V.- MODELO ESTÁTICO
5.8 RESULTADOS
Plant group North America Europe and Western Asia Eastern Asia
Fagaceae: Fagus Fagus grandiflora Fagus sylvatica Fagus crenata
Fagaceae: Castanea Castanea dentata Castanea sativa Castanea crenata
Fagaceae: Quercus
Section Lobatae Quercus rubra
Section Quercus Quercus alba Quercus ilex Quercus lanata
Section Cerris Quercus suber Quercus acutissima
Ulmaceae: Ulmus Ulmus americana Ulmus glabra Ulmus parvifolia
Betulaceae: Betula Betula nigra Betula pendula Betula utilis
Betulaceae: Alnus
Section Alnus Alnus serrulata Alnus cordata Alnus japonica
Section Clethropsis Alnus maritima Alnus nitida
Salicaceae: Salix Salix nigra Salix alba Salix tetrasperma
Salicaceae: Populus
Section Aegiros Populus deltoides Populus nigra
Section Populus Populus tremuloides Populus tremula Populus adenopoda
Aceraceae: Acer Acer saccharum Acer platanoides Acer palmatum
ern continents, and they remained largely separate from one another. By about a million years ago, the isthmus of Panama formed a land bridge over which placental mammals dis- persed from the Nearctic realm into South America, and mar- supial mammals in the opposite direction. Plants of temperate climates, not adapted to tropical conditions and less mobile than animals, did not disperse across this land bridge. As it turned out, placental mammals frequently drove marsupial mammals toward extinction by competition. The placental mammals of South America, now dominant, are the descen- dants of invaders from North America. Most of the marsu- pials that moved north into North America became extinct, except for the opossum. Meanwhile, Australia and New Guinea remained separate and the realm of marsupials. Few placental mammals made the journey into the Australasian realm. One exception was humans. Homoerectus ventured into the islands of Indonesia, becoming Java man, but there is no clear evidence that they traveled to Australia. Some of them became isolated on the island of Flores (see Flores Island people). Modern humans dispersed to Australia at least 50,000 years ago and apparently brought dogs with them. For thousands of years, humans and dogs were the main placental mammals of Australia. Australian marsupials included many species that were similar to placental species, but which had evolved separately (see convergence). Euro- peans brought placental mammals, such as rodents and live- stock, with them. Once again the placental mammals proved to be the superior competitors. Most of the Australian mar- supials, except various species of kangaroo, have become rare or extinct, largely as a result of competition with the placen- tal invaders.
When a continental fragment separates, it may some- times retain a collection of species that closely resemble their ancestors, because they are not exposed to many of the newly evolved continental species. This appears to be what happened with the island of New Caledonia. The forests consist largely of conifers more similar to those that domi- nated Mesozoic forests than those that dominate modern for- ests. This is why film crews who want a realistic backdrop to dinosaur movies choose New Caledonia for their film site. The species of flowering plant that most closely resembles the presumed ancestor of all flowering plants, Amborella trichop- oda, is found only on New Caledonia (see angiosperms, evolution of).
Some distribution patterns remain mysterious. One example is the plant family Empetraceae (crowberries), which is found in the cool regions of North America and Eurasia, but also the cool tip of South America, and in the warm cli- mate of Florida. This is difficult to explain in terms of conti- nental movements or dispersal to another habitat with similar climatic conditions. Islands Islands, starting with Darwin’s experience in the Galápagos Islands, have proven to be evolutionary showcases. When a new island forms, it receives only a genetic subset of organ- isms from the nearby mainland. In particular: • In any species some of whose members disperse to a new island, only a small portion of the mainland genetic vari- ability may be represented on the island. This founder effect may be followed by genetic drift if the island popu- lation remains small.
• The newly arrived species may have fewer parasites, and perhaps no predators or competitors. This may, if the spe- cies happens to be well suited to the physical environment of the island, allow a population explosion, leading to evo- lutionary change and diversification.
• Many small mammals remain small partly because their small size allows rapid population growth, giving them an advantage in competition; but on the island, with fewer or no competitors, the small mammals may evolve into larger forms. The same thing appears to happen in plants. Many small, weedy plants have an advantage due to rapid growth and early reproduction. In this way, they avoid competi- tion with large trees. When the plants disperse to an island and there are no trees, the weedy plants may evolve into a tree form. The plant family Asteraceae consists mostly of weedy herbaceous plants, except on some islands (such as the Channel Islands of California and St. Helena in the Atlantic) where they have evolved into small trees. The net- tle family Urticaceae consists mostly of weedy herbaceous plants, but on Hawaii there is a bush nettle. • Many large mammals remain large partly because their size allows them to resist the attacks of predators; but on the island, with no predators, the large mammals may evolve into smaller forms. Mammoths on Wrangel Island, north- east of Siberia, and on Santa Rosa Island off the coast of California evolved into smaller forms. Another example is the Flores Island people, Homo floresiensis. Some large mammals evolved into smaller forms on this island at the same time that the Flores Island folk lived there; the preda- tor release explanation, however, is unlikely to be true for the small size of these people. The explanation of the Flores Island people remains a mystery.
