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Acacia Ants and Acacia Trees: An Irreducibly Complex Symbiotic Relationship?


Symbiosis has been called “the most relevant and enduring biological theme in the history of our planet.” It can safely be said that symbiotic relationships quite often resist Darwinian explanation. According to Maureen A. O’Malley, “There is a long history of researchers who have theorized about symbiosis and evolution, and many of them have aligned themselves against Darwinian evolutionary theory.” One article puts it this way: “Mutualism among species is ubiquitous in nature but its evolution is not well understood.” Or, in the words of Doebeli and Knowlton: “Interspecific mutualisms are widespread, but how they evolved is not clear.” Thus, symbiotic relationships are problematic for evolutionary theory.

In a previous article, I proposed the use of power-sets for determining irreducible complexity. In this article, I am applying this method to a famous symbiotic relationship: the acacia ants and acacia trees. In the mutualistic-symbiotic relationship between the ants and acacia trees, “the ants receive shelter and food from the host plant, and they aggressively defend the plant against herbivores and competing plants.” The tree makes nectar for the ants, and the ants, in return, protect the tree from large herbivores and other plants. When it is time for the acacia tree to reproduce, however, animals must be able to have access to the flowers; otherwise, the plants could not reproduce. The acacia tree, therefore, produces ant repellent on its flowers during the time that its flowers bloom, making it safe for insects to land there. If a power-set is made for all the necessary traits in this relationship it looks like this:

  1. Acacia tree makes nectar for the ants
  2. Ants attack anything that tries to graze on the tree
  3. Trees produce ant repellent on the flowers
  4. Acacia tree makes nectar for the ants, and ants attack anything that tries to graze on the tree
  5. Acacia tree makes nectar for the ants, and trees produce ant repellent on the flowers
  6. Ants attack anything that tries to graze on the tree, and trees produce ant repellent on the flowers
  7. Acacia tree makes nectar for the ants, ants attack anything that tries to graze on the tree, and trees produce ant repellent on the flowers

(1-3) are the isolated traits need for the first evolutionary gradation. (4-6) are possible second modifications. (7) is the end-product. Some pathways are logically impossible. For example, the progression from (1) to (6) is not coherent. Thus, when scientists use this approach, they must only consider those progressions which are logically coherent. For the purpose of simplicity, I will only be focusing on (4-6). If (4-6) are considered, it becomes clear that any of these combinations would break down the entire system.

(4) would be dangerous; if the ants had access to the tree’s flowers, insects could not land on the flowers and the plant could not reproduce. This combination of traits would destroy the system.

(5) is also dangerous and seemingly impossible. The acacia trees rely on the protection of the ants for survival. The ants are a means of preventing other species from overtaking or out-competing the trees in the struggle for survival, and also provide protection from harmful pathogens. Moreover, the data strongly indicate that the resources provided by the tree are contingent upon the presence of large herbivores; when humans isolated the plants from large herbivores, they stopped producing the nectar, became sickly, and the entire system collapsed. Thus, the nectar provided by the tree is contingent upon large herbivores, which would threaten the tree if not for the ants.

(6) would not be possible without (1) since (1) is necessary to attract the ants to the trees. Thus, (6) simply would not happen in nature.

It seems that (4-6) would not physically occur in nature – to say nothing of (1-3). This indicates that this particular mutualistic-symbiotic relationship could not have evolved by numerous successive steps. All the parts are needed, otherwise the entire system breaks down completely. For these reasons, I believe that the mutualism between acacia ants and acacia trees is irreducibly complex.

If proponents of Darwinian theory wish to reject these findings, they must provide an alternative method for determining whether a system is irreducibly complex – that is, another method which could potentially either confirm or falsify irreducible complexity. Then, they would have to use this method to demonstrate at least one evolutionary pathway. Until then, the evidence leads me to think that this particular symbiotic relationship is truly irreducibly complex.

Jacob Pruse is a history major at California State University, Fresno. 

Pretty much all systems of plant and pollinator are irreducibly complex. The most impressive to me is this one: Some plants use negative charge intentionally to attract bees. When a flower is ready for bee service, the plant charges it negatively. Bees, who are always charged positively from the friction of flapping around in the air, sense the negative field and head for it. The contact of bee to flower discharges both sides, so the bee can tell which flowers are used up. Why would a plant go to the trouble of developing a flower-charging system if bees didn't have a sense to detect the charge? Why would bees have the sense if plants weren't sending electrostatic signals? polistra

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