Mutualism: Win-Win
Mutualism is relatively easy to explain in this model. The definition of mutualism is a relationship between organisms that benefits both organisms. Essentially, the relationship between these two organisms will save energy for both participants since they satisfy each other’s needs. One very extreme example of mutualism is addressed in my post on endosymbiosis, which talks about mutualism between ancient bacteria and primitive eukaryotic cells which redefined all life on Earth.
Commensalism: The Mooch
Commensalism is a relationship between organisms that benefits one while leaving the other unaffected. Once again, the benefiting organism is able to save energy by using the host while the host does not have to invest any energy to remove the other because it is not harmful. While most bacteria in our digestive tract have mutualistic relationships with us, there are some commentialistic ones which use us for survival without providing us anything.
Parasitism: Win-Lose
Finally, we turn to parasitism. This is a very complex relationship to explain because of the harmful effect of one organism on the other. From an energy perspective, this is close to a zero sum venture, although there are varying degrees of this. Most times, the benefit to the parasite greatly outweighs the detriment to the host. However, this is irrelevant to evolution; the moment energy is being spent on defense, there is a pressure to develop resistance over time.
Importance of Mode of Reproduction
One important aspect of the evolution of symbiosis proposed in The Selfish Gene is seen by observing the differences between how each participant in the relationship reproduces. Essentially, the more friendly a relationship is, the more common their method of reproduction. I covered this topic in length on my previous post, which can be accessed here. This is a very important aspect of inter-species relationships.
Symbiosis often occurs in terms of stable relationships called evolutionary stable strategies. What I mean by this is that there is a proportion of organisms that, when combined, reach a stable point. For example, a population of 50% overly selfless individuals and 50% overly selfish individuals would not be a stable population. There would inevitably be an increase in proportion of selfish individuals in the pool. The mathematics of such stability can be learned through studying the prisoners dilemma or by reading books about macroscopic biology.
To put it simply, the equilibrium of a group of organisms is like a ball in a valley. Even if you push the ball out of equilibrium, it will always fall back in. This is preferable to the “tip of the mountain” equilibrium where if a slight instability is introduced, the population distribution would swing around wildly forever.
Conclusion
Although the concept of symbiosis is easy to wrap one’s head around, there are a lot of factors to consider when thinking about relationships. Most importantly, it is important to see what benefit, if any, each organism is getting out of the partnership. Beyond these basic definitions, it is difficult to generalize every relationship between organisms due to the complexity present in each case. While there are countless examples of symbiosis I can refer to, I believe these examples are better served addressing more intricate issues.