• Many island species lose their defenses. Since defenses (spines or chemicals in plants, behaviors in animals) can be expensive, natural selection favors their loss in condi- tions where they are not useful. Many island plants have lost their defenses. For example, not only is the Hawaiian nettle unusual in its family for being a bush but also for its lack of stinging hairs. Numerous island animal species (reptiles, birds, and mammals) are notorious for their lack of fear of humans, with whom they have had little contact until recently.
• In order to get to the island in the first place, plants and animals needed to have superior dispersal abilities. Once on the island many of them have lost their dispersal abili- ties. One extreme example is the flightless cormorant of the Galápagos Islands. On some islands, plant families with small seeds have produced species with seeds larger than is normal for the family.
Consider new islands, such as the volcanic islands of Hawaii, as opposed to islands like New Caledonia that are fragments of continents. The new islands accumulate all of
their species through dispersal and subsequent evolution. Larger islands have a greater variety of microhabitats. More- over, on larger islands, populations within a species are more likely to avoid contact, resulting in greater speciation. For both of these reasons, a greater number of species evolves on large islands than on smaller islands. This is known as the species-area relationship. The source of the colonist species is the closest mainland. Islands that are closer to the mainland receive more immigrant species, which allows the evolution of more species native to the island. The first analysis of the balances among immigration, evolution, and extinction on islands, in which near versus far and large versus small islands were contrasted, was the theory of island biogeography devel- oped by ecologist Robert H. MacArthur and evolutionary biologist E. O. Wilson in 1967 (see Wilson, Edward O.).
Ice Ages
Genera and even species of plants are shared between the arctic and alpine tundras, enough to allow these two regions both to be called tundra. Arctic tundra is found around the Arctic Ocean, while alpine tundra is on mountaintops. This biogeographical pattern is explained mostly by the ice ages. When glaciers were at their maximum extent, the arctic tun- dra formed a band across what is now the northern United States. The Rocky Mountains, Sierra Nevada, and Cascades received tundra species from contact with this band of tun- dra, and some mountains that were not in direct contact with the tundra (e.g., the San Francisco Mountains of Ari- zona) were close enough to receive tundra species by disper- sal. When the glaciers retreated northward, the tundra plants retreated into the remaining zones of cold climate, either northward, where they are found today in the arctic tundra, or up the mountains, where they are today found on alpine peaks. The alpine tundra is now stranded, sometimes in very small patches (as in the San Francisco Mountains). This is not the entire explanation for tundra plant species. The tundra of the Sierra Nevada contains not only species affiliated with the arctic, but also the evolutionary descendants of desert species (such as the buckwheat Eriogonum). The tundra of the Sierra Nevada not only has drier soil than other tundras but also is close to the desert. Adaptations to cold and to drought are often similar, and this similarity allowed some desert plants to adapt to tundra conditions in California. This pattern is also evident in some animals: The caribou of the cold cli- mates of North America is the same species as the reindeer of the cold climates of Eurasia.
Most of the North American continent south of the tun- dra was boreal forest (mainly spruce) and pine forests. Nearly the entire area that is now the Great Plains was covered with white spruce (Picea glauca). There was a little bit of grass- land in what is now Texas, but there was no hot desert. It would have been an alien world to modern eyes. There were many modern mammals, but also many mammals such as mammoths and mastodons that have become extinct (see Pleistocene extinction). Most of the trees and other plants would have been familiar to modern observers, but they were in combinations that no longer exist; for example, in some
places there were spruce trees scattered in grassland, an arrangement uncommon today.
As the ice sheets retreated, each surviving species moved to new locations independently, some dispersing more rap- idly than others. The glacial retreats, and the movements of species, were not uniform; glaciers sometimes temporar- ily advanced again. The forests are still moving, though too slowly for human observers to notice without consulting long-term records. The tallgrass prairie did not exist before or during the ice ages. The tall, deep-rooted, warm-weather grasses of the prairie were established during the period of maximum warmth (the hypsithermal period, about 7,000 years ago). The prairies persisted even when conditions became cooler and wetter (until American agriculture and civilization destroyed most of them). The prairies persisted because periodic fires killed any forest trees that began to encroach. In North America, where most mountain ranges are north-south, there were few barriers to the primarily northward movement of plant and animal species. In Europe, however, the Mediterranean Sea and mountain ranges such as the Alps were barriers to northward movement. This helps to explain why British forests have fewer species of trees and spring wildflowers than do many American forests. The explanation for current patterns of distribution and diversity are rooted in the past, rather than in modern climatic condi- tions. The greenhouse effect may be altering the patterns of species movement that have been occurring for the past few millennia.
