How do symbiotic relationships develop between two different animal species over time?

555 views

How do symbiotic relationships develop between two different animal species over time?

In: Biology

Ate you asking because you saw Godzilla 2 ?

I am curious myself…

Co-evolution maybe. 2 Species have genetic variation in their populations because of mutations. Something happens in the environment which kills of a large proportion of each population, and by random chance organisms from each species survive after randomly depending on each other.

Since these 2 organisms which survived by living off each other are able to live long enough to reproduce, they can pass down these genes which made them capable of doing so down over generations. Eventually you get 2 different species which solely depend on each other. r/askscience is always available for further details

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.

My first ELI5 so bear with me.

Firstly, there are many different purposes for symbiotic relationships. These involve predation, Herbivory, mutualism, parasitism, and competition.

If you take the example of bees seeking pollen. This is a symbiotic relation as the bees get food and plants are able to reproduce.

To develop this relationship you have to think of how this benefits the plant. If the bee goes from a daisy to a weed, they will not spread the pollen from the daisy to the weed.

But! There are other pollinators. There are moths with extremely long tongues. But the plant can’t choose who picks up its pollen. Say there is a daisy flower with a really long flower. It takes a long tool to get to the pollen in the flower. So now, these moths with long tongues are the only things that can get to the pollen. So that moth now knows that if it goes to that flower, that it has a higher chance of getting pollen + food. So the plants that have longer flowers continue to reproduce, moths know that flowers that are longer have the food that they need and they start a “symbiotic” relationship.

I put symbiotic in quotes because there are so many examples of them, including caterpillars that eat sugary lollipops produced on the stems of some plants, that then make the caterpillars poop smell sweet, to attract wasps that come in to kill and eat the caterpillar.

It’s is all based on evolutionary traits that benefit a predator or prey. The new learned behavior changes that of another predator or prey and the cycle continues as one mutation out competes another.

Take an example, ants keeping pets. Some ants keep pets (*myrmecophiles*); aphids eat the ants’ plants with a higher “efficiency” than ants by themselves (a quarter of the aphids in the Rocky Mountains are ant “pets”–their species is partially defined by the ants). They drop honeydew, which they ants can use. I won’t go in to what the benefits are and what the costs for both species, but assume the relationship is certainly beneficial for the ants and likely beneficial for the aphid.

The ants evolved to learn to consume honeydew (some ants had mutations which made them better at consuming honeydew, these ants had a higher chance of survival, that is evolution–I shall use anthropomorphic words here for evolution, but it boils down to a mutation and increased chances of survival). The aphids gained from this, as the honeydew is sticky and gets everywhere. Aphids evolved to “help” ants so that the ants keep doing what they do and so that the ants’ lives are easier. The ants evolve to help the aphids in return.

Another example is closer to home: prehistoric humans and dogs. Humans have brains and their actions are very intentional, making this case slightly different from other symbiosis (for example, we intentionally evolved the wolves, or “bred” them, which isn’t as common amongst other symbiotic relationships). The dogs gain from it: humans working with dogs benefit more than the dogs and the humans individually. The humans can eat more, and the dogs are fed more than they would find undomesticated. Also, humans can help dogs, by sheltering them from the rain and other animals.

The wolves ate the carcasses of the animals the humans killed and started to live near the humans. Somehow, the humans gained from this relationship. Perhaps the humans could use the carcasses of the animals the wolves killed or the wolves scared off dangerous animals for the humans. After slow evolution favouring slight symbiosis, the humans could use the wolves and started breeding them.

*Ecology and Evolution of Aphid-Ant Interactions*, Bernhard Stadler and Anthony F. G. Dixon (Annual Review of Ecology, Evolution, and Systematics (Vol. 36 (2005), pp. 345-372) for the ants and aphids

Better ELI5 (than my prior comment)

An ice cream man has a car and a lot of ice cream. He needs money, you have money. He recognizes that he can trade you money for his ice cream.

How does he get your attention? He plays music to draw you in.

Never having met an ice cream man you are playing in the front hard with your friends. You hear music, they all go running towards the sound with their money in hand, and come back with a sweet treat.

You are now prepared, you hear the music, you go running, you give your money, and you get a sweet treat.

Now whenever you hear music, you know that you need money to get the treat. That is the relationship. You get a treat and the ice cream man gets money.

This holds true in every yard and whatever ice cream man drives around.