The mobility of members of a species, which may be modelled by diffusi
on, plays an important role in biology, though much remains to be disc
overed concerning its effects. It is therefore relevant to ask how dif
fusion itself may evolve, in particular in a spatially homogeneous env
ironment. The authors suggest that one mechanism driving the evolution
of a higher diffusion rate is connected with the effect of a deleteri
ous recessive gene which appears in the population. A reaction-diffusi
on model is proposed, and numerical and theoretical results are presen
ted which support the above hypothesis.