The Theme of Evolution in Ecology: Organisms and Their Environments
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.