In order to create his theory of evolution, Darwin had to take a revolutionary step in thinking about organism and environment. Previously there had been no clear demarcation between internal processes and external ones. Darwin created a dramatic rupture between the internal processes that generate the organism and the external processes, the environment, in which the organism must operate. Adapatation became both a process and a measure of "fitting" the organism to the environment. The organism's interaction with the environment -- with "nature" -- sets up a feedback process whereby nature "selects" one design over another, depending on how well it solves an adaptive problem. If a modification enhances the ability of a design to cause its own reproduction, a positive feedback occurs and an adaptive design will replace all alternative designs in a population. If it results in fewer (or no) offspring, a modification will set up a negative feedback, and a design will become extinct. According to strict usage in evolutionary biology, a character is an "adaptation" for a particular task only if it can be shown to have evolved in specific ways to make it more effective in the performance of that task, and that the change has occurred due to the increased fitness that results. Perhaps only some problems are adaptive problems , that is, problems whose solution can affect reproduction, however distally. Avoiding predation, choosing nutritious foods, finding a mate, and communicating with others are adaptive problems that our hominid ancestors would have faced.

Johannes von Uexküll, at the beginning of the twentieth century, criticized the Darwinist separation of organism and environment. For Uexküll, "Every animal carries its environment about with it like an impenetrable shell, all the days of its life." (quoted in Ernst Cassirer, The Problem of Knowledge, p. 202) Every creature has its own private world of perception and activity, prescribed once and for all by its structure, by its type of receptors and effectors. This environment-world is the animal's Umwelt. It is constituted by a more or less broad series of elements that he calls "carriers of significance" (Bedeutungsträger ) or of "marks" (Merkmalträger ), which are the only things that interest the animal. Every animal is bound to its environment by "the circle of functions." For Uexküll, "The protoplasm that made an earthworm made it for an earthworm's activities, and set it in an earthworm's world." For Uexküll, a unitary world in which all living beings are situated does not exist. -- nor do a space and a time that are equal for all living things. (see Agamben, Open, p.40) We may see an Umgebung, an objective space in which we see a living thing moving, but in reality the Umgebung is our own Umwelt , to which Uexküll does not attribute any particular privelege, and which can vary according to our point of view.

Uexküll sought to draw out the consequences of anatomical type defined as a structure of (static) relationships and to resurrect the program of an "idealistic morphology." In his Theoretical Biology and Umwelt und Innenwelt der Tiere, Uexküll describes the "theory of the living being" as a pure natural science with only one goal: "The research into the blueprint ( Bauplan or structural plan) of living beings, their origin, and their results." The relations between the functional spheres of the organism is the basic phenomenon of life. The stimuli of the external world, which an animal is able to receive by virtue of its blueprint, constitute the only reality present for it, and by virtue of this physical limit, it closes itself to all other spheres of existence. (Ernst Cassirer, The Logic of the Cultural Sciences, pp.23-24)

Whatever its context, adaptation is the accumulated output of selection and involves progressive modification of structures through the repeated actions of structural modifiers, or "operators." (eg. chromosomes are structures which are modified by mutation, recombination, etc.) Adaptation requires a specific performance measure (eg fitness) The defining feature of Darwinian adaptedness is that it results in the survival of more similar offspring. But Darwinian natural selection is not just differential reproduction but is differential reproduction due to the adaptive superiority of those who leave more offspring.

In Adaptation in Natural and Artificial Systems, John Holland describes the problems of adaptation as a form of optimization in which information must be exploited as it is aquired, so that performance improves apace. (see genetic algorithms) How can adaptation occur efficiently? Not through an "enumerative" plan, which simply tries out every structure in an order that is unaffected by the outcome of previous tests. It is extremely inefficient and the time required soon exceeds the age of the universe in systems of even modest complexity. Adaptation requires "robust" plans that make efficient use of the history of their interactions with an environment, within a framework of "enough time."

Relative adaptedness refers to the relation of one organism being better adapted than another (of the same interbreeding population) to a particular environment, and is the fundamental concept of neo-Darwinian theory. Adaptation works directly on phenotypes, not on genes or genotypes; so adaptedness is an organismic not a genetic quality. For mathematical genetics, the fitness of a phenotype is the number of its offspring which survive to reproduce.

One difficulty with the way adaptation is defined in population genetics is that increased reproductive success does not describe the correlations between traits and their contributions to organismic well-being on the basis of their design or function. In addition, because of the possibilities of conversions of functions, or " exaptations," adaptations must be described historically, and their histories are rarely simple teleologies directed towards contemporary fitness. Forms can just as well be vestigial or incidental.

Richard Lewontin criticizes the notion that organisms adapt to "fit" specific requirements of the environment through the use of terms such as "ecological niche," which imply a kind of ecological space with holes that are filled by organisms whose properties give them the right "shape" to fit the holes. In this framework, the study of the diversity of organisms becomes a map of the space of the evironment, just as sprinkled iron filings map a magnetic field. Lewontin argues that it is time to reconsider the relationship between outside and inside, between organism and environment and suggests that the actual process of evolution is better captured by the metaphor of construction. (The Triple Helix, p. 48) While there is a physical world outside of an organism that would continue to exist in the absence of the species, the environment does not exist without species. Organisms determine which elements of the external world are put together to make their environments and the relations between those elements that are relevant to them. The space of the environment is determined by the activities of the organism itself, not the other way around. In addition to this more selective process, organisms actively construct (and destroy), in the literal sense of the word, the world around them. Every species, not only Homo Sapiens , is in the process of detroying its own environment by using resources that are in short supply and transforming them into a form that cannot be used again by individuals of the species. But every such act of consumption is an act of production for other species.

Evolutionary biologists describe the relationship between adaptation and ongoing changes in the environment as the "Red Queen Hypothesis," named after the queen in Through the Looking Glass who found she had to keep running just to stay in place. According to Lewontin's constructionist view, the world is changing because the organisms are changing. The Red Queen's running only makes the problem worse. (p.58) For Lewontin, it is a general principle of historical development of biological systems that the state of conditions which make possible the coming into being of the system are abolished by it.