Convergent and divergent evolution

Biological species have been defined as populations of organisms that are capable of successfully interbreeding (producing fertile offspring) only with other members of the same species. Members of any species possess unique sets of biological characteristics, termed “characters.” These characters are physical expressions of a genetic code unique to members of that species. The code represents an extremely complex and thorough set of instructions for equipping an individual organism with the body and the behavioral knowledge it requires for success in the particular environment to which its species has adapted.

Thus, because of natural selection acting upon many past generations of that species, living members are fine-tuned to a specific ecological niche, or econiche, of the greater ecosystem of which the species is a member. When conditions within the ecosystem change (a general climatic change, for example) or when other scenarios occur, such as when a smaller subpopulation of the species migrates into a new, ecologically different territory or becomes isolated in some way, selective pressure is brought to bear upon members of the group or subgroup. Random mutation is a mechanism by which selective pressure is thought to be brought about. Such mutations are changes in the genetic code that occur spontaneously in some individuals within the species in an ongoing manner. Most random mutations are insignificant phenomena regarding the species because most have either a neutral or negative survival value: Either they do not help the individual possessing them to survive or they are counter-adaptive to an extreme degree and prove fatal. Consequently, mutations are not usually transmitted beyond the generation in which they occur or beyond the affected member or members. In certain scenarios, however, mutations with a positive survival value can spread throughout the population. This is believed to be especially true when a smaller, isolated subgroup of the population is dealing with a changed or new environmental situation. Such processes are thought, for example, to have been instrumental in the evolution of groups of closely related but now morphologically distinct species found in isolated, mid-ocean island groups. These adaptive radiations of monophyletic species, which thus share a relatively recent common ancestor, are good examples of evolutionary divergence at work.

The Case of the Galápagos Finches

In one of the best-known cases, studies have traced the presumed paths of divergence among a set of island bird species. This particular radiation produced a number of new species possessing novel adaptive morphologies evolved to exploit new econiches. This is the classic example of Darwin’s finches. Darwin’s finches are a group of closely related birds, numbering about fourteen species, found on the islands of the Galápagos Archipelago, which straddles the equator. The islands are remote from any large body of land that would typically harbor similar birds: South America lies about 960 kilometers (around 600 miles) to the east across an unbroken stretch of the Pacific Ocean. In 1835, Charles Darwin, author of the highly influential work on organic evolution, On the Origin of Species (1859), visited the islands while employed as a naturalist on a British scientific voyage. His studies of the flora and fauna of these islands provided him with many observations that directly influenced his later writings.

Darwin’s studies of the Galápagos finches convinced him and generations of subsequent scientists that the finches are a clear example of divergent evolution in operation. The scenario, he deduced, is that probably only one ancestral species arrived from South America by ocean currents or winds, established itself, and began to exploit the numerous, as yet unoccupied, econiches that the volcanic islands provided. In the relative ecological vacuum that the original finch species found among the islands, adaptive radiation occurred, resulting in the present, diverse species. The species of Darwin’s finches found today on the islands exhibit a great variety of beak types, many of which are atypical of finch-type birds, in general, but rather are typical of birds found among totally different avian family classifications. Typical finches are noted for beaks adapted for the crushing of seeds—the diet of the usual members of the finch family, such as the familiar North American cardinal. Among the dozen or so Galápagos finches can be found a wide assortment of beaks adapted for obtaining or processing a much greater variety of diets. Darwin’s finches include species with beaks and behaviors adapted for diets of insects, seeds, cacti, and other vegetal matter. Some species, like the woodpecker finch, evolved to use tools to thrive in its new environment.

The adaptive radiation in the case of the Galápagos finches was relatively easy to work out because of the obvious environmental factors involved (the islands’ remoteness and general barrenness) and the unusual variety of adaptations that the finches had made. Establishing the details of evolutionary divergence in other living ecosystems can be confusing because the numbers and types of econiches and interacting species are often far more numerous and diverse—for example, the lush and intricate ecosystem of a large tropical rainforest such as the Amazon.

