The midges in our opening example obtain the benefit of food from the cacao blossoms at the cost of unwitting pollen transport between flowers; the chocolate tree obtains the benefit of pollination at the cost of producing the sugars and amino acids in the flowers’ nectar.

The nitrogen-fixing bacteria that live in nodules on the roots of soybeans (Chapters 26 and 29) provide another illuminating example. The bacteria occur naturally in the soil and are attracted to the plant roots, where they take up residence in nodules produced by the plant. The bacteria provide their host plant with nitrogen in a biologically useful form, and the plant provides the bacteria with food and a stable environment. In response to signals from the bacteria, the plant builds the bacteria’s home—the nodules—out of root tissue, a cost outweighed by the benefit of enhanced access to nitrogen, a benefit measured in greater growth and reproductive output. In the interaction between plant and bacteria, both sides win.

Despite the observation that both partners in a mutualism benefit, it is important to remember that each side is acting in its own self-interest and bears costs that are weighed against benefits in terms of growth and reproduction. Associations that, overall, are beneficial to one partner are often also beneficial to the other partner and, therefore, more often represented in the next generation. In other words, mutualisms are subject to natural selection, just like any other adaptation (Chapter 21).