Coevolution

Coevolution is an extremely important and widespread phenomenon in the world of living things; it is a biological factor that is global in influence. When two or more different species experience a relationship in which any of the participating species’ evolution directly affects the evolution of the other members, 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.

The assumption of the interdependence of all organisms is a commonplace and fundamental concept in the twenty-first century, but the phenomenon of coevolution has not always enjoyed a more prominent position in evolutionary thinking. Many scientists seem to have considered sets of coevolved organisms as relatively unimportant phenomena, almost on the level of biological “curiosities.” The consensus appears to have been that while numerous examples of coevolution existed in both plant and animal kingdoms, overall, it was of relatively minor importance in comparison with other evolutionary phenomena, such as competition. This opinion has begun to change as researchers increasingly recognize the intrinsic and ubiquitous role that coevolution has played, and continues to play, in the evolution of life at all levels throughout Earth's history.

Gaia

Organisms do not evolve in a biological vacuum. All organisms exist in, and have evolved within, the framework of one of a great number of delicately balanced and self-tuning biological systems or living communities termed ecosystems. Indeed, the entire planet can be regarded as one huge, incredibly complex ecosystem in which all the lesser ecosystems fit together and work together harmoniously. This planetary ecosystem has been called Gaia by some biologists, in reference to the ancient Greek Earth goddess. In some respects, Gaia can be conceived of as actually one giant, worldwide organism. All the living communities in this huge ecosystem are products of coevolution. This phenomenon has been in effect over the vast expanses of geologic time and continues. The only period in history when coevolution was probably not operating was at the dawn of life, billions of years ago, when the very first species of organisms appeared and had not yet established interactive communities. The importance of coevolution as a factor affecting life cannot be overstated.

Some biologists use the term coevolution in a more restricted sense to describe coevolved relationships that have developed between plants and animals, particularly between plants and animals that are herbivores or pollinators. The coevolution between plants and animals is one of the aspects of the field that has traditionally received the most attention, so this aspect of coevolution provides a useful departure point in describing the phenomenon.

Coevolutionary Warfare

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 Earth’s crust. The most elemental relationship between plants and animals is that of plants as a food source. This relationship has an extremely long history, beginning with the evolution of microscopic, unicellular plants that were Earth’s first autotrophs (organisms that can produce their own food from basic ingredients derived from the environment). In conjunction with the appearance of autotrophs, microscopic, unicellular heterotrophs (organisms such as animals, which must derive food from organic sources such as autotrophs) evolved to exploit the simple plants. This ancient and basic relationship has resulted in uncounted numbers of plant and animal species evolving and coevolving over billions of years of Earth’s history.

As both plants and animals became multicellular and more complex, more elaborate defense mechanisms evolved among plants, as did more elaborate feeding apparatuses and behavior among animals. This biological “arms race” grew ever more intense as groups of plants and animals eventually adapted to the more rigorous demands of a terrestrial existence, leaving the marine environment behind. New ecosystems developed that culminated in the world’s first swamps, jungles, and forests. The plant-animal arms race engendered increasingly more sophisticated strategies of botanical defense and animal offense, and this coevolved interrelationship has continued unabated. This coevolutionary “warfare” between plants and animals has expressed itself partly through the evolution of botanical structures and chemicals that attempt either to discourage or to prevent the attentions of herbivores. These include the development of spines, barbs, thorns, bristles, and hooks on plant leaf, stem, and trunk surfaces. Cacti, holly, and rose bushes illustrate this form of plant strategy.

Another type of deterrence evolved in the form of chemical compounds that can cause a wide spectrum of negative animal responses. These compounds range in effect from producing a sensation of mild distaste, such as bitterness, to more extreme effects, such as actual poisoning of herbivore metabolisms. Plants that contain organic compounds such as tannin are examples of the chemical defensive strategy. Tannins produce several negative results in animals, including partially inactivating digestive juices and creating cumulative toxic effects that have been correlated with cancer. Plants containing tannin include trees, such as members of the oak group, and shrubs, such as those that produce the teas used as human beverages. Other plants have developed more lethal poisons that act more rapidly. Plants have also developed other strategies, such as possession of a high silica content (as found in grasses), that act to wear down the teeth of plant eaters. Animals have counter adapted to these plant 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.

Coevolutionary Alliances

Not all coevolution is characterized by having an adversarial nature; mutually beneficial relationships are also very common. Sometime during the latter 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, 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 result was several plant and animal species that gained mutual benefit from the new type of relationship. Such relationships are, in general, termed mutualisms.

Eventually some of these 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 involved other types of animal partners besides insects. Vertebrate partners such as birds, reptiles, and mammals also became involved in mutualisms with plants. Contemporary ecosystems, such as the United States’ southwestern desert, include mutualisms between aerial mammals, such as bats, and plants, such as the agave and the saguaro cactus. The bats involved are nectar drinkers and pollen eaters. They have evolved specialized feeding structures such as erectile tongues like those found among moths and other insects with similar lifestyles. In turn, the plants involved with the pollinating bats have evolved either reciprocal morphologies or behavior patterns to accommodate their warm-blooded visitors. For example, angiosperms coevolutionarily involved with bats have developed such specializations as bat-attractive scents, flower structures that minimize the chance of injury to bats, and petal openings timed to the nocturnal activity of bats.

