As you think about all of the amazing interactions that can occur between organisms of different species, you might begin to wonder about the evolutionary consequences of such interactions. Actually, evolution and ecology are closely connected. One of the newest and hottest fields in biology is even called Evolutionary Ecology, or Evo-Eco. As you read in the section on predation, predators and their prey are often engaged in an evolutionary arms race, with each species adapting to the attacks or defenses of the other. When one species evolves in response to the evolutionary path of another, and vice versa, the process is called coevolution. This unique, though not uncommon, evolutionary process is based on the ecological interactions experienced by the coevolving species. In fact, many of the interactions we discussed in community ecology can lead to coevolutionary relationships. 

Let’s look at a couple of interesting examples:

First, there is a mutualistic relationship between a moth and an orchid plant in Africa that has resulted in a coevolutionary event. The orchid plant has a flower with an 11-inch tube leading to the nectar below. The only organism that can reach the nectar and provide the pollination services needed by the orchid is a moth with—you guessed it—an 11-inch tongue. Through evolutionary time, natural selection has acted on the mutualistic relationship between the orchid and the moth, leading to alternating and incremental increases in both flower tube length and moth tongue length. This type of coevolution could occur if moths with longer tongues were more successful at obtaining nectar than shorter-tongued moths in the same population, leading to longer-tongued moths having more offspring. In the same manner, longer-tubed flowers in the population were more successfully pollinated, leading to more offspring.

A little closer to home, coevolution has and continues to occur between humans and one of our more unfortunate and devastating parasites, the human immunodeficiency virus (HIV). This virus is able to rapidly mutate and adapt to human immune defenses and other aspects of the cellular environment. It is unlikely that HIV could completely eradicate the human population, though, because human response to HIV infection varies dramatically from person to person. That is, some people are affected more negatively than others by the disease, leading to evolutionary changes in the worldwide human population. In addition, humans are exerting evolutionary pressure on HIV by developing antiviral drugs and other defenses. It will be interesting to see how this evolutionary arms race plays out in the future.

Lastly, even competitive interactions can lead to coevolution. When two competing species partition their limited resources, they usually do so through an coevolutionary process called character displacement. In fact, character displacement is what makes resource partitioning possible in the first place. This process occurs when some members of the competing species are able to use a slightly different resource than others. Those that are able to use these slightly different resources become more successful at reproducing and have more offspring, leading to a large portion of the population being able to use the alternative resource.

Through evolutionary time, the two competing species diverge from each other enough so they can happily coexist. A great example of character displacement has been shown in two species of finches on the Galapagos Islands. Where these two species live on separate islands they both have medium-sized beaks that are useful for opening seeds of different sizes. However, where the two species occupy the same island, one species has evolved a larger beak size—good for opening larger seeds—and the other has evolved a smaller beak size—good for opening smaller seeds. Originally, the two species occupying the same island competed for seeds of all sizes. Through time, however, they were able to partition the resources through the evolutionary process of character displacement.

These examples, and many others, show the intimate relationship between ecological interactions and evolutionary processes. You can think of evolution as the long-term outcome of shorter-term ecological interactions. These interactions can be between different species, within species, or even between organisms and their ever-changing environments.

We have structured this entire unit using the natural levels of organization seen in ecological interactions. We started by discussing how individual organisms of the same species interact with one another in populations. These interactions led to the distinctive characteristics of populations that ecologists measure and observe, including growth rates, sizes, distributions, and so on. We then moved to a discussion of how populations of different species interact with one another in communities. These interspecific interactions, as well as the intraspecific interactions seen inside populations themselves, are the basis for much of the evolutionary process in the world. Last, we discussed the interactions between communities and their habitats in ecosystems. These biotic-abiotic interactions provide the rest of the substance upon which evolutionary forces generally act.

Each level of organization in ecology has its unique attributes and characteristics, most of which cannot be measured, or even understood, at the other levels. For example, an individual cannot have a growth rate, as this term is understood for a population. A growth rate is calculated by subtracting the death rate from the birth rate. Since an individual only has one birth and one death, the idea of an individual growth rate in this sense is absurd. Absurd, we tell you!

At the same time, it is impossible to measure the life history of a population or community. These are characteristics of individuals. Therefore, recognizing and understanding the different levels of organization in ecology can help the researcher or student focus on the most important aspects of that level. It is impossible to study and comprehend everything at once. You know it, and we know it.

For this reason, too, it is important to identify which level of organization is under consideration. In addition, having a good concept of one level can help you better understand the others. We saw this when we transitioned from populations to communities. Without a good understanding of what a population is and how it functions, your understanding of a community would be greatly reduced. The same is true for understanding an ecosystem. Recognizing the natural levels of organization in the other areas of biology can be equally useful.