Coevolution in plants

Categories: Ecology; ecosystems; evolution

When two or more different species evolve in a way that affects one another’s evolution, coevolution is taking place. This interactive type of evolution is characterized by the fact that the participant life-forms are acting as a strong selective pressure upon one another over a period of time. The coevolution of plants and animals, whether animals are considered strictly in their plant-eating role or also as pollinators, is abundantly represented in every terrestrial ecosystem throughout the world where flora has established itself. Moreover, the overall history of some of the multitude of present and past plant and animal relationships is displayed (although fragmentally) in the fossil record found in the earth’s crust.

Beginnings

The most common coevolutionary relationships between plants and animals surround plants as a food source. Microscopic, unicellular plants were the earth’s first autotrophs (organisms that can produce their own organic energy through photosynthesis, that is, from basic chemical ingredients derived from the environment). In conjunction with the appearance of autotrophs, microscopic, unicellular heterotrophs (organisms, such as animals, that must derive food from other sources, such as autotrophs) evolved to exploit the autotrophs.

Sometime during the later part of the Mesozoic era, angiosperms, the flowering plants, evolved and replaced most of the previously dominant land plants, such as the gymnosperms and the ferns. New species of herbivores evolved to exploit these new food sources. At some point, probably during the Cretaceous period of the late Mesozoic era, animals became unintentional aids in the angiosperm pollination process. As this coevolution proceeded, the first animal pollinators became more and more indispensable as partners to the plants.

Eventually, highly coevolved plants and animals developed relationships of extreme interdependence, exemplified by the honeybees and their coevolved flowers. This angiosperm-insect relationship is thought to have arisen in the Mesozoic era by way of beetle predation, possibly on early, magnolia-like angiosperms. The fossil record gives some support to this theory. Whatever the exact route along which plant-animal pollination partnerships coevolved, the end result was a number of plant and animal species that gained mutual benefit from the new type of relationship.

Coevolutionary Relationships

Coevolved relationships include an immense number of relationships between plants and animals, and even between plants and other plants. Among these coevolved situations can be found commensalisms, in which different species have coevolved to live intimately with one another without injury to any participant, and symbioses, in which species have coevolved to literally “live together.”

Such intertwined relationships can take the form of mutualism, in which neither partner is harmed and indeed one or both benefit—as in the relationships between fungi and algae in lichens, fungi and roots in mycorrhizae, and ants and acacia trees in a symbiotic mutualism in which the ants protect the acacias from herbivores. In parasitism, one partner benefits at the expense of the other; a classic example is the relationship between the mistletoe parasite and the oak tree. Another coevolutionary relationship, predation, is restricted primarily to animal-animal relationships (vertebrate carnivores eating other animals, most obviously), although some plants, such as Venus’s flytrap, mimic predation in having evolved means of trapping and ingesting insects as a source of food. Some highly evolved fungi, such as the oyster mushroom, have evolved anesthetizing compounds and other means of trapping protozoa, nematodes, and other small animals.

One of the most obvious and complex coevolutionary relationships are the mutualisms that have evolved between plants bearing fleshy fruits and vertebrate animals, which serve to disperse the seeds in these fruits. Over time, plants that produce these fruits have benefited from natural selection because their seeds have enjoyed a high degree of survival and germination: Animals eat the fruits, whose seeds are passed through their digestive system (or regurgitated to feed offspring) unharmed; at times the seeds are even encouraged toward germination as digestion helps break down the seed coat. Furthermore, dispersal through the animals’ mobility allows the seeds to enjoy more widely distributed propagation. The coevolutionary process works on the animals as well: Birds and animals that eat the fruits enjoy a higher degree of survival, and so natural selection favors both fleshy-fruit-producing plants and fleshy-fruit-eating animals. Similar selection has favored the coevolution of flowers with colors and smells that attract pollinators such as bees.

