Saag, Michael: Dr. Saag is director of the AIDS Outpatient Clinic and associate professor of medicine at the University of Alabama, Birmingham. He is also associate director for clinical care and therapeutics at the UAB AIDS Center. Dr. Saag's research activities focus on both clinical and basic aspects of the human immunodeficiency virus. He serves on several state and national advisory panels, including the NIH/NIAID AIDS Clinical Trials Group Executive Committee.
sagittal crest: A ridge of bone projecting up from the top midline of the skull, running from front to back. It serves as a muscle attachment area for the muscles that extend up the side of the head from the jaw. The presence of asagittal crest indicates extremely strong jaw muscles.
Schneider, Chris: A biologist and professor at Boston University whose research focuses on the evolution of vertebrate diversity in tropical systems and the scientific basis for conservation of tropical diversity. He uses a variety of molecular genetic methods, such as DNA sequencing, to study speciation, systematics, and biogeography of terrestrial vertebrates, with an emphasis on reptiles and amphibians.
Schultz, Ted R.: An ant systematist at the Smithsonian Institution, Dr. Schultz studies the evolution of the symbiosis between fungus-growing ants and the fungi they cultivate.
science: A way of knowing about the natural world based on observations and experiments that can be confirmed or disproved by other scientists using accepted scientific techniques.
Scopes, John: The 24-year-old teacher in the public high school in Dayton, Tenn., who was the defendant in the "monkey trial" of 1925. He agreed to be thefocus of a test case attacking a newly passed Tennessee state law against teaching evolution or any other theory denying the biblical account of the creation of man, and was arrested and tried, with the American Civil Liberties Union backing his defense.
Scott, Eugenie C.: A human biologist specializing in medical anthropology and skeletal biology. As executive director of the National Center for Science Education, Scott is an advocate of church/state separation in schools, ands peaks widely about science, evolution, and natural selection.
Scott, Matthew P.: A professor and researcher whose work in developmental biology explores how homeotic genes orchestrate differentiation and multicellular organization.
sedimentary rocks: Rocks composed of sediments, usually with a layered appearance. The sediments are composed of particles that come mostly from the weathering of pre-existing rocks, but often include material of organic origin; they are then transported and deposited by wind, water, or glacial ice. Sedimentary rocks are deposited mainly under water, usually in approximately horizontal layers (beds). Clastic sedimentary rocks are formed from the erosion and deposition of pre-existing rocks and are classified according to the size of the particles. Organically formed sedimentary rocks are derived from the remains of plants and animals (for example, limestone and coal). Chemically formed sedimentary rocks result from natural chemical processes and include sedimentary iron ores. Many sedimentary rocks contain fossils.
selection: Synonym of natural selection.
selectionism: The theory that some class of evolutionary events, such as molecular or phenotypic changes, have mainly been caused by natural selection.
selective pressures: Environmental forces such as scarcity of food or extreme temperatures that result in the survival of only certain organisms with characteristics that provide resistance.
Senut, Brigitte: An anatomist at France's National Museum of Natural History. In2000, Senut and Martin Pickford discovered Orrorin tugensis, a proto-hominid dated at 6 million years old.
separate creation: The theory that species have separate origins and neverc hange after their origin. Most versions of the theory of separate creation are religiously inspired and suggest that the origin of species occurs by supernatural action.
sexually dimorphic: When males and females of a species have considerably different appearances, which may include size, coloration, or other features, such as special plumage.
sexual selection: A selection on mating behavior, either through competition among members of one sex (usually males) for access to members of the other sex or through choice by members of one sex (usually females) of certain members of the other sex. In sexual selection, individuals are favored by their fitness relative to other members of the same sex, whereas natural selection works on the fitness of a genotype relative to the whole population.
sex chromosome: The chromosome or chromosomes that influence sex determination. In mammals, including humans, the X and Y chromosomes are the sex chromosomes (females are XX, males XY). Compare with autosome.
Shubin, Neil: A paleontologist who is known for his work on early tetrapods (any creature with four limbs). He presented a hypothesis of general patterns of the development of tetrapod limbs which changed the way scientists think about this field. The study of limbs is crucial to evolutionary science; one example of why this is important is that human development would have been impossible without limbs.
sickle cell anemia: A disease in which poorly formed red blood cells cannot bind correctly to oxygen, resulting in low iron, blood clotting, and joint pain.
