Eventhough the wild type genes mutate at a very low rate,typically in the range of 10-4 to 10-6 new mutations per gene per generation, the genetic diversity for wild plants will be higher,What were the factors behind it?
See, all genomes are not expressed in any individual at a time, I think only up to 30 % may get expressed at a time and this differs from individuals to individuals, showing more diversity in the wild. In the wild there is no difference such as desirable or non desirable characters expressed in a population.
But cultivars and land races are often selections by man, for his needs and here case of desirable character vs. non desirable come into picture. In the process of selection always desirable character ( desirable to man) come into picture and non desirable characters are often neglected and they get slowly eroded generations after generations.
For example genes for resistance to diseases etc are not found in high yielding crops, both plants and animals and often a disease will wipe out the whole crop. (story of Grassy stunt disease in rice, during 1970 s is a good example.)
Here is the relevance of Conservation of Wild Crop Relatives, where such genes may become useful as that of Oryza nivara which saved rice from Grassy stunt disease.
Mutation may be a slow process, but expression of differing traits make the wild populations more diverse.
Wild plants, due to broader genetic base can grow in different agro-climatic conditions. But landraces/cultivars are often confined to a specific geographic location. Wild plants are the progenitors of landraces/cultivated plants. Due to natural selection, domestication events and artificial selection exercised by human (to suit their cultural practices and ethnic values) there is a huge genetic drift and bottleneck in landraces followed by cultivated plants. In nature, fixation of genetic combination is always in favorable direction, hence major portion of alternates are often eliminated. So naturally diversity is always higher in wild relatives. Though the rate of mutation is low, we are talking of millions of years of evolution which in fact has resulted in narrow genetic diversity in present day landraces.
Land races are selections of one are two wild types and further evolved to adapt to particular niche, so there shall be similarity between land races, whereas wild types remain distant as originally hence diversity is more in wild than land races which are evolved from one or two wild species.
The 'landraces' have been through 'limited' selection by farmers, and can leave out some genes from the gene pool. The 'cultivars' have been intensively (systematically) selected and bred before release for growing.
The wild plants are part of a population of diverse genetic composition and continuously interbreeding with equally diverse population while cultivars and land races are mostly isolated into specific environments They breed to similarly homogenous population.
Because wild plant populations have had millions of years to diversify while selective breeding has often been done from a fairly small genetic base over only a few thousand years.
Phenomic diversity is the adaptation to environment and diseases. The genomic variability can be related to it if reproductive type and generation time are sufficient to sustain heterozygosity. Wilderness does not grants it a priori.
With forest trees, which come within my expertise, various practices of convenience can operate to narrow the genetic base in the course of even limited domestication. Such practices need not even involve deliberate selection. Some species, especially poplars, are most conveniently propagated vegetatively, as cuttings, which can lead to clonal monocultures with all their vulnerability, whereas sexual propagation may figure prominently in the wild. In some cases, individual genotypes vary widely in their ease of vegetative propagation, which may lead to unconscious selection for that property and preferential propagation of particular clones. With eucalypts, land-race stocks are widely subject to genetic bottlenecks, since the felling of even a single tree may meet current seed requirement, and when this sort of thing has happened for more than one generation the narrowing of the genetic base and level of inbreeding can be extremely deleterious compared with seed collections made in the wild. With confers, however, the opposite can happen, with large seed collections bringing in genotypes from over quite wide ranges, and the mixing up in the nursery leading to a breakup of 'neighborhood inbreeding' structures and a consequent increase in both diversity and vigor.
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