Two kinds of host propagation

When the host is propagated as a pure line, clone, or hybrid variety, it 'breeds true', without any significant variation. This can be referred to as non-diverging propagation, and it is essential in commercial agriculture if the desirable qualities of the cultivar are to be maintained. Non-diverging propagation means that the host population is genetically uniform, genetically inflexible, and that it cannot respond to selection pressures during the cultivation process. It can respond to selection pressures only during the breeding process.

Wild hosts, and many subsistence crop hosts, have diverging reproduction. These hosts do not 'breed true'. They are heterozygous and heterogeneous. They are also genetically flexible, and they respond to selection pressures. In wild host populations, diverging propagation produces different ecotypes. In subsistence crops, diverging propagation produces different local landraces, which can equally be called agro-ecotypes. And these changes result from selection pressures that operate during the cultivation process. This is what happened with the maizes in tropical Africa (see 7.2.6).

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