Adaptive radiation

In 1898, Henry F. Osborn developed the concept of adaptive radiation. According to Osborn, many different forms of evolutionary adaptations may occur among plants and animals that started with a common ancestor. In this way, evolutionary divergences can take place, and the occupation of a variety of ecological niches is made possible according to the adaptive nature of the invading species. As may be seen with certain forms of animal life, however, the ability to adapt is not shared by all species. Therefore, in many instances, either evolutionary divergence has been modest, or the species involved has become extinct.

The Principles of Natural Selection

To understand how adaptive radiation operates, it is necessary to become familiar with the principles of natural selection. The concept of natural selection, frequently expressed as “survival of the fittest,” is at the core of Charles Darwin’s theory of evolution. Darwin did not mean to suggest that there was a physical struggle among organisms to survive. Instead, he meant that organisms compete for food, space, shelter, water, and other things necessary for existence. Only those organisms best adapted for a particular habitat will survive. According to the concept of natural selection, all organisms of a given species will show variation in color, size, physiology, and many other characteristics; in nature, all organisms produce more offspring than can survive, so the offspring must, therefore, compete for the limited environmental resources. Organisms that are the best adapted (most fit) to compete will live to reproduce and pass their successful traits on to their offspring. The others, which are less fit, will die without reproducing. When different parts of an animal population are faced with slightly different environments, they will diverge from one another and, in time, will become different enough to form new species. Natural selection also has the effect of producing different patterns of evolution. It may bring about widely different phenotypes (variable characteristics) in closely related animals, for example, or similar phenotypes in distantly related organisms. The organisms themselves may also become forces of selection through their interrelationships with other species.

The process of adaptive radiation illustrates how natural selection operates. The most frequently cited example is the evolution of Darwin’s finches on the Galápagos Islands, off the west coast of South America. The islands were formed from volcanic lava about one million years ago. At first, they were devoid of life, but bit by bit, several species of plants and animals migrated to them from the South American mainland. Since the nearest island is about 950 kilometers (590 miles) from the coast of Ecuador, it is unclear how the different species arrived. It has been suggested that the birds may have been carried to the islands by strong winds since finches are not known for their lengthy flights. Other organisms may have been carried by floating debris. In any event, the islands became populated. The mainland ancestor of the finches is not known, but it was no doubt a nonspecialized finch (a finch is about the size of a sparrow). Since there were no other birds with which to compete on the islands, the original population of finches began to adapt to the various unoccupied niches. The early offshoots of the original population were repeatedly modified as adaptations continued. This process resulted in the evolution of fourteen species of finches. The main feature that makes each species different is the size of their beaks, which have adapted for the various types of available foods. Today, the finches live on fifteen different islands. Some of the species are found in the same area (sympatric), while others occur in different areas (allopatric). The most noteworthy example of an adaptation to a particular niche is the woodpecker finch. A true woodpecker has an extremely long tongue that it uses to probe for insects. Since the woodpecker finch does not have a long tongue, it has learned to use a cactus spine for insect probing, and it can, therefore, occupy a niche normally filled by true woodpeckers.

A more recent example of adaptive radiation in its early stages has taken place in an original population of brown bears. The brown bear can be found throughout the Northern Hemisphere, ranging from the deciduous forests up into the tundra. During one of the glacier periods, a small population of the brown bear was separated from the main group; according to fossil evidence, this small population, under selection pressure from the Arctic environment, evolved into the polar bear. Although brown bears are classified as carnivores, their diets are mostly vegetarian, with occasional fish and small animals eaten as supplements. On the other hand, polar bears are mostly carnivorous. The polar bear is different from the brown bear in many ways, including its streamlined head and shoulders and the stiff bristles that cover the soles of its feet, which provide traction and insulation, enabling it to walk on ice. The polar bear’s translucent, pigment-free, hollow tube-like hairs reflect the snow, making them appear white, allowing them to blend into their environment.

Adaptive radiation research concerning opossums in the first decades of the twenty-first century offered the field increasing insight into the evolutionary processes and physical and behavioral adaptations of marsupials. Marsupials, like opossums, first evolved in South America, but in the twenty-first century, they inhibit most of the world’s terrestrial biomes. In the Amazonian Rainforest, opossums are the third most ecologically diverse creatures, behind bats and rodents. Among the many species living worldwide, each species developed a range of adaptations to accommodate specific ecosystems. These include eating venomous snakes like rattlesnakes and pit vipers, faking death or “playing opossum,” living in swamps, eucalyptus forests, and cities, and developing sensory organs on their hands that are not fully understood. While these traits are specific to the opossums’ environment, all opossums have spinal cords that can regenerate for a few days following birth and are born with reptile-like features, such as their jaw bones and middle ear, that provide an important evolutionary clue about modern mammals’ ancestors.

