Genetic Resources in Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 194

genetic basis of a crop or breed, the greater
the chance that some individuals will have innate
resistance to disease, unusual variations in envi-
ronmental conditions, or herbivore attack in the
case of crops, preventing total crop loss or herd
decimation if one of these events should occur.

• Genetic diversity is also a reservoir of potential

environmental resistance. A few individuals
in a genetically diverse crop variety or livestock
breed may have genes or gene combinations
that may confer resistance to future events or
conditions, such as the spread of a new disease.
These genes may be selected in a population to
provide it with resistance.

• Genetic diversity in crops ensures a reservoir of

traits with potential value for satisfying
(in developed countries) the growing consumer
demand for organically grown foods with higher
nutritive value. Varieties with innate disease
and pest resistance are much easier to grow in
more-sustainable low-input organic systems.

FIGURE 14.7 An endangered variety of corn from rural
Mexico. Many pressures have pushed farmers away from using their
local varieties, and many of those that are left are being contami-
nated by genetic material from genetically engineered varieties.

• Genetic diversity gives a system overall long-

term flexibility, the ability to adjust and adapt
to changes in conditions from season to season
and from decade to decade.

All higher organisms have very complex genetic struc-
tures. A great many genes — a single plant can contain
upward of 10 million — all work together in complex ways
to control the way the organism functions and interacts with
its environment. Some genes act alone, but most appear to
act in complex combinations with others. In nature, each
species' genetic totality, or genome, is the product of a very
long evolutionary process, as described above. The genome
as a whole is typically very diverse because it is made up
of many individual genotypes, many, or all of them unique.
Traditional methods of mass selection, though chang-
ing the content of a species' genome, tend to preserve
much of its genetically rich structure (Figure 14.7). Mod-
ern crop breeding, in contrast, tends to both alter and
narrow a crop variety's genome by focusing on the opti-
mization of one or a few genotypes of the variety.
Although this process creates plants that perform exceed-
ingly well in specific, highly altered modern agricultural
environments, it also greatly restricts a variety's genetic
basis. At the most uniform end of the scale, the genetic
diversity of a crop variety is restricted to a single genome —
that of the hybrid seed of that variety. At the most diverse
end of the scale, the genetic diversity of a mass-selected,
open-pollinated variety is the product of countless unique
individual genomes. Figure 14.8 illustrates this contrast in
the structure of genetic diversity.
Commercially produced, hybrid, high-yielding varieties
(HYVs), have captured the seed market and are now
planted over large areas in genetically uniform fields.
Their dominance is challenged only by equally uniform

Some farmers, geneticists, plant breeders, and others
saw, several decades ago, the dangers of losing genetic
diversity in our food crops. One response was the estab-
lishment of “gene banks,” where the seeds of varieties
and cultivars not in general use would be stored for
possible later use. These gene banks serve an important
purpose (Qualset and Shands, 2005), but are limited in
what they can do to stem genetic erosion. First, the vast
majority of current gene banks only maintain stocks of
crops that have national and international research pro-
grams supporting them, and even then, only a fraction
of the genetic diversity of protected crops has been col-
lected. Second, management and evaluation of genetic
resources within gene banks are often lacking, so that
deterioration of material occurs. Third, germplasm col-
lections are really static, with no incorporation of the
processes that maintain and create genetic diversity in
the first place, including both environmental and cultural
selection pressures. Unfortunately, we may never know
how many varieties have already been lost, especially
for the large number of minor crops that meet local needs
around the world, but are not part of current germplasm
preservation efforts.

G REATER G ENETIC U NIFORMITY IN C ROP V ARIETIES

The erosion of diversity at the level of the variety or breed
deserves a closer look. Therefore, we examine this subject
in greater detail as it applies to crop plants.