Eventually some plant-animal mutualisms became so intertwined that one or both participants reached a point at which they could not exist without the aid of the other. These obligatory mutualisms ultimately involve other types of animal partners besides insects. Vertebrate partners such as birds, reptiles, and mammals became involved in mutualisms with plants. In the southwestern United States, for example, bats and the agave and saguaro cactus have a special coevolutionary relationship: The bats, nectar drinkers and pollen eaters, have evolved specialized feeding structures such as erectile tongues similar to those found among moths and other insects with similar lifestyles. In turn, angiosperms coevolutionarily involved with bats have developed such specializations as bat-attractive scents, flower structures that match the bats’ feeding habits and minimize the chance of injuring the animals, and petal openings timed to the nocturnal activity of bats.

Defense Mechanisms

Coevolution is manifested in defense mechanisms as well as attractants: Botanical structures and chemicals (secondary metabolites) have evolved to discourage or to prevent the attention of plant eaters. These include the development of spines, barbs, thorns, bristles, and hooks on plant leaves, stems, and trunk surfaces. Cacti, hollies, and rose bushes illustrate this form of plant strategy. Some plants produce chemical compounds that are bitter to the taste or poisonous. Plants that contain organic tannins, such as trees and shrubs, can partially inactivate animals’ digestive juices and create cumulative toxic effects that have been correlated with cancer. Grasses with a high silica content act to wear down the teeth of plant eaters. Animals have counteradapted to these defensive innovations by evolving a higher degree of resistance to plant toxins or by developing more efficient and tougher teeth with features such as harder enamel surfaces or the capacity of grinding with batteries of teeth.

Bibliography

Bakker, Robert T. The Dinosaur Heresies. Reprint. Secaucus, N.J.: Citadel, 2001. Although the emphasis of the book is on dinosaur evolution, chapter 9, “When Dinosaurs Invented Flowers,” discusses coevolved morphologies and strategies among angiosperm plants and extinct reptiles. A wonderful, thought-provoking book for all reading audiences. Illustrated.

Barth, Friedrich G. Insects and Flowers: The Biology of a Partnership. Translated by M. A. Biederman-Thorson. Princeton, N.J.: Princeton University Press, 1991. A delightful study of the coevolution of insects and flowers, highlighting both the complex sensory world of insects and the sometimes bizarre ways in which angiosperms have evolved to take advantage of insect senses in order to assure their own pollination.

Gilbert, Lawrence E., and Peter H. Raven, eds. Coevolution of Animals and Plants. Austin: University of Texas Press, 1980. A collection of scientific papers on the subject of evolved plant and animal relationships. The majority of the papers involve analyses of plant and animal biochemical relationships and mechanisms or ecological processes and patterns. Written for professionals in the field, though understandable by the general reader.

Gould, Stephen Jay. The Panda’s Thumb. Reprint. New York: W. W. Norton, 1992. Chapter 27, “Nature’s Odd Couples,” focuses on a conspicuous example of a possible coevolution—an inferred mutualistic relationship between the extinct dodo bird and a large tree (Calvaria major) that is found on the island of Mauritius. Gould presents the evidence objectively, both for and against the dodo’s possible role in facilitating the tree’s germination.

Grant, Susan. Beauty and the Beast: The Coevolution of Plants and Animals. New York: Charles Scribner’s Sons, 1984. This is one of the best books devoted solely to the subject of coevolution written for a general audience. Each of the ten chapters is devoted to a major subtopic within the phenomenon of coevolution, such as specific animal and plant symbioses or antagonisms.

Hughes, Norman F. Paleobiology of Angiosperm Origins. Reprint. New York: Cambridge University Press, 1994. The chapter on Cretaceous land fauna sketches out much of what is known about the earliest coevolved relationships between animals and the flowering plants (angiosperms) during the late Mesozoic period. Appropriate for readers who have some familiarity with botany, entomology, and evolutionary theory.

Thompson, John N. The Coevolutionary Process. Chicago: University of Chicago Press, 1994. Thompson advances a new conceptual approach to the evolution of species interactions, the geographic mosaic theory of coevolution. Thompson demonstrates how an integrated study of life histories, genetics, and the geographic structure of populations yields a broader understanding of coevolution.