  In both population genetics and forensic genetics it is important to know how
haplotypes are distributed in a population. Simulation of population dynamics
helps facilitating research on the distribution of haplotypes. In forensic
genetics, the haplotypes can for example consist of lineage markers such as
short tandem repeat loci on the Y chromosome (Y-STR). A dominating model for
describing population dynamics is the simple, yet powerful, Fisher-Wright
model. We describe an efficient algorithm for exact forward simulation of exact
Fisher-Wright populations (and not approximative such as the coalescent model).
The efficiency comes from convenient data structures by changing the
traditional view from individuals to haplotypes. The algorithm is implemented
in the open-source R package 'fwsim' and is able to simulate very large
populations. We focus on a haploid model and assume stochastic population size
with flexible growth specification, no selection, a neutral single step
mutation process, and self-reproducing individuals. These assumptions make the
algorithm ideal for studying lineage markers such as Y-STR.