Symbiosis, Commensalism, and Parasitism

Coevolved relationships are not restricted to beneficial or nonbeneficial relationships between plants and animals. They also include an immense number of relationships between animals and other animals, and even between plants and other plants. Among these various types of coevolved situations can be found subcategories such as symbioses, commensalisms, and parasitism. The first two involve relationships beneficial to varying degrees that feature interactions of increasing physical intimacy between or among two or more species. Parasitism involves an intimate relationship produced through coevolution in which one participant, the host, experiences serious harm or even death through exploitation by the parasite. Predation is probably the most obvious form of coevolution among higher animals such as vertebrates. Modern carnivores such as the canines and felines and their prey are a dramatic example of coevolution at work. Animal hunters, over time, responded to the improved defenses of their prey by evolving better senses, such as stereoscopic, three-dimensional vision, hearing with expanded range of frequency response, and more effective body structures, such as multifunctional teeth. Such teeth are termed heterodont and represent a great improvement over the simple dental array of the more primitive vertebrates, such as fish and amphibians.

Beginning with the more advanced reptiles appearing in the late stages of the Paleozoic era, teeth began to differentiate into specialized components—incisors, canines, premolars, and molars—that enhanced food acquisition and improved mastication. This, in turn, improved digestion and allowed quicker energy acquisition from food. This evolutionary advantage has reached a zenith of adaptive success among the mammals. Mammalian predators evolved fangs and efficient claws, sometimes retractable, to minimize injury and wear. Along with improved hunting senses and better dentition came increased speed from the evolution of improvements in pelvic and limb arrangements. In response to this process, vertebrate herbivores also became generally swifter or better defended, more alert, attained higher metabolic rates, and were thus better able to elude or defend against predation. Advanced predators placed an intense selective pressure on their prey herbivores, spurring ever more efficient and acutely tuned responses among the herbivore populations. Herbivores evolved either as swift forms, such as deer, or became efficiently defended, walking fortresses, such as porcupines or armadillos. Because of the pervasive effect of coevolution, the overall relationship between predator and prey has been a reciprocal one in which all participants affect one another in an interactive manner.

Unraveling the Intricacies of Coevolution

Field research and laboratory research are pursued concurrently in the effort to unravel the intricacies of the subject of coevolution. Field research involves actual observation in nature of animals and plants, their behavior, and, especially, their interaction with other species. Special attention is given to useful clues that can be employed to establish evolutionary relationships, either presently existing or previously in effect. For example, cooperative behavior between or among several different species of animal or plant is often indicative of an established, coevolutionary relationship. If this behavior is consistent over time, and can also be traced or inferred through the agency of the fossil record, more useful data are acquired concerning a possible, evolved, reciprocal relationship. Of particular importance is the confirmation of specialized physical structures that are unique to the members of the observed relationship. Examples are the specialized feeding apparatuses of pollinating animals and the specialized, accommodating flower structures of their angiosperm partners. Such physical structures are strong evidence for the handiwork of the coevolutionary process. Direct human observation is preferable in ascertaining coevolutionary behavior; however, this is not always possible because of the rapidity of the animals involved, their habitat, their extremely small size, their preference for nocturnal activity, or their determined avoidance of humans. Consequently, electronic and mechanical aids are sometimes indispensable. These include remote-controlled still and video cameras, microscopic or telephoto lenses, infrared or ultraviolet lighting units, sonar or radar sensors, trip wires and other mechanical triggering devices, and sound recording equipment with high-gain or long-range microphones.

Laboratory research in the field of coevolution involves investigations heavily reliant on modern, sophisticated laboratory equipment and techniques. High-powered conventional, optical microscopes are employed to determine tissue and cellular structures. Scanning electron microscopes (SEMs) are employed for study of extremely small unicellular animals or plants, such as planktonic organisms or extremely small organic structures. In addition to these tools of laboratory specimen observation, there are the analytical equipment and techniques used to determine the genetic codes and blood protein complexes of animals and plants to establish the degree of relatedness or divergence between various species.

Maintaining Balances

Coevolutionary studies are increasingly important in the biological sciences. One of the aims is to determine the degree of interdependence between various species, whether the relationship is between animals and plants, animals and other animals, or plants and other plants. A key factor to be determined in all these coevolved relationships is that of the nature and degree of balance attained. Although most of the biological world is forever in a state of flux, some categorical, coevolved relationships have been of long duration and can be reasonably assessed as having been in existence for tens of millions of years, such as that of flowering plants and vertebrate and invertebrate pollinators, or even hundreds of millions of years, such as the oceanic, planktonic food chain.

The degree to which these large-scale, coevolved relationships, involving entire planetary ecologies, continue to enjoy their former degree of health and well-being is of the utmost importance to human society. The present depth of understanding of the biological sciences clearly indicates the interrelatedness of all nature. Many angles of study agree that the global life system is experiencing great stress from human intervention: industrialization, urbanization, and overpopulation. It becomes increasingly urgent to know with the utmost precision all facets of the way the global life system operates, and has operated with general stability, over geological expanses of time. Every detail that contributes to this knowledge—every coevolved relationship, no matter how seemingly insignificant—adds to the total effect. This information can be used as an important resource to help maintain the stability of the entire system for all humanity.

Principal Terms

Antagonism: Any type of interactive, interdependent relationship between two or more organisms that is destructive to one of the participants

Coevolution: The interactive evolution of two or more species that results in a mutualistic or antagonistic relationship

Commensalism: A type of coevolved relationship between different species that live intimately with one another without injury to any participant

Parasitism: A type of coevolved relationship between different species in which one species exploits the other to its physical detriment

Phytophagous: Animals, also referred to as herbivorous, that feed on plants

Reciprocal Relationship: Any type of coevolved, highly interdependent relationship between two or more species

Selective Pressure: Evolutionary factors that favor or disfavor the genetic inheritance of various characteristics of a species

Symbiosis: A type of coevolved relationship between two species in which both participants benefit; a type of mutualism

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