  When simulating biological populations under different evolutionary genetic
models, backward or forward strategies can be followed. Backward simulations,
also called coalescent-based simulations, are computationally very efficient.
However, this framework imposes several limitations that forward simulation
does not. In this work, a new simple and efficient model to perform forward
simulation of populations and/or genomes is proposed. The basic idea considers
an individual as the differences (mutations) between this individual and a
reference or consensus genotype. Thus, this individual is no longer represented
by its complete sequence or genotype. An example of the efficiency of the new
model with respect to a more classical forward one is demonstrated. This
example models the evolution of HIV resistance using the B_FR.HXB2 reference
sequence to study the emergence of known resistance mutants to Zidovudine and
Didanosine drugs
