TOEFL Academic Passage Practice 018Coevolution of Pinyon Pines and Pinyon Jays (Variation A)

Species in nature often evolve alongside one another in a way that makes them interdependent. One example of this is the relationship between trees and certain fungi. Mycorrhizal fungi live in tree roots, assisting the tree in absorbing nutrients from the soil, while the fungi rely on the tree’s energy-rich sugars for survival. This mutual dependence is a result of coevolution—a process where two species adapt to each other over time to such an extent that they cannot thrive without one another.

At first glance, the interaction between pinyon pines and pinyon jays does not seem to qualify as coevolution. Pinyon pines produce cones with seeds that attract animals, particularly pinyon jays. Since these seeds contain the plant embryos for the next generation of trees, it may seem counterintuitive for the trees to produce seeds that attract predators. Other plants, for example, have developed toxic seeds to prevent animals from eating them. Pinyon pines, however, appear to do the opposite. Their cones grow in prominent locations, their seeds are highly nutritious, and they even have thin, easily digestible seed coats. Moreover, the seeds differ in color to indicate which are viable, essentially guiding jays to select only seeds that can grow into new trees.

This seemingly risky strategy actually benefits the trees. Pinyon jays gather and bury large quantities of seeds to save for winter. Many of these buried seeds go uneaten, allowing them to germinate and grow into new trees. For this system to work, though, the pines must produce seeds in such abundance that jays cannot consume them all. Individual trees cannot manage this on their own, so pinyons across a region coordinate their seed production. Every few years, they produce massive crops of seeds—a phenomenon known as a “mast year.”

Producing large crops of seeds requires significant energy, and annual mast years would be unsustainable. Additionally, regular seed production would make it easy for seed predators to expand their populations, which could result in the loss of all seeds. By varying the intervals between mast years, usually every six years, pinyons keep seed predator populations lower during lean years, ensuring some seeds survive during heavy production cycles.

Pinyon jays have adapted to this cyclical pattern. Unlike most birds, which breed based on seasonal changes in daylight, pinyon jays also use seed abundance as a cue for reproduction. When pinyon seeds are plentiful, the jays breed earlier and more frequently. This increase in jay populations is temporary and never fully matches the abundance of seeds during a mast year, leaving many seeds buried and able to germinate. The relationship between pinyon pines and pinyon jays is, therefore, an intricate example of coevolution, where each species depends on the other for survival.