The fungi in mycorrhizal associations often show a lack of specificity
and are acquired by horizontal rather than vertical transmission. Mod
els of the development of symbiosis suggest that such a situation is m
ore likely to lead to parasitism than mutualism. The fact that in many
plants most seeds only disperse 1-2 canopy diameters means that seedl
ings are likely to come into contact with fungi genetically identical
to those associated with their parent plant. This creates conditions s
imilar to vertical transmission of the symbiont and makes mutualism mu
ch more likely to evolve.