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in biology, the process of organisms dispersing into a new environment.
radioactive decay
the decay of an unstable parent isotope through one or many steps to a stable daughter isotope, releasing radioactivity as it decays.
the emission of radiation during radioactive decay.
radiocarbon dating
an absolute dating technique that measures the amount of 14Carbon present in a sample. See also Carbon-14 dating.
radiometric dating
an absolute dating method that measure the amount of radioactive decay in a sample.
recessive allele
an allele that is not expressed in the phenotype of an individual because it is masked by its dominant alternative.
the random distribution of parental chromosomes resulting in new gene combinations in the offspring.
the process by which raw materials are turned into lithic tools; this may involve hard or soft percussion (i.e., striking) and/or pressure flaking.
relative date
a date assigned to an artifact or site based on the temporal relationship of surrounding materials. Relative dates can be established by determining the stratigraphic position of the item or identifying its evolutionary history; contrasts with absolute dating.
the process by which a copy of DNA is made.
reproductive advantage
a feature that enables an organism of a breeding population to produce more offspring than other organisms in the same breeding population that lack that particular feature.
reproductive isolation
the inability of two populations of a species to successfully interbreed. The most common reason for reproductive isolation is geographic separation. For example, two groups of one species may be separated from one another by a river.
reproductive success
the contribution of genes to the next generation.
the process by which a worn stone tool, such as a biface, is reflaked to provide a new, sharp cutting surface.
the destruction of bone by osteoclasts.
the process by which a stone tool or flake is reflaked into a new shape or tool type.
ribonucleic acid
an important molecule found within the chromosomes that does not carry genetic information but assists in the duplication of DNA; a nucleic acid made up of a series of four bases (adenine, guanine, thymine and uracil), a ribose sugar and a phosphate group.
a place where the Earth's crust and lithosphere are being pulled apart as a result of plate tectonics.
rift valley
a long narrow valley surrounded by faults on either side. In the East African Rift System, numerous primate and hominid fossils have been found.
describes a region bordering a river.
river bed
ground area surrounding the path of a river.
the abbreviation for ribonucleic acid.

a rugged build; usually characterized as being large bodied and/or having heavily-structured anatomy; the opposite of gracile.

[Russian: "Homeland"] a late Proterozoic super-continent that approximately dates from 1 billion years ago to 700 million years ago; during this time, life consisted mainly of single celled-organisms and the oceans were at approximately 95 percent of their present volume.