The intimacy of such symbiotic relationships varies considerably. In `ectosymbionts' the associations between organisms are purely external, whereas in `endosymbionts' one of the organisms is actually incorporated internally into the other (the `host').
In the extreme, it is possible that the relationship between host and (endo)symbiont becomes so strong that the previously independent organisms are totally dependent on each other for their survival and reproduction, and effectively become a single organism. This process, the evolutionary origin of new morphologies and physiologies by symbiosis, is called `symbiogenesis', a term coined by the Russian biologist Merezhkovsky and subsequently developed by others such as Kozo-Poliansky (see [Khakhina 79]).
It is now widely accepted that the process of symbiogenesis played a key role in the transition from prokaryotic2.10 cells to eukaryotic2.11 cells (e.g. [Margulis 81], [de Duve 96], [Maynard Smith & Szathmáry 95]). This has led a growing number of people to question whether symbiosis also played a key role in other major events during the evolutionary history of life [Maynard Smith 91].
The significance of symbiogenesis with respect to neo-Darwinism is that Darwin's arguments concerning descent with modification seem to imply that evolutionary novelty comes about `vertically', by mutations in the genes passed on from parents to offspring. In contrast, symbiogenesis is a mechanism for `horizontal' gene transfer, where the genetic material from essentially unrelated organisms can be brought together in a single descendant. Whether this process is considered to be consistent with neo-Darwinism is a matter of debate. If one thinks about Darwinian evolution occurring at the fundamental level of the genes, it can certainly be argued that it is consistent (e.g. [Dawkins 76], [Maynard Smith 91]). For example, Maynard Smith says:
``Whether [symbiosis constitutes a challenge to neo-Darwinism] depends, of course, on what one understands by `neo-Darwinism'. I interpret it to mean the hypothesis that mutation (change in the genetic material) is not, except occasionally and by accident, adaptive to its causative agent, and therefore, insofar as organisms are adapted to their ways of life, that adaptation must have arisen by natural selection acting on originally nonadaptive genetic variation. In this sense I am a neo-Darwinist. However, the ways in which genetic material from different ancestors is united in a single descendant has profound effects.'' [Maynard Smith 91] (p.26).
Even if we accept that symbiogenesis is consistent with neo-Darwinism, we should recognise that this particular mechanism for producing evolutionary novelty has apparently played a key role at certain stages in the history of life on Earth. In terms of building artificial evolutionary systems, then, we should certainly give consideration to how symbioses and symbiogenesis might be achieved.