R

R plasmid: A bacterial plasmid carrying genes that confer resistance to certain antibiotics.
Radial cleavage: A type of embryonic development in deuterostomes in which the planes of cell division that transform the zygote into a ball of cells are either parallel or perpendicular to the polar axis, thereby aligning tiers of cells one above the other.
Radial glia: In an embryo, supporting cells that form tracks along which newly formed neurons migrate from the neural tube; can also act as stem cells that give rise to neurons and other glia.
Radial symmetry: Characterizing a body shaped like a pie or barrel, with many equal parts radiating outward like the spokes in a wheel; present in cnidarians and echinoderms; also can refer to flower structure.
Radiation: The emission of electromagnetic waves by all objects warmer than absolute zero.
Radicle: An embryonic root of a plant.
Radioactive isotope: An isotope, an atomic form of a chemical element, that is unstable; the nuclei decays spontaneously, giving off detectable particles and energy.
Radiolarian: Member of a phylum of radially symmetric eukaryotes in the Rhizaria kingdom. They have a strong fossil record.
Radiometric dating: A method of dating samples based on analysis of radioactive isotopes and the products of their decay.
Radula: A straplike rasping organ used by many molluscs during feeding
Random (genetic) drift: The random change in genotype frequency caused by random variation in individual reproduction. Looking backward in time, this process causes coalescence of lineages.
Random segregation: During meiosis, the two chromosomes of a pair are distributed randomly to the gametes, each gamete having an equal chance of receiving either chromosome.
ras gene: A gene that codes for Ras protein, a G protein that relays a growth signal from a growth factor receptor on the plasma membrane to a cascade of protein kinases that ultimately results in the stimulation of the cell cycle.
Ratite: Member of the group of flightless birds.
Ray initials: Cells within the vascular cambrium that produce xylem and phloem rays, radial files that consist mostly of parenchyma cells.
Ray-finned fish: Member of the class Actinopterygii, aquatic osteichthyans with fins supported by long, flexible rays, including tuna, bass, and herring.
Reactant: A starting material in a chemical reaction.
Reaction center: Complex of proteins associated with two special Chlorophyll a molecules and a primary electron acceptor. Located centrally in a photosystem, this complex triggers the light reactions of photosynthesis. Excited by light energy, one of the chlorophylls donates an electron to the primary electron acceptor, which passes an electron to an electron transport chain.
Reaction norm: The set of phenotypes expressed by a single genotype across of a range of environments.
Reading frame: The way a cell’s mRNA-translating machinery groups the mRNA nucleotides into codons.
Receptacle: The base of a flower, the part of the stem that is the site of attachment of the floral organs.
Reception: In cellular communication, the target cell’s detection ( by binding to a receptor protein) of a signal molecule from outside the cell.
Receptor- mediated endocytosis: The movement of specific molecules into a cell by the inward budding of membranous vesicles containing proteins with receptor sites specific to the molecules being taking in; enables the cell to acquire bulk quantities of specific substances.
Receptor potential: An initial response of a receptor cell to a stimulus, consisting of a change of voltage across the receptor membrane proportional to the stimulus strength. The intensity of the receptor potential determines the frequency of action potentials traveling to the nervous system.
Receptor tyrosine kinase: A receptor protein in the plasma membrane that responds to the binding of a signal molecule by catalyzing the transfer of phosphate groups from ATP to tyrosine on the cytoplasmic side of the receptor. The phosphorylated tyrosines activate other signal transduction proteins within the cell.
Recessive allele: An allele is recessive with respect to a certain phenotype if it produces that phenotype when present in two copies, that is, as a homozygote.
Reciprocal altruism: Altruistic behavior between unrelated individuals, whereby the current altruistic individual benefits in the future when the current beneficiary reciprocates.
Reciprocal cross: If a cross is made between A males and B females, then the reciprocal cross is between B males and A females.
Reciprocal translocation: A translocation mutation in which parts of two different chromosomes are exchanged.
Recombinant: An offspring whose phenotype differs from that of the parents, also called recombinant type.
Recombinant chromosome: A chromosome created when crossing over combines the DNA from two parents into a single chromosome.
Recombination: The generation of a new combination of genes.
Recombination hot spot:  A localized region with exceptionally high recombination rate.
Recombination load: The loss of mean fitness caused by recombination.
Recombination rate: The proportion of recombinant gametes.
Recruitment: The process of progressively increasing the tension of a muscle by activating more and more of the motor neurons controlling the muscle.
Rectum: The terminal portion of the large intestine where the feces are stored until they are eliminated.
