Consequences of domestication

Just as the diversity of species we depend on is a small fraction of the species available to us, so is the genetic diversity with those species a small fraction of the genetic diversity actually present in them. The species we depend on have become more and more genetically uniform. Well over half of the soybean crop in the United States is genetically engineered, and it's likely that the diversity of soybean germplasm in Iowa is less now than it was even five years ago.⁹ Tanksley and McCouch [4] have a figure that illustrates this nicely (Figure 1):

Figure 1: Loss of genetic diversity during crop domestication (from [4])

I mentioned some extreme examples at the beginning of this lecture. Let's consider the more typical case of barley. As with many other crop plants, the genetic resources can be divided into five categories:

1. ``Advanced'' varieties in current commercial use and bred varieties no longer in commercial use.

2. Genetic stocks, i.e., lines that carry particular mutations, cytogenetic rearrangements, or linkage markers.

3. Bulk populations or composite crosses developed from crosses from a wide variety of cultivars.

4. ``folk'' varieties of land races associated with traditional, pre-scientific agriculture

5. wild progenitors or relatives of potential use in crop breeding (or as new crops)

As illustrated in Table 1, a single, wild population of barley in Israel harbors more electrophoretic diversity than an advanced generation of a composite cross. A relatively small number of populations from a small part of the wild range contains far more electrophoretic diversity than a broadly based composite cross. Notice that about half of the diversity present is due to differences among populations. There is, of course, an important few caveat: patterns of variation at electrophoretic loci need not match those of economically or agronomically important traits. And barley is not unique. Rice and tomatoes show much the same pattern (Figure 2).

Table 1: Genetic diversity in barley as based on allozyme loci.

	Within samples	Total collection
32 Swedish cultivars	0.005	0.095
C.C.-21, $F_17$		0.100
C.C.-31, $F_4$		0.140
12 Iranian land races	0.081	0.170
28 Israel wild populations	0.118	0.230

Figure 2: Relative amounts of genetic diversity in cultivated rice and tomato and wild relatives (``exotics'') with which they are cross-compatibile (from [4]).