While Darwin's finches are well-known, there are many other examples of convergent and divergent evolution. Pitcher plants, cacti, and euphorbia plants all adapted to their environment to ensure survival, retaining water and adapting their diet requirements. The present-day elephant and the extinct woolly mammoth shared a common ancestor but looked very different as the mammoth grew fur to stay warm. Opposable thumbs are also important examples of convergent evolution. Only humans, primates, opossums, koalas, giant pandas, and chameleons have opposable thumbs. They likely helped our ancestors climb through trees but continued to evolve as a method of using tools.

Tracing the Fossil Record

Myriad examples of evolutionary divergence exist between both living plant and animal species throughout existing ecosystems in the modern world. The fossil record, however, also can be studied, to examine the phenomenon between extinct animal groups. This record of past life-forms preserved in the crustal rocks of Earth provides numerous examples of diverging species as organisms adapted to changing general conditions or spread into novel environments. An example is the many species of ceratopsian dinosaurs found in the latter part of the Cretaceous period of the Mesozoic era of Earth’s history. Although the earlier ancestral forms appear to be bipedal and possess no significant armor, the later radiation of ceratopsians is well known by way of such impressive animals as Triceratops, a typical ceratopsian: a heavy, quadrupedal herbivore with a large, horned, and beaked skull with a defensive, bony frill. Many variations on the basic late ceratopsian body architecture evolved through divergence. Varieties included such forms as Pentaceratops, Torosaurus, Styracosaurus, Chasmosaurus, and Centrosaurus, among many others. In all these later animals, the basic morphology regarding body, tail, and limbs remained the same. All forms also retained the typical massive, beaked head. What diverged were such morphological features as the number and the length of facial horns and length and degree of ornamentation of the frill. The study of such fossil forms shows that past examples of evolutionary divergence eloquently underscore the continuity of the evolutionary process through time down to the present living world. This continuity further reinforces the validity of basic evolutionary theory in general.

Evolutionary Convergence

A related phenomenon concerning adaptive evolution is the phenomenon of evolutionary convergence. This process can be described briefly as the evolution of similar body structures in two or more species that are only quite distantly related; they, therefore, come to resemble each other, sometimes to a startling degree of at least outward sameness. These sets of similar-looking but polyphyletic species frequently even display similar behavioral characteristics. All similarities found in convergence cases are believed to be attributable to the fact that the various species involved have adapted to a similar econiche within a similar ecosystem. Because in nature, form follows function and the morphology of an animal or plant is the product of environmental pressures that continuously favor the better-adapted organism, it is easy to understand how convergence can take place. Like divergence, the evidence for convergence can be traced not only among the many participants in contemporary biospace but also over vast stretches of past time.

One of the classic examples of convergence is a threefold example that, conveniently, not only includes representatives from three different classes of vertebrates but also spans many millions of years of time and includes an extinct group. This is the textbook example that compares the morphology of sharks, a type of cartilaginous fish; ichthyosaurs, an extinct type of marine reptile; and dolphins, marine mammals like whales. All three groups possess numerous member species, both fossil and alive (except for ichthyosaurs), which resemble each other in body plan and lifestyle. All three groups include species which lead (or led) an open ocean, fish-eating existence. Consequently, the forms of their bodies came to follow the functions dictated by their environment—sometimes termed their environmental constraints. All three groups’ general body plan began to approach a hydrodynamic ideal for a water-living animal: a streamlined fusiform, or spindle shape, efficient in passing through an aqueous medium. Besides this feature, pelagic, or open ocean-living sharks, ichthyosaurs, and dolphins all evolved a dorsal fin to act as a vertical stabilizer for water travel. In addition, each group evolved a propulsive tail and a pectoral fin necessary for the demands of constant swimming and steering in water. Even more remarkable in this comprehensive example, dolphins’ and ichthyosaurs’ ancestors were both originally land-dwelling vertebrates that returned to the marine environment. This case presents an inclusive and persuasive argument for the reality of the phenomenon of convergent evolution.

