How do symbiotic relationships develop between two different animal species over time?
In: Biology
Symbiosis has evolved enough times across the tree of life that it’s hard to make any broad generalizations about how these relationships develop. In a lot of cases though, it probably works pretty similarly to any other kind of natural selection. As a purely hypothetical example though, let’s say two organisms find themselves in a situation where they can help each other, even if they are mostly just acting in their own interest. Species A lives in trees and has easy access to nuts, but it takes a lot of work to open the shells and get at the edible part inside. Species B is stuck on the ground, but is much better at breaking nutshells. Initially, members of species A stay in trees and eat whatever they can find; maybe mostly insects, but they’ll occasionally go to the effort of spending a while gnawing their way through a nutshell. Members of species B walk around on the ground eating various plants or whatever they can find.
At some point, a pair of individuals from each species start working together. This doesn’t need to be a conscious decision or anything; maybe this particular species A just happens to be very clumsy and drops a lot of nuts, while this one species B individual likes nuts more than other foods. These two animals start working together, gathering nuts on the ground, breaking them open, and eating them. This easy access to food is beneficial for both individuals, giving them more time to focus on reproduction, and so they are both more successful than the average of their species in terms of how many babies they have. Now, if whatever aspect of their behaviour made them start working together in the first place is inherited, the offspring of these two individuals might also work together at least some of the time. In this way, the behaviour of collaborating with members of the other species will spread through the population just like any other beneficial trait, until eventually all members of both species at least have the capacity to co-operate.
For a while, individuals from these two species have options; nuts are a bigger part of their diet than before, but they still both use other sources of food. To use a more technical term, we can call them “facultatively symbiotic”. Eventually though, additional changes occur that make both species more specialized on eating nuts, at the cost of their ability to eat other things. This can eventually lead to the two species becoming completely dependent on each other for survival (i.e., “obligately symbiotic”), although in reality not all symbiotic relationships progress to this point.
So there’s a (completely hypothetical) example of how a symbiotic relationship might develop. Besides spontaneously starting to work together though, there are other ways mutually beneficial relationships can arise. For example, it’s actually quite possible for a mutualistic relationship to develop from one that is initially parasitic. Parasites are already often well-adapted to living with (or on) a specific host. If they become specialized enough, then the fitness of host and parasite can become sufficiently intertwined that the relationship evolves so that both start to receive benefits. [This article](https://yaledailynews.com/blog/2018/02/06/yale-researchers-show-that-mutualism-can-come-from-parasitism/) talks about an example of this with a virus and bacterium; I’m not sure if this has also been directly shown with animal-animal relationships, but it definitely wouldn’t surprise me.
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