Actually, there has been a great deal of controversy regarding the role of symbiosis in evolution. It goes back as far as about nine years after the publication of "The Origin of the Species." But I believe that in many ways, symbiosis has been a major driving force in evolution.
For example, it had been assumed that fungus was essentially non-productive, and if it entered a relationship with some other organism, the relationship was one between parasite and host. But while investigating the roots of an apparently healthy tree at one time, it was discovered that those roots were infested with a fungus. More investigation lead to the discovery that different species of tree typically play host to several different species of fungus at the same time. And in fact, such trees are incapable of living without their fungal symbionts: the roots of the trees, once stripped of their fungal symbionts, are stubby and relatively small, whereas the fungal symbionts are much larger, with a far more extensive web of fine branches extending out, increasing the tree's uptake of neutrients and water typically by somewhere between a factor of 100 and a 1000.
We now know that mitochondria were originally bacteria which entered into an endosymbiotic relationship with eukaryotes approximately two billion years ago, and chloroplasts have a similar bacterial origin -- as indicated by their circular genes. At the time that mitochondria entered the endosymbiotic relationship, they had about 3000 genes. Today I believe they have about 35, with most of their genetic material having gradually migrated to the nucleus of the eukaryotic cell. The closest known non-endosymbiotic relative of the mitochondria is the rickettsia bacteria, which is responsible for typhoid. The closest known non-endosymbiotic relative of the chloroplast would be a species of cyanobacteria, which is more commonly known as blue-green algae.
Of course, viruses have historically been viewed simply as parasites, both reproductively and, genetically. It had been common to assume that if the host of a virus and the virus itself contained some of the same genetic material found nowhere else, then surely this genetic material originally belonged to the host, and the virus had simply acquired it from the host. (In fact, a common view at one time was that viruses were originally part of the genomes of hosts, then somehow became separated, but "return" to the nucleus for the purpose of reproduction.)
What this does not take into account is the fact that viruses typically mutate approximately 1000 times more quickly than their more conservative, less error-prone hosts, and when under stress are able to increase the speed at which they mutate by approximately another factor of a 1000. It is a safe bet that if a genetic innovation is present in both, the innovation was the product of the virus, not the host.
Thus, for example, we are fairly confident nowadays that retrotransposons originated with retroviruses. And currently, the only hypothetical origin being discussed for transposons (as far as I have been able to gather) are DNA viruses.
Likewise, in the human genome, we appear to have roughly 30,000 endogenous retroviruses -- three of which are at a "low" level of activity in normal placental and embryonic tissue development. Some of course are implicated in various kinds of disease, such as leukemia, cancers, and schizophrenia, but the fact that most people are able to live quite well with 30,000 endogenous retroviruses suggests that the vast majority are well-behaved, after having existed in a symbiotic relationship with their host over perhaps millions or tens of millions of years. And it has recently been suggested that some play a regulatory role in the expression of other genes -- although this is fairly tentative at present.
However, the fact that they are well-behaved now does not mean that their ancestors were similarly well-behaved. One of the endogenous retroviruses involved in human placental and embryonic development is HERV-K, which entered the genome for the first time approximately 30 MYA (shortly after the old world monkeys separated from the new world monkeys), and most recently about 5 MYA. Currently, we have approximately 50 HERV-K proviruses. From what they have been able to gather, HERV-K is closely related to HIV-1.
While endogenous retroviruses account for only about one to three percent of the human genome, roughly half of the human genome consists of retroelements -- most likely the wreckage of earlier ERV proviruses or artifacts of earlier retroviral infections.
In essence, if we are talking about a given endogenous retrovirus, a retrotransposon or transposon, what we are talking about is lateral gene transfer. Thus viruses can have a far greater impact upon host evolution than the simple predator/prey-relationship in which they were first identified would first suggest. (Similar lateral gene transfer between host and bacteria had been suggested as a driving force in eukaryotic host evolution. However, currently the literature seems to be turning against this.)
Here are a few more articles which may be of interest...
Periodic Explosive Expansion of Human retroelements Associated with the Evolution of the Hominoid Primate
Tae-Min Kim, Seung-Jin Hong, Mun-Gan Rhyu
J Korean Med Sci 2004; 19: 177-85
http://jkms.kams.or.kr/2004/pdf/04177.pdf
'Punctuated' evolution in the human genome (popularized)
Medical and health information
An ancient family of human endogenous retroviruses encodes a functional homolog of the HIV-1 Rev protein
Jin Yang, Hal P. Bogerd, Sheila Peng, Heather Wiegand, Ray Truant, and Bryan R. Cullen
Howard Hughes Medial Institute and Department of Genetics
Approved September 28, 1999
Just a moment...
Constructing primate phylogenies from ancient retrovirus sequences
Welkin E. Johnson and John M. Coffin
Proc. Natl. Acad. Sci.
Vol. 96, pp. 10254-10260, August 1999
Just a moment...
Helpful junk
Helpful junk | The Economist
(NOTE: this is a popularized article dealing with a topic covered at a technical level in a recent article in Nature -- where retrotransposons have been implicated in the chaotic over-generation of neurons within the mammalian brain, and a process akin to natural selection eliminates roughly fifty percent of the neurons -- those which apparently do not make the right connections. It is also suggested that the retrotransposons may be responsible for there being a greater variety of cells in the brain than in any other organ.)
Transposable elements as the key to a 21st century view of evolution
http://shapiro.bsd.uchicago.edu/Shapiro_1999_Genetica.pdf
Finally, for readers who are interested in the role of symbiosis in evolution and the history of its treatment in evolutionary thought, a good book for laymen would be "Darwin's Blindspot" by Frank Ryan.
This message has been edited by TimChase, 06-19-2005 01:01 AM
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