Why Mutualism Is Good - the Evolutionary Significance of Symbiosis

Katelyn Lee

Dr. Ely

November 6th, 2015

                                The Evolutionary Significance of Symbiosis

The Theory of Evolution can heavily be attributed to Charles Darwin’s idea that all forms of living organisms descended from one common ancestor therefore making all forms of living organisms somehow related. Darwin described evolution as descent with modification meaning that over time organisms faced environmental circumstances that lead to changes in their genetic makeup (Watson and Pollack, 1995). These genetic changes were either beneficial or defective in aiding in survival, a process known as natural selection. Natural selection is driven by the effects selective pressures have on any organism and as a result creates evolutionary change within an organism’s population. The direction in which a species will evolve over time is determined by the selective pressures they face in their specific environment. Environmental factors such as food, resources and temperature serve to be selective pressures that determine which organisms are selected for and will have increased fitness and which organisms are selected against meaning their fitness will be drastically reduced (Hooper, 2001). Symbiosis is an example of a selective pressure in which a long-term relationship is formed between organisms belonging to different species. Mutualism is a form of symbiosis where a relationship that is benefitting to every member is established between two or more organisms and often times results in the coevolution of those organisms. Mutual symbiotic relationships are a strong selective pressure for evolutionary change because these relationships are responsible for rapidly shaping each organism’s adaptions to their environment which in turn improves their survival and fitness faster than environmental pressures.

In any ecosystem, organisms that possess specific traits necessary for survival will have a greater fitness than those who lack some or all of the necessary traits. In some cases, organisms lack certain abilities and therefore depend on the formation of a mutual symbiotic relationship with a symbiont that carries the specific trait(s). Mutualism is a continuous relationship where both organisms cooperate with one another to fill in for the other’s inadequacies (Hooper, 2001). Forming a mutual relationship in an ecosystem with minimal variation is challenging when compared to forming a relationship in an ecosystem that has abundant symbionts because of the fact that only a few symbionts carry the rare combination of allele frequencies that an organism requires for survival(Watson and Pollack, 1995). Once a mutual relationship is formed, both organisms depend on each other to act as the other’s symbiont and to continuously provide the desired traits. In an ecosystem where finding a symbiont is challenging, there is a greater chance that one of the organisms can become independent as a result of acquiring enough learned traits from the symbiont that it is able to keep using those traits to its advantage. This new independence can be looked at as a potentially strong force for evolutionary change within a population because an organism is quickly forced to change its behavior and use those acquired traits in order to survive (Watson and Pollack, 1995). Even in situations where the symbiont does not leave, mutualism can still be looked at as a strong force for evolutionary change because the benefiting mutual relationship occurs continuously which results in the rapid integration and an eventual change in each organism’s behavior. This behavioral change increases each organism’s fitness by allowing them to be selected for in their environment. Although mutation is usually noted as the ultimate source of genetic variation, a mutual symbiotic relationship can also be responsible for genetic change (Watson and Pollack, 1995). Rather than depending on the direct exchange of genetic information to produce a heritable variation, mutualism focusses on the idea that with the help of a symbiont, an organism can acquire enough learned traits that it could produce a heritable variation in genes. Mutual symbiotic relationships are a driving force behind evolutionary change because with the help of one another, each organism is able to quickly adapt benefiting behaviors not commonly found within their individual population and respectively increase their chances of producing offspring who have inherited the same behaviors for survival.

        One example of a mutual symbiotic relationship that has quickly lead to the evolution of two different species is the relationship between Acacia cornigera, more commonly known as Swollen-Thorn Acacias and a species of ants known as Pseudomyrmex jerruginea. This relationship exists due to the ants’ dependency on the Acacia for food and housing while the Acacia depends on the ants for preservation (Janzen, 1996). Since the Acacia remains green all year round, it is susceptible to predation from rodents in the area. In the past, unoccupied acacias were often destroyed by rodents and therefore became less frequent in some environments. Once the ants discovered the Acacia, they were able to prevent predation and help preserve the Acacia’s fitness. While living in the thorns of the Acacia, the ants have evolved techniques to remove harmful fungal spores from the plant’s leaves as well as defense mechanisms, such as biting or stinging, used to help the plant void off predators. The ants also learned to attack any foreign plants that come anywhere from 50-150 cm close to the Acacia’s foliage (Janzen, 1996). The ants exhibit these behaviors as a result of their evolutionary change that occurred over a short period of time due to their desire to protect their most beneficial means of housing and food. The evolution of these positive behaviors in the ants has led to the increased fitness of the Acacias which in turn has benefited the ant population. Over time, the ants have continued to flourish and grow as a species as a result of shaping the evolutionary direction of the Acacia by selecting certain individual Acacia plants for housing and food. For example, plants that exhibited more swollen, hollow thorns were selected for because the bigger the thorn was the more likely the queen ant was to choose and protect that specific plant to build her colony in. Also, the Acacias that produced extra floral nectaries were selected for because the ants occupied those specific plants and in turn protected that allele frequency from being deleted due to environmental factors (Janzen, 1996). This example of a mutualistic symbiotic relationship is evidence that strong symbiotic relationships can affect evolution because there is no way to guarantee that these changes would have evolved on their own over time but because both species were readily available within the environment, they were able to take advantage of one another which helped speed up the evolution of each new behavior and characteristic. As long as the Acacias and the ants continue to support and protect one another, this mutual relationship will continue to evolve these benefiting behaviors and characteristics that will be naturally selected for.

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