Red algae: A photosynthetic marine protist that contains the accessory pigment phycoerythrin, most are multicellular.
Red blood cell: A blood cell containing hemoglobin, which transport oxygen, also called an erythrocyte.
Red Queen: Continual evolution between two species (e.g. between host and parasites).
Redox reaction: A chemical reaction involving the transfer of one or more electrons from one reactant to another, also call oxidation-reduction reaction.
Reducing agent: The electron donor in a redox reaction.
Reduction: The addition of electrons to a substance involved in a redox reaction.
Reduction principle: If selection is the only process acting, then the recombination rate will tend to decrease.
Reductionism: Reducing complex systems to simpler components that are more manageable to study.
Reflex: An automatic reaction to a stimulus mediated by the spinal cord or lower brain.
Refractory period: The short time immediately after an action potential in which the neuron cannot respond to another stimulus, owing to an increase in potassium permeability.
Regeneration: The regrowth of body parts from pieces of an organism.
Regulator: A regulator uses mechanisms of homeostasis to moderate internal changes in the face of external fluctuations.
Regulatory gene: A gene that codes for a protein, such a repressor, that controls the transcription of another gene or a group of genes.
Reinforcement: The strengthening of prezygotic isolation through selection against cross-matings that produce unfit hybrid offspring.
Relative abundance: Differences in the abundance of different species within a community.
Relative fitness: The contribution of one genotype to the next generation compared to that of alternative genotype for the same locus.
Renal artery: The blood vessel bringing blood to the kidney.
Renal cortex:  The outer portion of the vertebrate kidney
Renal medulla: The inner portion of the vertebrate kidney, beneath the renal cortex.
Renal pelvis: Funnel-shaped chamber that receives processed filtrate from the vertebrate kidney’s collecting ducts and is drained to the ureter.
Renal vein: The blood vessel draining the kidney.
Renin-angiotensin-aldosterone system (RAAS): A part of a complex feedback circuit that helps regulate blood pressure and blood volume.
Repeated reproduction: A life history in which adults produce large numbers of offspring over many years; also know as interoparity.
Repetitive DNA: Nucleotide sequences, usually noncoding, that are present in many copies in a eukaryotic genome. The repeated units may be short and arranged tandemly (in series) or long and dispersed in the genome.
Replicase: An enzyme that copies any form of genome (i.e. in the origin of life, genome may not have been DNA or RNA based).
Replication fork: A Y-shaped region on a replicating DNA molecule where new strands are growing.
Replicative DNA transposon: A DNA-based transposable element that moves itself to a new place in the genome and also leaves a copy in the original location.
Replicator: Any entity that can replicate. It usually refers to a DNA-based genome but can include prions.
Repressor: A protein that suppresses the transcription of a gene.
Reproductive assurance: The assurance that an individual can fertilize its eggs or ovules by selfing.
Reproductive isolation: The separation of distinct gene pools, as a result of genetic differences that prevents successful interbreeding.
Reptile: Member of the clade of amniotes that includes tuatara, lizards, snakes, turtles,  crocodilians, and birds. This classification is now call Class Sauropsida.
Residual volume: The amount of air that remains in the lungs after forceful exhaling.
Resilience: The power or ability to return to a previous form or position after a disturbance. To recover readily.
Resource partitioning: The division of environmental resources by coexisting species such that the niche of each specie differs by one or more significant factors from the niches of all coexisting species.
Respiratory medium: The source of oxygen; air for terrestrial animals and water for aquatic organisms.
Respiratory pigment: A protein that transport most of the oxygen in blood.
Respiratory surface: The part of an animal where gases are exchanged with the environment.
Response: In cellular communication, the change in a specific cellular activity brought about by a transduced signal from outside the cell.
Resting potential:  The membrane potential characteristic of a non-conducting, excitable cell, with the inside of the cell more negative than the outside.
Restriction enzyme: An enzyme that cuts DNA at specific sites, typically four to six bases long. The natural function of restriction enzymes is to destroy foreign DNA that enters the cell.
Restriction fragment length polymorphism (RFLP): A method of detecting DNA sequence variation that detects variation in the length of fragments cut by restriction enzymes.
Restriction site: A specific sequence on a DNA strand that is recognized as a “cut site” by a restriction enzyme.
Reticular fiber: A very thin and branched fiber made of collagen. Reticular branches form a tightly woven fabric that is continuous with the collagenous fibers of the extracellular matrix.
Reticular formation: A system of neurons, containing over 90 separate nuclei, that passes through the core of the brainstem.
