Nagel, Ronald: A hematologist and professor at Albert Einstein College of Medicine. His research includes molecular, biochemical, and physiological studies of genetic red blood cell defects, including sickle cell.
natural selection: The differential survival and reproduction of classes of organisms that differ from one another in one or more usually heritable characteristics. Through this process, the forms of organisms in a population that are best adapted to their local environment increase in frequency relative to less well-adapted forms over a number of generations. This difference in survival and reproduction is not due to chance.
Neanderthal: A hominid, similar to but distinct from modern humans, that livedin Europe and Western Asia about 150,000 to 30,000 years ago.
Nelson, Craig: A professor of biology and environmental affairs at Indiana University in Bloomington. His research focuses on evolutionary ecology.
neo-Darwinism: (1) Darwin's theory of natural selection plus Mendelian inheritance. (2) The larger body of evolutionary thought that was inspired by the unification of natural selection and Mendelism. A synonym of the modern synthesis.
nervous system: An organ system, composed of a network of cells called neurons, that allows an animal to monitor its internal and external environment, and to move voluntarily or in response to stimulation.
neural: Related to nerves and neurons.
neutral drift: Synonym of genetic drift.
neutral mutation: A mutation with the same fitness as the other allele or alleles at its locus.
neutral theory (and neutralism): The theory that much evolution at the molecular level occurs by genetic drift.
Newton, Isaac: An English physician and mathematician, considered the culminating figure of the scientific revolution of the 17th century. He is best known for his explanation of gravity and for laying the foundation for modern physical optics.
niche: The ecological role of a species; the set of resources it consumes and habitats it occupies.
Nilsson, Dan-Erik: Professor Nilsson heads the Functional Morphology Division of the Department of Zoology at Lund University in Sweden. His main research interest is the optics and evolution of invertebrate eyes.
Nilsson, Lennart: A Swedish photographer who began as a photojournalist, Nilsson soon began exploring new techniques such as the use of endoscopes and electron microscopes to photograph the inner mysteries of the human body. He published a book entitled A Child is Born of his photographs of the beginning of life, and made a number of films, including the mini-series Odyssey of Life, a coproduction between WGBH/NOVA and SVT Swedish Television.
nitrogen fixation: A chemical process by which nitrogen in the atmosphere is assimilated into organic compounds. Only certain bacteria are able to fix atmospheric nitrogen, which then becomes available to other organisms through the food chains.
nomadic: Having to do with nomads, people who live in no fixed place but move in search of food or grazing land for their animals; of a wandering lifestyle.
notochord: A flexible skeletal rod running the length of the body in the embryos of the chordates (including the vertebrates). In some simpler types, such assea-squirts, only the free-swimming larva has a notochord; in others, such as the lancelets and lampreys, the notochord remains the main axial support, and invertebrates it is incorporated into the backbone as the embryo develops.
Novacek, Michael J.: Paleontologist with the American Museum of Natural History. Dr. Novacek's research interests include evolution of and relationships among organisms, particularly mammals. He is the author of Dinosaurs of the Flaming Cliffs, an account of AMNH's Gobi Desert expeditions.
nucleotide: A unit building block of DNA and RNA. A nucleotide consists of a sugar and phosphate backbone with a base attached.
nucleus: A region of eukaryotic cells, enclosed within a membrane, containing the DNA.
numerical taxonomy: In general, any method of taxonomy using numerical measurements. In particular, it often refers to phenetic classification using large numbers of quantitatively measured characters.
Nurse, Paul: A pioneer in genetic and molecular studies who revealed the universal machinery for regulating cell division in all eukaryotic organisms, from yeasts to frogs to human beings.
O'Brien, Stephen J.: A geneticist at the National Cancer Institute whose research interests include the evolutionary history of the immunological response in mammals to retroviruses like HIV. With his colleagues, he discovered a mutation that can protect individuals from infection by HIV, the virus that causes AIDS.
order: The taxonomic classification level between class and family. For example, within the class Mammalia, there are several orders, including the meat-eaters, who make up the order Carnivora; and the insect-eaters, grouped together in the order Insectivora. The orders in turn are divided into families; the order Carnivora includes the families Felidae (the cats), Canidae (the dogs), andUrsidae (the bears), among others. See also taxon.
organelle: Any of a number of distinct small structures found in the cytoplasm(and therefore outside the nucleus) of eukaryotic cells (e.g., mitochondrion andchloroplast).
organisms: Living things.
orthogenesis: The erroneous idea that species tend to evolve in a fixed direction because of some inherent force driving them to do so.
Owen, Richard: A 19th-century British comparative anatomist, who coined the word "dinosaur" to describe a breed of large, extinct reptiles. He was the first to propose that dinosaurs were a separate taxonomic group. Owen opposed Darwin's theory of evolution, but ultimately his work helped support evolutionary arguments.
ozone layer: The region of the atmosphere, generally 11-26 km (7-16 miles) above Earth, where ozone forms in high concentrations. The ozone layer absorbs ultraviolet radiation, shielding Earth from its damaging effects.