As with the use of both living and extinct examples in the discussion of divergent evolution, the existence of fossils, as well as contemporary species that display convergent morphology, is convincing evidence for the process of adaptive evolution. Again, a continuity across vast stretches of time exists that connects evolutionary phenomena in a continuum.

Observation, Comparison, and Classification

Research of adaptive evolution, especially the phenomena of divergence and convergence of species, began centuries ago with the simple process of recognizing relationships in the surrounding environment between living plants and animals. The search for a unifying order to tie the complex web of animal and plant life together in some meaningful manner was, for a long time, a part of natural science. The modern theory of organic evolution fulfills this goal admirably in many respects. The methods used to illuminate the intricacies of evolution still encompass the type of keen, analytical observation of phenomena and reflection on their causes and effects that characterized Darwin’s studies on the voyage of HMS Beagle. Observation, collection of specimens for comparison, classification of specimens according to a meaningful scheme, and, finally, an attempt to sort out the processes involved in a way that agrees with the dictates of strict logic are hallmarks of the scientific method at work.

Researchers investigating evolutionary divergence and convergence have powerful aids in the form of increasingly sophisticated technology. The focus of their work is the correct interpretation of the path that various lineages took over time to arrive at known, living forms or extinct forms. In the case of living forms, technology originally developed in the field of medicine has been pressed into service to help establish relationships. For example, detailed analyses of various body tissues and fluids have been employed. Blood types have been traced with varying degrees of success, as have various proteins. Powerful optical microscopes are employed to analyze various tissue types and their structures. Since the invention of scanning electron microscopes, these more powerful instruments have further aided in probing the compositions and textures of animal and plant tissues to determine affinities among various species. In addition to these methods, very sophisticated laboratory techniques are now used to unravel and analyze deoxyribonucleic acid (DNA) strands to try to determine the actual genetic encoding possessed by a particular organism. All these methods help establish more clearly the picture of biological relationships regarding ancestries.

This physiological approach is obviously of limited utility regarding fossil species. Except for such instances as the various ice age animals that were frozen in such environments as the tundra, extinct life-forms cannot be analyzed by medical means, as the original tissue has been transformed or destroyed by geological processes. In the case of most fossil forms, hard body parts such as bones and teeth (for vertebrates) and exoskeletons (for invertebrates) must be analyzed in a more structural way to determine possible evolutionary relationships.

The clarification of the paths that various animal and plant lineages took during the process of their evolution further confirms the validity of basic organic evolutionary theory, such as natural selection and adaptation. The study of divergent and convergent species is part of the ongoing study of living organisms that make up the functional ecosystems of which humankind is also a part. Learning more about these ecosystems and the parts that all the member species play within them is extremely important in light of the contemporary world picture of pollution, overpopulation, and industrialization. The increased insight into how ecosystems operate from the species interaction approach is one of the positive by-products that studies of divergent and convergent evolution among species provide.

Principal Terms

Adaptive Radiation: The successful invasion by a species into a number of previously unavailable ecological niches

Analogue: An individual structure shared by two or more species that is of only superficial similarity; thus, it is not indicative of a common ancestor

Clade: A type of grouping of living or extinct species along lines of shared, unique structures, or homologues, indicative of a common ancestor; helpful in establishing evolutionary relationships

Convergence: The evolution of a similar morphology by unrelated or only distantly related species caused by both having adapted to similar lifestyles in similar environments

Divergence: The evolution of increasing morphological differences between an ancestral species and offshoot species caused by differing adaptive pressures

Environmental Constraints (Pressures): The physical demands placed upon any species by its surroundings that ultimately determine the success or failure of its adaptations and consequently its success as a species

Homologue: An individual structure shared by two or more different species that is indicative of a common ancestor

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