Evolution

All the genes of any population of living organisms at any given time make up its gene pool, and the ratio of alternative characteristics (alleles) in the gene pool can change because of selection pressures during the passage of time. As the ratio of alleles changes, evolution occurs. Evolution may be a random change, or it may occur because of the directive influences of natural selection. In the former case, occasional and unpredictable permanent random changes called mutations take place in the DNA molecules that compose the genes. These mutations also may be selected for by the environment or selected against by the environment. It is simply an accident if the newly mutated genes help the organism to become better adapted to its particular habitat niche. Genes may not change or become mutated through several generations (the Hardy-Weinberg principle) but may change in terms of survival value if the environment changes or the species population is subjected to new mutations or natural selection. The relative numbers of one form of allele decrease in a divergent population, while the relative numbers of a different gene increase. This progressive change is all-important in the evolutionary process that takes place between the origin of a new gene by random mutation and the replacement of the original form of the gene by descendants having the newer, better-adapted form of the gene. The result in the long term is that enough of the DNA changes, either slowly or rapidly, through divergent populations or organisms, that the new generations have become so different from the original population that they are considered new species. Many times in Earth’s history, a single parental population has given rise not to one or two new species but to an entire family of species. The rapid multiplication of related species, each with its unique specializations that fit it for a particular ecological niche, is called adaptive radiation, or divergent evolution.

Studying Adaptive Radiation

Not all scientific information is gained by experimentation: A considerable portion of science is descriptive and is based on observation. In determining that adaptive radiation has occurred and is indeed taking place among living species, much supporting evidence has come from the study of fossils and from observations of the structural, physiological, and behavioral adaptations of modern animals. Clearly, wide-scale experimentation is out of the question. No matter how well an experiment may be designed to test the concept of adaptive radiation, the scientist could not be around thousands or millions of years from now to gather the data. Therefore, scientific observation of animal remains is the best method.

Scientific observations have established that the phenomenon known as adaptive radiation is a general feature of the evolution of most organisms. Studies of the morphological features of fossilized remains help determine relationships among prehistoric animals and enable scientists to trace adaptive radiations from a more primitive ancestral stock. To establish time intervals, scientists use techniques such as radioactive carbon dating, potassium-argon dating, and fluorine dating.

Zoologists have also made use of the uneven distribution of blood groups (A, B, AB, and O) among different groups of animals. As more blood subgroups were discovered, they became useful in helping chart migrations and indicating relationships between species.

The Evidence and Its Implications

Adaptive radiation, as an important aspect of evolution, means that modern organisms have attained their diversity in form and behavior through hereditary modifications after having been separated from ancestral populations. Adaptive radiation, therefore, is attributable to the genetic changes in isolated groups of organisms or, more specifically, to a change in the relative frequency of their genes from one generation to the next that eventually results in the formation of new species.

Evidence in several areas supports the concept of adaptive radiation as an important aspect of evolution: the fossil record (the most direct evidence), biogeographic distribution of organisms, comparative anatomy and embryology, homologous and analogous structures, vestigial organs, and comparative biochemistry. Regarding comparative biochemistry, scientists agree that blood group similarities confirm evolutionary relationships among nonhuman primates. It has been shown that the blood of higher primates, such as orangutans and chimpanzees, is closer to human blood than that of the more primitive monkeys.

Principal Terms

Allele: An alternative form of a gene that is located at the same position on a chromosome

Fossil: Any recognizable remains of an organism preserved in the earth’s crust; it may be a footprint, bones, or even feces

Gene: The biological unit of heredity, which is composed of DNA and is located on a chromosome

Genotype: The total genetic composition of an organism

Habitat: The place where an organism normally lives or where individuals of a population live

Natural Selection: The process of evolution whereby organisms that are the best adapted are the most successful in reproducing and, therefore, in passing along their genotypes to successive generations

Niche: The role of an organism in an ecological community—its unique way of life and its relationship to other biotic and abiotic factors

Phenotype: The visible expression of the genetic makeup of an individual

Species: A taxonomic subdivision of a genus containing populations of similar organisms that interbreed and that usually do not interbreed with other species

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