Engines of Diversity

A species does not just adapt to its physical environment but also to its biological environment-the species with which it shares intimate ecological relationships. What makes the biological environment different from the physical environment is that it can also evolve. As a species adapts to its ecological partners, its partners can adapt to it as well. Biologists refer to this reciprocal evolutionary change in interacting species as coevolution.

Coevolution can help drive some populations to evolve into distinct species. One of the few tree species to survive in the Mojave Desert, the Joshua tree (Yucca brevifolia), is pollinated by a species of yucca moth called Tegeticula synthetica in the western part of its range; in the east, it is pollinated by another species called Tegeticula antithetica. Biologist Olle Pellmyr and his colleagues have reconstructed the evolution of the tree and the moths on which it depends to carry its pollen to other trees for fertilization. They found that the ancestor of both moth species was already pollinating Joshua trees. Only later did the two species divide, each specializing on one population of the trees. In response, the scientists have found, Joshua trees have evolved canals in their flowers to match the length of the tongues of their own species of yucca moth. If the trees and moths continue to diverge, there will be less and less opportunity for pollen to move from one population of Joshua trees to another. Ultimately this could lead to the development of different species.

Over millions of years, this kind of coevolution can have a profound impact on biodiversity. Biologist Szabolcs Lengyel and his colleagues recently took a look at the diversity that arises from the coevolution of flowering plants and the ants that spread their seeds. About 11,000 known plant species around the world grow fleshy handles on their seeds called elaiosomes that serve as food for the ants. After ants bring the seeds to their nests and eat the elaiosomes, they discard the seeds in a special room in their colony. There the seeds can sprout, protected from being eaten by other animals. Elaiosomes have evolved independently at least 101 times, as Lengyel and his colleagues reported in 2009. They also found that the ant lineages that disperse plant seeds contain over twice as many species as the most closely related lineages of plants. Ants may foster plant diversity by protecting seeds and by keeping them growing in a small range around their colonies, causing them to become isolated and therefore unable to reproduce with plants in other locations.

The world's biodiversity is now experiencing mass extinctions on a scale rarely seen in the history of life. Some scientists are trying to understand how coevolution will affect which species survive and which disappear. If one species depends on another one for its survival, then it will not be able to endure after the other species becomes extinct. If it can shift to a new partner, however, it may be able to survive. Mass extinctions in the past offer some clues to how coevolution makes species vulnerable. Some species of corals live mutualistically with algae-that is, the two species exist in a relationship that benefits each-while some do not. In the last major mass extinctions, 66 million years ago, the mutualist corals suffered about four times more extinctions than the nonmutualists. The mass extinctions coincided with a huge asteroid impact that blocked out the light of the Sun for months. It is possible that this blackout killed off algae as well as the corals that depended on them for survival.

Today we can see a growing number of species that have lost their coevolutionary partners. In Central America and South America, a number of trees grow giant fruits with massive seeds at their core. Biologist Daniel Janzen has argued that these giant fruits are the result of coevolution with giant mammals, such as ground sloths. The mammals ate the fruits and spread the seeds in their droppings. Around 12,000 years ago, giant sloths and other big mammals disappeared, probably hunted by newly arrived humans. The trees survived, still producing their giant fruits. They can be dispersed today by cows and rats, but not as successfully as they were in the past.