quantitative character: A character showing continuous variation in a population.
radioactivity: The emission of energy due to changes in the nucleus of an atom. Such spontaneously released radiation is a characteristic of certain elements and at some levels can be harmful.
radiometric dating: A dating technique that uses the decay rate of radioactive isotopes to estimate the age of an object.
Rak, Yoel: An Israeli paleoanthropologist and anatomist whose research interests include facial morphology of fossil hominids. Rak was part of the team that found a 2.3-million-year-old skull fragment from the genus Homo at Hadar, Ethiopia.
random drift: Synonym of genetic drift.
random mating: A mating pattern in which the probability of mating with another individual of a particular genotype (or phenotype) equals the frequency of that genotype (or phenotype) in the population.
recanted: Withdrew a statement or opinion; disavowed a former assertion.
recapitulation: A partly or wholly erroneous hypothesis stating that an individual, during its development, passes through a series of stages corresponding to its successive evolutionary ancestors. According to the recapitulation hypothesis, an individual thus develops by "climbing up its family tree.
"receptors: Proteins that can bind to other specific molecules. Usually on the surface of a cell, receptors often bind to antibodies or hormones.
recessive: An allele (A) is recessive if the phenotype of the heterozygote (Aa)is the same as the homozygote (aa) for the alternative allele (a) and different from the homozygote for the recessive (AA). The allele (a) controls the heterozygote's phenotype and is called dominant. An allele may be partly, rather than fully, recessive; in that case, the heterozygous phenotype is nearer to, rather than identical with, the homozygote for the dominant allele.
recognition species concept: A concept of species according to which a species is a set of organisms that recognize one another as potential mates; they have a shared mate recognition system. Compare with biological species concept, cladistic species concept, ecological species concept, and phenetic species concept.
recombination: An event, occurring by the crossing-over of chromosomes during meiosis, in which DNA is exchanged between a pair of chromosomes of a pair. Thus, two genes that were previously unlinked, being on different chromosomes, can become linked because of recombination, and linked genes may become unlinked.
reinforcement: An increase in reproductive isolation between incipient species by natural selection. Natural selection can directly favor only an increase in prezygotic isolation; reinforcement therefore amounts to selection for assortative mating between the incipiently speciating forms.
relative dating: The process of ordering fossils, rocks, and geologic events from oldest to youngest. Because of the way sedimentary rocks form, lower layers in most series are older than higher layers, making it possible to determine which fossils found in those layers are oldest and which are youngest. By itself, relative dating cannot assign any absolute age to rocks or fossils.
reproductive character displacement: The increased reproductive isolation between two closely related species when they live in the same geographic region (sympatry) as compared with when they live in separate geographic regions. A kind of character displacement in which the character concerned influences reproductive isolation, not ecological competition.
reproductive isolation: Two populations or individuals of opposite sex are considered reproductively isolated from one another if they cannot together produce fertile offspring. See prezygotic isolation and postzygotic isolation.retina: The back wall of the eye onto which images are projected. From the retina, the information is sent to the brain via the optic nerve .
ribosomal RNA (rRNA): The kind of RNA that constitutes the ribosomes and provides the site for translation.
ribosome: The site of protein synthesis (or translation) in the cell, mainly consisting of ribosomal RNA.
ring species: A situation in which two reproductively isolated populations (see reproductive isolation) living in the same region are connected by a geographic ring of populations that can interbreed.
RNA: Ribonucleic acid. Messenger RNA, ribosomal RNA, and transfer RNA are its three main forms. These act as the intermediaries by which the hereditary code of DNA is converted into proteins. In some viruses, RNA is itself the hereditary molecule.