Humans have had a tremendous effect on biogeo- graphical patterns. First, humans have carried species of plants and animals from one place to another. In some cases these plants and animals have become invasive spe- cies that have displaced native species and greatly altered the original environment. Humans have allowed species to disperse, especially by ship and airplane, more effectively than almost any species could previously have dispersed. Second, humans have created a great deal of disturbed habitat (such as farming, road building, and construction). Species that specialize in disturbed areas (especially weeds) have found a worldwide network of suitable habitats, cour- tesy of humankind. Some of the most widespread species, such as dandelions and barn swallows, live in areas altered by human activity.
Further Reading
Bonnicksen, Thomas M. America’s Ancient Forests: From the Ice Age to the Age of Discovery. New York: John Wiley and Sons, 2000. Gillespie, Rosemary G. “The ecology and evolution of Hawaiian spi-
der communities.” American Scientist 93 (2005): 122–131. Grant, Peter R. Evolution on Islands. Oxford: Oxford University
Press, 1998.
MacArthur, Robert H., and Edward O. Wilson. The Theory of Island Biogeography. Princeton, N.J.: Princeton University Press, 1967. MacDonald, Glen M. Biogeography: Space, Time, and Life. New
York: John Wiley and Sons, 2003.
Whitfield, John. “Biogeography: Is everything everywhere?” Science 310 (2005): 960–961.
bioinformatics
Bioinformatics is the computer-based analysis of the information content of biological molecules. This includes the amino acid sequences of proteins and the nucleotide sequences of nucleic acids such as RNA and DNA (see DNA [raw material of evolution]). Bioinformatics has many applications for the study of molecular biology and medicine. Its particular application to evolutionary science is that it allows the comparison of amino acid sequences in the corresponding proteins of different species, or the nucleotide sequences in the corresponding genes or noncoding DNA of different species. Determining the nucleotide sequence of a DNA molecule used to be a lengthy and expensive process. The process devel- oped by molecular biologist Frederick Sanger considerably shortened the work, which has now been entirely automated. The process uses nucleotides that cause DNA replication to stop, and that also have components that produce differ- ent colors of fluorescent light. When DNA fragments from this process are separated out according to size, they form a series of colored bands that look like a supermarket bar code. The sequence of red, green, blue, and yellow bands directly represents the sequence of the four nucleotides in the origi- nal DNA molecule. Thousands of DNA sequences have now been determined in laboratories all around the world. When- ever scientists publish papers that include reference to these sequences, the journal requires that the sequence be submit- ted to an electronic database that is accessible to scientists (or to anyone else) throughout the world. Two major databases are at the National Center for Biotechnology Information (NCBI), sponsored by the U.S. National Institutes of Health, and the Expert Protein Analysis System (ExPASy) in Switzer- land. NCBI has databased DNA from more than 165,000 organisms.Comparing nucleotide sequences also used to be a lengthy process, in which the nucleotides of each DNA mol- ecule were compared to the corresponding nucleotides of another, one by one. Automated programs now line up the DNA molecules, identify the nucleotides that differ among the molecules, and calculate the percent similarities among them. Perhaps the most commonly used system is the Basic Local Alignment Search Tool (BLAST), also available at the NCBI web site. The single-nucleotide differences among DNA molecules, known as single nucleotide polymorphisms (SNPs), are often used as the data in the construction of phy- logenetic trees (see cladistics).
The construction of phylogenetic trees is a mainstay of modern evolutionary science. Phylogenetic trees allow the reconstruction of the evolutionary history of everything from single strains of virus (see AIDS, evolution of) to families (see Linnaean system) to the entire living world (see tree of life). Modern phylogenetic studies would be unthinkable without the databases and analyses used in bioinformatics.
Further Reading
National Institutes of Health. “National Center for Biotechnology Information.” Available online. URL: http://www.ncbi.nih.gov. Accessed April 17, 2006.
Swiss Institute of Bioinformatics. “ExPASy Proteomics Server.” Avail- able online. URL: http://us.expasy.org. Accessed July 12, 2005.
biology
Biology is the scientific study of life. Other disci- plines study life, and life-forms, from other viewpoints, but biology employs the scientific method. Subdisciplines within biology are presented in the table.Biology has been transformed by the emergence of evolu- tionary science. Many of the characteristics of organisms make little sense in terms of operational design (see intelligent