Adaptive Radiation

A Adaptive Radiation FIELDS OF STUDY phenotype: the visible expression of the genetic makeup of an individual Biology; Genetics; Environmental Sciences; Ecology; species: a taxonomic subdivision of a genus, con- Botany taining populations of similar organisms that interbreed and that usually do not interbreed with ABSTRACT other species Many different forms of evolutionary adaptations may occur among animals that started with a common ancestor. ADAPTATION In this way, evolutionary divergences can take place, and the In 1898, Henry F. Osborn developed the concept of occupation of a variety of ecological niches is made possible. adaptive radiation. According to Osborn, many dif- The ability to adapt is not shared by all species. Therefore, ferent forms of evolutionary adaptations may occur in many instances, either evolutionary divergence has been among animals that started with a common ancestor. modest or the species involved has become extinct. In this way, evolutionary divergences can take place, and the occupation of a variety of ecological niches is KEY CONCEPTS made possible according to the adaptive nature of the invading species. As may be seen with certain forms allele: an alternative form of a gene that is located at of animal life, however, the ability to adapt is not the same position on a chromosome shared by all species. Therefore, in many instances, evolution: the process of change through time by either evolutionary divergence has been modest or which the characteristics of a species of plant or the species involved has become extinct. animal are altered by adaptation to the environment and produce a new species distinct from the THE PRINCIPLES OF NATURAL SELECTION parent species In order to understand how adaptive radiation oper- fossil: any recognizable remains of an organism pre- ates, it is necessary to become familiar with the prin- served in the earth’s crust; it may be a footprint, ciples of natural selection. The concept of natural bones, or even feces selection, frequently expressed as “survival of the gene: the biological unit of heredity, which is com- fittest,” is at the core of Charles Darwin’s theory of posed of DNA and is located on a chromosome evolution. Darwin did not mean to suggest that there genotype: the total genetic composition of an was a physical struggle among organisms in order to organism survive. Instead, he meant that organisms compete habitat: the place where an organism normally lives for food, space, shelter, water, and other things neces- or where individuals of a population live sary for existence. Rather, he meant only that those natural selection: the process of evolution whereby organisms best adapted for a particular habitat will organisms that are the best adapted are the most have the greatest ability to survive in a particular successful in reproducing and therefore in passing environment. According to the concept of natural along their genotypes to successive generations selection, all organisms of a given species will show niche: the role of an organism in an ecological variation in color, size, physiology, and many other community— its unique way of life and its relation- characteristics; in nature, all organisms produce ship to other biotic and abiotic factors more offspring than can survive, so the offspring must 1 Adaptive Radiation Principles of Zoology therefore compete for the limited environmental resources. Organisms that are the best adapted (most fit) to compete are most likely to live to reproduce and pass their successful traits on to their offspring. The others, which are less fit, will be most likely to 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, typically unable to interbreed. 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 them- selves may also become forces of selection through Darwin's finches or Galapagos finches. Darwin, 1845. Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. their interrelationships with other species. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. 2d edition. The process of adaptive radiation illustrates how natural selection operates. The most frequently is the woodpecker finch. A true woodpecker has cited example is the evolution of Darwin’s finches an extremely long tongue that it uses to probe for on the Galápagos Islands, off the west coast of insects. Since the woodpecker finch does not have a South America. The islands were formed from vol- long tongue, it has learned to use a cactus spine for canic lava about one million years ago. At first they insect probing, and it can therefore occupy a niche were devoid of life, but bit by bit, several species of normally filled by true woodpeckers. plants and animals arrived at them from the South A more recent example of adaptive radiation in American mainland. Since the nearest island is its early stages has taken place in an original popula- about 950 kilometers from the coast of Ecuador, it tion of brown bears. The brown bear can be found is anybody’s guess how the different species arrived. throughout the Northern Hemisphere, ranging from It has been suggested that the birds may have been the deciduous forests up into the tundra. During one carried to the islands by strong winds, since finches of the glacier periods, a small population of the brown are not known for their lengthy flights. Other organ- bear was separated from the main group; according isms may have been carried by floating debris. In any to fossil evidence, this small population, under selec- event, the islands became populated. The mainland tion pressure from the Arctic environment, evolved ancestor of the finches is not known, but it was no into the polar bear. Although brown bears are classi- doubt a nonspecialized finch (a finch is about the fied as carnivores, their diets are mostly vegetarian, size of a sparrow). Since there were no other birds with occasional fish and small animals eaten as with which to compete on the islands, the original supplements. On the other hand, the polar bear is population of finches began to adapt to the various unoccupied niches. The early offshoots of the orig- mostly carnivorous. Besides its white coat, the polar inal population were modified again and again as bear is different from the brown bear in many ways, adaptations continued. This process resulted in the including its streamlined head and shoulders and evolution of fourteen species of finches. The main the stiff bristles that cover the soles of its feet, which feature that makes each species different is the size provide traction and insulation, enabling it to walk of their beaks, which have adapted for the various on ice. types of available foods. Today the finches live on fif- teen different islands. Some of the species are found EVOLUTION in the same area (sympatric), while others occur in All the genes of any population of living organisms at different areas (allopatric). The most noteworthy any given time make up its gene pool, and the ratio of example of an adaptation to a particular niche alternative characteristics (alleles) in the gene pool 2 Principles of Zoology Adaptive Radiation can change because of selection pressures during the designed to test the concept of adaptive radiation, passage of time. As the ratio of alleles changes, evo- the scientist could not be around thousands or mil- lution occurs. Evolution may be a random change, lions of years from now to gather the data. Therefore, or it may occur because of the directive influences scientific observation of animal remains is the of natural selection. In the former case, occasional best method. and unpredictable permanent random changes Based upon scientific observations, it has been called mutations take place in the DNA molecules well established that the phenomenon known as that compose the genes. These mutations also may be adaptive radiation is a general feature of the evolu- selected for by the environment or selected against by tion of most organisms. Studies of the morphological the environment. It is simply an accident if the newly features of fossilized remains help determine rela- mutated genes help the organism to become better tionships among prehistoric animals and enable the adapted to its particular habitat niche. Genes may not scientist to trace adaptive radiations from a more change or become mutated through several genera- primitive ancestral stock. In order to establish time tions (the Hardy- Weinberg law), but may change in intervals, techniques such as radioactive carbon terms of survival value if the environment changes or dating, potassium- argon dating, and fluorine dating the species population is subjected to new mutations have been used. or natural selection. Zoologists have also made use of the uneven The relative numbers of one form of allele distribution of blood groups (A, B, AB, and O) decrease in a divergent population, while the relative among different groups of animals. As more blood numbers of a different gene increase. This progres- subgroups were discovered, they became useful in sive change is all- important in the evolutionary

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