Simpson, George Gaylord: One of the most influential paleontologists of the 20thcentury and a leading developer of the modern synthesis. He wrote hundreds of technical papers in addition to many widely read popular books and textbooks, and was a leading expert on Mesozoic, Paleocene, and South American mammals.S mall, Meredith: A professor of anthropology. Her research interests include primate behavior and ecology; mating strategies; reproduction; and the evolution of human behavior. Small's publications include Female Choices: Sexual Behaviour of Female Primates, What's Love Got to Do With It?, and The Evolution of Human Mating.
Smith, John Maynard: An eminent evolutionary biologist and author of many books on evolution, both for scientists and the general public. A professor emeritus at the University of Sussex, his research interests include evolution of human mitochondrial DNA sequences and investigation of evidence for extensive recombination.
Smith, Tom: An ornithologist and conservation biologist, Smith is executive director of the Center for Tropical Research at San Francisco State University. His work combines basic research in ecology and evolutionary science with applied research in conservation biology. Among other issues, Smith is interested in the role of ecological gradients in speciation and maintaining species diversity.
social Darwinism: A doctrine that applies the principles of selection to the structure of society, asserting that social structure is determined by how well people are suited to living conditions.
spacer region: A sequence of nucleotides in the DNA between coding genes.
speciation: Changes in related organisms to the point where they are different enough to be considered separate species. This occurs when populations of one species are separated and adapt to their new environment or conditions(physiological, geographic, or behavioral).
species: An important classificatory category, which can be variously defined by the biological species concept, cladistic species concept, ecological species concept, phenetic species concept, and recognition species concept. The biological species concept, according to which a species is a set of interbreeding organisms, is the most widely used definition, at least by biologists who study vertebrates. A particular species is referred to by a Linnaean binomial, such as Homo sapiens for human beings.
sponge: A member of the phylum Porifera, marine and freshwater invertebrates that live permanently attached to rocks or other surfaces. The body of a sponge is hollow and consists basically of an aggregation of cells between which there is little nervous coordination, although they do have specialized sets of cells that perform different functions. One set of cells causes water to flow in through openings in the body wall and out through openings at the top; food particles are filtered from the water by these cells. Other cells construct a stiffening skeletal framework of spicules of chalk, silica, or fibrous protein to support the body.
stabilizing selection: A form of selection that tends to keep the form of a population constant. Individuals with the mean value for a character have high fitness; those with extreme values have low fitness.
stepped cline: A cline with a sudden change in gene or character frequency.
stromatoporoid: Stromatoporoids, once thought to be related to the corals, are now recognized as being calcareous sponges. Sponges similar to fossil stromatoporoids are found in the oceans today. Like modern sponges, stromatoporoid created currents to pump water in and out of their body, where they filtered out tiny food particles. Fossil stromatoporoids can be massive, chocolate-drop in shape, tabular, encrusting, cylindrical, or even arm-shaped("ramose"). There are two main groups of fossil stromatoporoids that lived in different eras, the Paleozoic and the Mesozoic. After their appearance in the Ordovician, the Paleozoic stromatoporoids were dominant reef builders for over100 million years. The second group of stromatoporoids, from the Mesozoic, may represent a distinct group with a similar growth form. They were also important contributors to reef formation, especially during the Cretaceous.
subduction zone: A zone where rocks of an oceanic plate are forced to plunge below much thicker continental crust, along margins between adjoining plates. As the plate descends it melts and is released into the magma below Earth's crust. Such a zone is marked by volcanoes and earthquakes. See plate tectonics.
substitution: The evolutionary replacement of one allele by another in apopulation.
supernatural: Relating to phenomena that cannot be described by natural laws, cannot be tested by scientific methodology, and are therefore outside the realm of science.
symbiosis: A relationship of mutual benefit between two organisms that live together.
sympatric speciation: Speciation via populations with overlapping geographic ranges.
sympatry: Living in the same geographic region. Compare with allopatry.
syntax: The rules by which words are combined to form grammatical sentences.
systematics: A near synonym of taxonomy.