Some rain forests in the Amazon region occur on white sand soils. In these locations, the physical environment consists of clean white sand, air, falling water, and sunlight. Embedded within this relatively simple physical context we find one of the most complex ecosystems on earth, containing hundreds of thousands of species. These species do not represent hundreds of thousands of adaptations to the physical environment. Most of the adaptations of these species are to the other living organism. The forest creates its own environment.
Life is an auto-catalytic process that builds on itself. Ecological communities are complex webs of species, each living off of others, and being lived off of by others. The system is self-constructing, self-perpetuating, and feeds on itself. Living organisms interface with the non-living physical environment, exchanging materials with it, such as oxygen, carbon-dioxide, nitrogen, and various minerals. However, in the richest ecosystems, the living components of the environment predominate over the physical components.
With living organisms constituting the predominant features of the environment, the evolutionary process is primarily concerned with adaptation to the living environment. Thus ecological interactions are an important driving force for evolution. Species evolve adaptations to exploit other species (to eat them, to parasitize them, to climb on them, to nest on them, to catch a ride on them, etc.) and to defend against such exploitation where it creates a burden.
This situation creates an interesting dynamic. Evolution is predominantly concerned with creating and maintaining adaptations to living organisms which are themselves evolving. This generates evolutionary races among groups of species that interact ecologically. These races can catalyze the evolution of upwardly spiraling complexity as each species evolves to overcome the adaptations of the others. Imagine for example, a predator and prey, each evolving to increase its speed and agility, in capturing prey, or in evading capture. This coupled evolutionary race can lead to increasingly complex nervous systems in the evolving predator and prey species.
This mutual evolutionary dynamic is related to the Red Queen hypothesis , named after the Red Queen from Alice in Wonderland. This hypothesis suggests that in the face of a changing environment, organisms must evolve as fast as they can in order to simply maintain their current state of adaptation. ``In order to get anywhere you must run twice as fast as that'' .
If organisms only had to adapt to the non-living environment, the race would not be so urgent. Species would only need to evolve as fast as the relatively gradual changes in the geology and climate. However, given that the species that comprise the environment are themselves evolving, the race becomes rather hectic. The pace is set by the maximal rate that species may change through evolution, and it becomes very difficult to actually get ahead. A maximal rate of evolution is required just to keep from falling behind.
What all of this discussion points to is the importance of embedding evolving synthetic organisms into a context in which they may interact with other evolving organisms. A counter example is the standard implementations of genetic algorithms in which the evolving entities interact only with the fitness function, and never ``see'' the other entities in the population. Many interesting behavioral, ecological and evolutionary phenomena can only emerge from interactions among the evolving entities.