R

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.
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.
Reaction norm: The set of phenotypes expressed by a single genotype across of a range of environments.
Recessive: 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 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.
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.
Red Queen: Continual evolution between two species (e.g. between host and parasites).
Reduction principle: If selection is the only process acting, then the recombination rate will tend to decrease.
Reinforcement: The strengthening of prezygotic isolation through selection against cross-matings that produce unfit hybrid offspring.
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).
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.
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.
Resilience: The power or ability to return to a previous form or position after a disturbance. To recover readily.
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.
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.
Rhizaria: Another name for the kingdom of eukaryotes known as Cercozoa.
Rhyniophyte: Member of an early group of vascular plants.
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.
Ribozyme: An RNA with catalytic activity.
Ring species: A chain of interbreeding populations whose ends overlap without interbreeding.
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 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.
Root: The most ancient branch in a phylogenetic tree.
Runaway process: Arises from the positive feedback between the evolution of a male trait and the female preference for that trait.