Josiah, a frequent commenter on this blog, asked an excellent question in a post on my previous entry. I started to reply to his question, but I realized the answer would make a good blog entry. Josiah asked whether or not “cooperative relationships in the animal world” are a problem for evolution. He doesn’t think so, but the author of one of his homeschool books thinks it is. What do I think?
Well, let’s start with the terminology. A relationship between two or more individuals from different species is called symbiosis. However, that word has grown to refer to different kinds of relationships. It can refer to a relationship in which all participants benefit, a relationship in which only one participant benefits but the others are not harmed, or a relationship in which one benefits and another is harmed. Thus, to specifically talk about cooperative relationships, we use the term mutualism, which refers specifically to a relationship in which all participants benefit.
Josiah mentioned a couple of examples of mutualistic relationships. One was the tick bird, which eats ticks off a rhino’s skin. This is beneficial to the rhino, which becomes relatively “tick free,” and it is obviously beneficial to the bird, as the bird has a fairly safe place to find food. The tick birds also warn each other (and the rhinos) about any incoming danger. The other example was the single-celled protistans (flagellates) that live in a termite’s gut and digest cellulose. The termite (indeed, all multi-celled animals of which I am aware) cannot digest cellulose on its own, and since wood is 50% cellulose by mass, this would be a problem for an animal that eats wood. However, the flagellates in a termite’s gut digest the cellulose, which allows the termite to eat wood. Obviously, both participants benefit in this relationship. Are these kinds of situations a problem for evolution?
My coauthor and I talk a lot about mutualistic relationships in our biology book, and we point out that some of them are fairly easy to understand in an evolutionary framework. For example, the relationship between the clownfish and sea anemones can be explained in an evolutionary framework. Both the sea anemone and clownfish could have existed for a long time before some mutant clownfish masked the amino acid that triggers the sea anemone’s nematocysts. However, once a mutant did develop, it could have swum into a sea anemone and not been stung. Over time, it would probably learn that it was safer there than when it was not in the anemone. As a result, it would tend to hang around the anemone, and thus it would be more likely to survive so it could reproduce and pass that mutation on to its offspring.
So simple mutualistic relationships such as the one between the clownfish and sea anemones are not a problem for evolution. Neither is the tickbird example that Josiah mentions. However, in our biology book, my coauthor and I say that there are many mutualistic relationships that are not so easy to explain in an evolutionary framework. I actually think the flagellate/termite relationship is one of them.
The problem with explaining flagellate/termite mutualism rests in the fact that the termite would have needed another food source before the relationship developed, because it probably could not have eaten much plant matter at all without the flagellate in its gut. So it would have needed to be mostly a carnivore. That’s not such much of a problem, except that there is now a really big timing issue here. The termite would have to ingest something that contained flagellates (or their spores), and those flagellates would have to be able to evade its immune system. Thus, they would have needed something that gave them immunity already, or the termite’s immune system would have needed to be very weak. Then, they could live in the gut, but without a good food supply, they would not live for long. Thus, the termite would suddenly have to develop the “craving” for wood. That seems like a lot of coincidences all at once. Of course, the relationship between multi-celled herbivores and cellulose-digesting microorganisms could have evolved before the termite evolved, but no matter which multi-celled herbivore it began in, you have essentially the same problem.
But even that relationship is fairly simple compared to some mutualistic relationships. Consider Acacia trees and Cregaster ants. The ants live in the hollow thorns of the tree and eat nectar that the tree makes especially for them. As a way of saying “thank you,” the ants viciously attack anything that tries to graze on their tree. This means they attack large herbivores (like giraffes), but it also means they attack other insects. This works really well for most of the year, but when it’s time for the tree to reproduce, this is a problem. After all, the tree must reproduce by getting its pollen to another tree. This typically happens when insects visit the flowers to eat the nectar in them. They get some pollen on them, and then when they go to the next tree, the pollen is transferred.
Well, if the ants had access to the flowers, this couldn’t happen. So as the flowers form, they produce ant repellant that tells the ants to stay away from the flowers. In the end, this makes the flowers the only safe place for an insect to land.1 Now…if a mutant tree just happened to make nectar for ants, and the ants just happened to find the tree, you would have the makings of this mutualistic relationship. However, without the ant-repelling flowers, such a tree would have a hard time reproducing. As a result, it seems that the tree would have to have a lot of mutations all at once, or there must be a whole lot of intermediates that eventually built up to this relationship, but those intermediates are all missing.
Not surprisingly, evolutionists recognize that mutualism presents difficulties for evolution. As a result, there is a lot of literature on the subject. For example, Yamamura and colleagues suggest a model in which spatial effects are important in the evolution of mutualism. However, they readily admit:
Mutualism among species is ubiquitous in natural ecosystems but its evolution is not well understood.”2
In addition, Doebeli and Knowlton propose a model that indicates overcoming host immune systems is a fairly big obstacle in the development of mutualistic relationships. In their paper, they say:
“Interspecific mutualisms are widespread, but how they evolve is not clear.” 3
Of course, none of the authors mentioned above think that mutualism demonstrates that evolution didn’t occur. They just admit that it does present a problem. Going back to Josiah’s question, then, mutualism (at least in some of its forms) does present a problem for evolution. If it did not, there would be papers explaining how these relationships evolve instead of papers that admit there is a problem and propose models that produce only partial, possible explanations.
In my own view, I don’t think a specific mutualistic relationship here or there poses a problem for evolution. After all, evolution relies on random chance that is acted on by natural selection. Gamblers rely on random change acted on by their betting strategies. Just as some gamblers can “get lucky” from time to time and have some success, you might expect some species to “get lucky” from time to time, allowing for a mutualistic relationship to develop every now and again. In my view, however, the real problem is mentioned in both of the quotes I gave above. Mutualistic relationships are amazingly common in nature. They are literally everywhere. Thus, until evolutionists can come up with a model that explains not only how they happen in some cases, but why they are so common, I do think that mutualism is one of the other problems you can thrown on top of the mountain of other problems associated with evolution.
1. Willmer PG and Stone G, “Ant deterrence in Acacia flowers: how aggressive ant-guards assist seed-set,” Nature 388:165-167, 1997.
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2. Norio Yamamura, Masahiko Higashi, Narayan Behera and Joe Yuichiro Wakano, “Evolution of mutualism through spatial effects,” Journal of Theoretical Biology 226:421-428, 2003.
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3. Michael Doebeli and Nancy Knowlton, “The evolution of interspecific mutualisms,” Proc Natl Acad Sci 95:8676–8680, 1998.
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