B chromosome
See Supernumerary chromosome.

balancing selection
Diversifying selection that maintains polymorphism resulting from such mechanisms as frequency-dependent selection, spatially heterogeneous selection, or heterozygous advantage.

Barr bodies
Inactivated X chromosomes in female mammals that condense to form a darkly colored structure in the nuclei of somatic cells.

Bayesian inference
A procedure of statistical inference in which observed data are interpreted not as frequencies or proportions, but rather are used to compute the probability that a hypothesis is true, given what was observed. Bayesian inference also allows for the incorporation of prior data or information. Bayes’ theorem is named after the Reverend Thomas Bayes.

binomial proportion
A population will be in binomial proportions when it conforms to the binomial distribution so that the occurrence of a given event X, ri times with a probability (pi) of success, in a population of n total events, is not significantly different than that which would be expected based on random chance alone.

biogeography
The study of the geographic distribution of species and the principles and factors influencing these distributions.

biological species concept (BSC)
Groups of naturally occurring interbreeding populations that are reproductively isolated from other such groups or species.

BLAST
Basic Local Alignment Search Tool. Software program to search a DNA sequence data base for sequence similar to the one in hand.

Bonferroni correction
A correction used when several statistical tests are being performed simultaneously (since while a given á-value may be appropriate for each individual comparison, it is not for the set of all comparisons). In order to avoid a lot of spurious positives, the á-value needs to be adjusted to account for the number of comparisons being performed. Suppose we are testing for Hardy–Weinberg proportions at 20 loci. Instead of using the traditional 0.05 á-level, we would test at á of 0.05/20 = 0.0025 level. This insures that the overall chance of making a Type I error is still less than 0.05.

bootstrap analysis
A nonparametric statistical analysis for computing confidence intervals for a phylogeny or a point estimate (e.g., of FST). Re-sampling with replacement to estimate the proportion of times an event (such as the positioning of a node on a phylogenetic tree) appears during multiple re-sampling of a data set.

bottleneck
A special case of strong genetic drift where a population experiences a loss of genetic variation by temporarily going through a marked reduction in effective population size. In demography, a severe transient reduction in population size.

branch length
Length of branches on a phylogenetic tree. Often proportional to the amount of genetic divergence between species or groups.

broad sense heritability (HB)
The proportion of phenotypic variation within a population that is due to genetic differences among individuals.

BSC
See biological species concept.