bacteria1 (see prokaryotes)

Bacteria2 (eubacteria) Prokaryotes that incorporate muramic acid into their cell walls. One of the three primary lines of descent in the living world (see urkingdom).

back mutation (backward mutation, reversal) A mutation that reverts a nucleotide site to a previous character state.

bacteriophage (abbreviated as phage) A virus that parasitizes bacteria.

balanced polymorphism A polymorphism that is stable over time and is maintained by balancing selection.

balancing selection A selection regime that results in the maintenance of two or more alleles at a locus in a population (see overdominance).

banding Areas of light and dark staining on chromosomes.

base (see nucleotide)

base pair (1) A pair of nucleotides on a double-stranded nucleic acid that hydrogen-bond with one another according to the pairing rules between a purine and a pyrimidine. (2) A unit of measurement of double-stranded DNA length.

biased codon usage (see unequal codon usage)

bifurcation (dichotomy) The graphical representation in a phylogenetic tree of an evolutionary speciation event whereby an ancestral taxon splits into two.

bootstrapping A statistical method based on repeated resampling with replacement from an original sample to provide a collection of new estimates of some parameter, from which confidence limits can be calculated.

bottleneck A severe reduction in population size.

box A short DNA sequence, adjacent to or residing within a gene, that performs a regulatory function (e.g., TATA box).

branch (link, segment, internode, edge) (1) In a rooted phylogenetic tree, the graphical representation of the evolutionary relationship between an ancestral and a descendant taxonomic unit. (2) In an unrooted phylogenetic tree, the graphical representation of an evolutionary relationship between two taxonomic units, either between an ancestral and a descendant taxonomic unit, or between two immediate descendants of the common ancestor of all the taxonomic units under study.