (S.E. Luria Life: the Unfinished Experiment, quoted in Yates , p.6) Eg. symmetry breaking, once it has occurred, remains in place.
An example of symmetry breaking: a pole stands on a horizontal plane. Prior to falling, its range of possible directions to fall is the full circle. After it falls, it points in some specific direction. By falling, the pole has broken the circular symmetry of the system and come to a specific orientation.
Darwin's theory of evolution requires that the notion of natural kind, the kind of event found in nature and hooked up to other events by laws of nature, be replaced by that of historical lineage and that the concept of essential nature be replaced by that of the accidental collocation of properties. In traditional terms, the forma essentialis is replaced by that of forma accidentalis.
What are the real entities that are the objects of scientific evolution theory? Darwinism is generally understood to be a theory about species, not about individuals. What, then, are species? According to David Hull, species taxa are not classes at all, not natural kinds, but are rather individuals -- species individuals conceptualized through the theory of descent. In this view, the relation between an individual organism and a species taxon is not that of a member and class, but that of a part and a whole. According to this theory, the act of naming a species is like a baptism. It does not give us morphological information, for example, and is not even based on it. A name is not the same as a description. According to Hull, "Evolutionnary theory...is false. It is unfalsifiable. One of its basic principles, the claim that fitter organisms tend to survive to reproduce themselves more frequently that those organisms that are less fit, is tautological..It does not provide the necssary basis for predictions about the future development of particular lineages but can be used to explain these events once they have occurred." (quoted in Elizabeth Lloyd, The Structure and Confirmation of Evolutionnary Theory. p. 1)
Brian Goodwin argues for the fundamental nature of organisms as the primary source of the emergent properties of life revealed in evolution. He argues that although Darwin took organisms to be primary, modern biology thinks of them as complex molecular and information-processing machines controlled by the genes carried within them. Mechanistic biology describes articulated parts and how they fit together to carry out particular functions. Instead, Goodwin argues, organisms should be thought of as a distinct level of emergent biological order achieved through morphogenesis. Goodwin proposes that the term natural selection could simply be replaced with the word dynamic stabilization, the emergence of stable states in a dynamic system.
Critics of neo-Darwinism point out that scientist still rely on their ability to identify specimens as types, and that Darwinism has looked only on the conditions of evolution while overlooking its actions. For Michael Polanyi, for example, "Evolution can be understood only as a feat of emergence." (P.390)
For Gerald Edelman, there appear to be only two deeply fundamental ways of patterning thought: selectionism and logic.
The American psychologist James Mark Baldwin outlined a subtle variation on Darwinism, often called "Baldwinian evolution." Baldwin suggested that learning and behavioral flexibility play a role in amplifying and and biasing natural selection because these abilities enable individuals to modify the context of natural selection that affects their future kin. This provides a means by which behavioral changes can be subject to what Conrad Waddington called "genetic assimilation." An well-known example of Baldwinian evolution is the rise of human lactose tolerance. Most adult animals lose the ability to digest lactose, but once humans began to herd animals and to use animal milk as a food source, selective pressures strongly favored the reproduction of those most tolerant to it. For Terrence Deacon, behavioral adaptations tend to precede and condition the major biological changes evident in human evolution because they are far more facile and responsive than genetic and morphological changes. "More than any other group of species, hominids' behavioral adaptations have determined the course of their physical evolution, rather than vice-versa. Stone and symbolic tools, which were initially aquired with the aid of flexible ape-learning abilities, ultimately tied the tables on their users and forced them to adapt to a new niche opened up by these technologies. Rather than being just useful tricks, these behavioral prostheses for obtaining food and organizing social behaviors became indispensible elements in a new adaptive complex. The origin of "humanness" can be defined as that point in our evolution where these tools became the principal source of selection of our bodies and brains. It is the diagnostic trait of Homo symbolicus. " (The Symbolic Species, p.345)
Should the history of technology be compared to evolution? This is the basis for Deleuze and Guattari's term " machinic phylum." (see also machine )
Some recent issues about evolution include the question as to whether evolution evolves, (Langton, Dawkins) and especially whether evolution is constrained by its own internal growth. (Kaufmann, Packard) These questions address the questions of the dynamics of the process of evolution, how spontaneous order may mingle with Darwin's mechanism of evolution by natural selection. If Darwinian evolution is crudely reduced to a question of fitness to an environment, these questions reflect an awareness that the environment is at least partly a result of the evolution itself. The " fitness landscape" changes right along with the species. It consists of other products of the same process.
Stuart Kauffman's project is to "incorporate self-organization into the weave of evolutionary theory." (The Origins of Order, Preface)For Kauffman, natural selection is not the sole source of order in organisms. Some systems adapt readily, whereas others are so badly disrupted by minor modifications that adaptive improvements by random mutation and natural selesction can hardly occur. Are there characteristic features so deeply requisite for the capacity to adapt in a coevolutionary process that their presence in organisms is itself a lawlike consequence of selection operating on complex coevolving systems? For Kauffman, "Selection achieves and maintains complex systems poised on the boundary, or edge, between order and chaos. These (poised) systems are best able to coordinate complex tasks and evolve in a complex environmnent." (P.XV) (see phase boundary) For ensembles of these systems, selection may be unable to avoid their spontaneous order.
Dawkins: "perhaps there is a sense in which a form of natural selection favors, not just adaptively successful phenotypes, but a tendency to evolve in certain directions, or even just a tendency to evolve at all." (Artificial Life 2, p.219) Selection among embryologies for the properties of evolvability would be an example of this higher order selection process. Terrence Deacon suggests that learning biases are genetically assimilated.
John Holland's genetic algorithms are based on analogies to evolution and to market mechanisms. They are a "theoretical tool" of artificial life -- as much a product of metaphorical transference as an enabling technology.