[ In a Google+ post, someone asked whether symbiosis was a good example of group selection. I responded in a comment, and another comment asked me to expand my response a little bit in my own post. So here is a copy of that post (with a few more hyperlinks). ]
Part 1: What is group selection?
Typically "group selection" doesn't cross species boundaries. That is, group selection refers to the proliferation of a particular form of a gene, otherwise known as an "allele", due to its benefits to groups of individuals which share that allele despite the individual costs of having that allele. It may help to consider the basic group-selection argument for the evolution of altruism (i.e., the evolution of behaviors that are costly to an individual and yet beneficial to a different unrelated individual). Before that, consider why we wouldn't expect altruistic alleles to have strong representation in a population.
For every gene or group of genes, there can be many different variations (alleles). Some of those variations will be deleterious to an individual, and so you would expect the relative representation of those deleterious variations to decrease over generations. So imagine if one of those alleles encoded an altruistic trait that caused an individual to do something costly for the benefit of another (e.g., helping a stranger understand group selection with no expectation of future payoff). Individuals with that allele are suckers. Those without that allele instead focus on tasks that return direct benefit to themselves, and that direct benefit would payoff with greater productivity of offspring that share that non-altruistic allele. When an altruist met a non-altruist, the benefit from the altruist would increase the non-altruist's alleles representation in the next population while decreasing its own alleles' representations. So we would expect that altruistic alleles would fade away into obscurity. Moreover, the benefit from all of the altruists would diffuse across the variety of alleles rather than being concentrated on just the altruistic ones.
However, what if that altruistic allele also encoded a behavior that would seek out others with that same allele. This non-random association means that each individual who helps another does actually help to increase the productivity in that allele. That is, even though there is a cost to the individual doing the altruistic task, the benefit going to the other individual is felt by the other copy of the same allele in the different (and unrelated) person. So when these altruists group together, altruistic benefits do not diffuse. They are captured within the group. Moreover, the group's synergy can cause it to be more productive than the remaining groups of non-altruists. Consequently, the altruistic allele not only persists in the population, but its representation can grow because there is a differential benefit between altruistic and non-altruistic groups. It is this differential benefit between groups that is group selection.
Part 2: Symbiosis and Mutualism
A symbiotic relationship between members of different species is not group selection (in general) because it does not posit that there is a mutual allele that may be deleterious in an individual but beneficial in a group. That is, there is no group synergy that is mitigating individual costs by generating benefits elsewhere that help to support alleles that would otherwise naturally decay. When species are mixed within a population of interest, the analysis is a bit different because alleles cannot flow across the species barrier (except for special cases).
For example, consider an allele that existed across species (e.g., an allele for a gene shared between humans and bonobos), the speciation in general would prevent the sort of group selection gains because there would be no way for increased numbers of alleles in one species to transfer to the other species. Imagine that altruists in one species seek out altruists in the other species. The result could lead to more increases in the altruist representation in one species than another, and so there would be an altruist surplus. Those surplus altruists would have no choice but to associate with non-altruists in the other species. However, if the group was all of one species, then there would not be surplus altruists. Altruistic benefit need not diffuse across non-altruists too.
However, most examples of symbiosis are not altruistic. Instead, they are mutualistic. That is, the behavior does benefit another, but that is a possibly unavoidable side effect of an action that benefits the individual doing the behavior. For example, if I'm driving through a parking lot looking for an empty space to park, I am revealing information to my competitors (other drivers) about where empty spots are not. I don't want to help the competing drivers, but it is unavoidable because they can see me go down an isle of the parking lot and not find a spot. Consequently, they do not go down that same isle. Of course, I use their searching behavior to inform my choices of the next isle. So we are doing "cooperative search" only because the behaviors have mutual benefits. The same goes for many symbiotic relationships among individuals of different species.
Consider a remora ("sharksucker"). It's a small fish that essentially attaches to another host (fish, whale, turtle, etc.). It can receive nutrients from on or around the host. It can also be protected from predators that avoid the host. In some cases, the host could eat the remora, but the remora is so small that it may not be worth the effort. Some hosts actually receive a small benefit (cleaning, for example) from the remora. Regardless, the remora experiences very little cost and plenty of benefit. Moreover, the host experiences very little cost and possibly some benefit. So there's no surprise that this behavior evolved. You don't need any fancy mathematical model to show how this is possible – when the benefits align like this, it's natural to assume that it is going to be favored by natural selection.
Part 2.5: Symbiosis and Co-evolution
Having said all of that, symbiosis can lead to elegant examples (or at least suggestions) of co-evolution, which describes how a change in one species can lead to a change in other species. In particular, natural selection on different species creates a feedback across species. One species is the ecological background for another species, and so as each species changes it creates new niches (and destroys old ones) for other species. So the evolution of one species can guide the evolution in another. But I think this post is long enough. :)
Wikipedia does a pretty good job on these particular subjects. Check 'em out there.
( I have also mirrored this content on a post on my website. )