UCMP Glossary: R
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radiation -- Event of rapid cladogenesis, believed to occur under conditions where a new feature permits a lineage to move into a new niche or new habitat, and is then called an adaptive radiation.
radicle -- The end of a plant embryo which gives rise to the first root.
radio-carbon dating -- Method for determining the age of an organic substance by measuring the amount of the carbon isotope, carbon-14, remaining in the substance; useful for determining ages in the range of 500 to 70,000 years.
radiospermic -- Having seeds which are round or ovoid. Contrast with platyspermic.
rain shadow -- n. The dry region on the leeward side of a mountain range, where rainfall is noticeably less than on the windward side. For example, the White Mountains in east central California are in the rain shadow of the Sierra Nevada.
rank -- In traditional taxonomy, taxa are ranked according to their level of inclusiveness. Thus a genus contains one or more species, a family includes one or more genera, and so on.
rbcL -- a gene which is located in the chloroplast of photosynthetic organisms. It codes for the large subunit of the protein rubisco, and its sequence has been useful in plant phylogenies.
rebound -- v. To spring back after a weight has been removed.
red bed -- n. Sedimentary layers composed primarily of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of iron oxides; often used in reference to the Permian or Triassic sediments of the western U.S.
reef -- n. A large ridge or mound-like structure within a body of water that is built by calcareous organisms such as corals, red algae, and bivalves; barrier reef- n. A reef growing offshore from a land mass and separated by a lagoon or estuary, e.g, the Great Barrier Reef of Australia; patch reef- n. A discontinuous reef growing in small areas, separated by bare areas of sand or debris, often part of a larger reef complex.
regression -- (n) A drop in sea level that causes an area of the land to be uncovered by seawater.
relatedness -- Two clades are more closely related when they share a more recent common ancestor between them than they do with any other clade.
repeat sequences -- The length of a nucleotide sequence that is repeated in a tandem cluster.
replacement -- Fossilization process in which the original material in an organism is broken down and replaced by different minerals.
reproduction -- The manufacture of offspring as part of an organism's life cycle. This is not the same as dispersal. Reproduction may be sexual, involving the fusion of gametes, or asexual.
reticulation -- Joining of separate lineages on a phylogenetic tree, generally through hybridization or through lateral gene transfer. Fairly common in certain land plant clades; reticulation is thought to be rare among metazoans.
reticulopodia -- Long thread-like pseudopodia that branch apart and rejoin, forming a fine network. They are characteristic of forams.
rhizoid -- n. A cellular outgrowth of a plant that usually aids in anchoring to the surface and increasing surface area to acquire water or nutrients; found in mosses, liverworts, and hornworts.
rhizome -- n. A horizontal underground stem, such as found in many ferns, where only the leaves may stick up into the air; sphenophytes (horsetails and their relatives) spread via rhizomes, but also produce erect stems.
rhyolite -- Highly felsic igneous volcanic rock, typically light in color; rough volcanic equivalent of granite.
rift -- n. A long, narrow crack in the entire thickness of the Earth's crust, which is bounded by normal faults on either side and forms as the crust is pulled apart; v. To split the Earth's crust; rift zone- n. The area on continents where a trough bounded by normal faults is forming; the site of crustal extension, similar to that which occurs at mid-oceanic ridges; rift basin or rift valley- n. The long, and fairly wide trough that has formed as a section of the Earth's crust has dropped down along faults, e.g., African Rift Valley in East Africa.
riparian -- Having to do with the edges of streams or rivers.
RNA -- "ribonucleic acid". The nucleic acid which carries the DNA message into parts of the cell where it is interpreted and used. The 18S ribosomal RNA sequence has been used in many groups of organisms to reconstruct phylogeny.
roche moutonee -- n. A rock formation created by a glacier. The front and sides of the rock are rounded where ice moved over it, and the back is angular, where freezing and thawing broke off pieces; the name comes from the French for "fleecy rock," also called a sheepback.
rock cycle -- The process through which one type of rock (igneous, sedimentary, or metamorphic) is converted into another.
Rodinia -- n. A supercontinent that existed during the Late Precambrian before the supercontinent Pannotia; the oldest supercontinent for which we have a good record; Russian for "homeland."
root -- Usually the below ground portion of a plant. Contrast with shoot.
rosette -- A series of whorls of leaves or leaf-like structure produced at the base of the stem, just above the ground.
rubisco -- protein which fixes carbon in photosynthetic organisms. It binds molecules of carbon dioxide to a five-carbon molcule. Rubisco is the most common protein on earth.
