Figure 4.

Mean frequency of the B allele in worms during 20 years of simulation using either the full (A-C) or reduced (D) model. Under scenario 5, there is a 25% survival advantage (s_h) and 10% pasture mortality (s_p) for the B worm allele. The mechanisms by which sheep reduce faecal worm egg count (WEC) are (A) reduced worm establishment, (B) increased worm mortality, or (C) reduced worm fecundity; the frequency of the low WEC A allele in sheep is investigated at 11 levels, from low (light blue) to high (dark blue) frequencies and also when the allele is absent (black).