Retina: The innermost layer of the vertebrate eye, containing photoreceptor cells (rods and cones) and neurons. transmit imagens formed by the lens to the brain via the optic nerve.
Retinal: The light absorbing pigment in rods and cones of the vertebrate eye.
Retroposon: A DNA transposable element that replicates through an RNA intermediate and does not have long terminal repeats.
Retrotransposon: A DNA transposable element that replicates through an RNA intermediate and have long terminal repeats on both sides.
Retrovirus: A virus that have an RNA genome and replicates it through a DNA intermediate. DNA can be inserted into the genome of host species.
Reverse genetics: Term used to describe any of a variety of molecular methods that allow a wild-type allele of a gene to be targeted and replaced by an engineered mutant allele.
Reverse transcription: Some viruses produce enzymes that reverse the transcription process by copying RNA back into a complimentary DNA sequence. This process produces a complementary DNA copy of an RNA molecule and used by retroviruses and retrotransposons.
Rh factor: A protein antigen on the surface of the red blood cell designated as Rh-positive. If an Rh-negative mother is exposed to blood from an Rh-positive fetus, she produces anti-Rh antibodies of the IgG class.
Rhizaria: Another name for the kingdom of eukaryotes known as Cercozoa.
Rhizoid: Long tubular single cell or filament of cells that anchors bryophytes to the ground. They are not compose of tissues, lack conducting cells, and do not play a role in water or mineral absorption.
Rhodopsin: A visual pigment consisting of retinal and opsin. When rhodopsin absorbs light, the retinal changes shape and dissociates from the opsin, after which it is converted back to its original form.
Rhyniophyte: Member of an early group of vascular plants.
Ribonucleic acid (RNA): Type of nucleic acid consisting of nucleotide monomers with a ribose sugar and the nitrogen bases adenine (A), cytosine (C), guanine (G), and uracil (U); usually single stranded; functions in protein synthesis and as the genome of some viruses.
Ribose: The sugar component of RNA.
Ribosomal DNA (rDNA): The DNA sequence that codes for the ribosomal RNAs, which form the core of the ribosome.
Ribosomal RNA (rRNA): The highly conserve RNA molecules that are found within the ribosomes. They are widely used for estimating phylogenies.
Ribosome: The protein-RNA complex responsible for translating the genetic code into proteins in the cytoplasm. Ribosomes are constructed in the nucleolus
Ribozyme: An RNA with catalytic activity. catalyzes reactions during splicing.
Ring species: A chain of interbreeding populations whose ends overlap without interbreeding.
River: A flowing body of water.
RNA-mediated interference (RNAi): Mechanism of RNA-based regulation of gene function that results from the inhibition of gene expression through the formation of double stranded RNA.
RNA polymerase: An enzyme that links together the growing chain of ribonucleotides during transcription.
RNA processing: Modification of RNA before it leaves the nucleus, a process unique to eukaryotes.
RNA splicing: The removal of noncoding portion (introns) of the RNA molecule after initial synthesis.
RNA world: The stage before the evolution of the genetic code when RNA was responsible for both heredity and catalysis.
Robust: Used to describe early human fossil — specifically the large rear teeth of certain Australopithecines. It contrasts with gracile.
Rod cell: One of two kinds of photoreceptors in the vertebrate retina; sensitive to white and black and enables night vision.
Root: The most ancient branch in a phylogenetic tree. Also an organ in vascular plants that anchors the plant and enables it to absorb water and nutrients from the soil.
Root cap: A cone of cells at the tip of a plant root that protects the apical meristem.
Root hair: A tiny extension of a root epidermal cell, growing just behind the root tip and increasing surface area for absorption of water and minerals.
Root pressure: The upward push of xylem sap in the vascular tissue of roots.
Root system: All of a plant’s roots that anchor it to the soil, absorb and transport  minerals and water and store food.
Rough ER: That portion of the endoplasmic reticulum studded wit ribosomes.
Round window: The point of contact between the staples and the cochlea. It is where the vibrations of the staples create a traveling series of pressure waves in the fluid of the cochlea.
r-selection: the concept that in certain populations, a high reproductive rate is the chief determinant of life history.
Rubisco: Ribulose carboxylase, the enzyme that catalyzes the first step in the Calvin cycle i.e. the addition of CO2 to rubilose bisphosphate (RuBP).
Ruminant: An animal, such a cow or a sheep, with an elaborate, multicompartmentalized stomach specialized for an herbivorous diet.
Runaway process: Arises from the positive feedback between the evolution of a male trait and the female preference for that trait.