Genetic Resources in Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 188

human health, and the degradation of the soil resource. If
steps are taken to restrict the use of many of the practices
and materials that humans have developed to protect and
promote the growth of our crops and stock animals, yields
and production can suffer.
This problem is particularly troubling with regard
to pesticide use on crop plants. Plants' natural abilities to
withstand herbivory — through morphological adapta-
tions, mutualistic interactions, the production of obnox-
ious compounds, and other methods — have been largely
lost at the expense of the development of other traits.
Agroecosystems become dependent on pesticide use to
prevent loss of the crop through herbivory, but pesticide
use becomes a selective pressure on the herbivore popu-
lations, resulting in their evolution toward pesticide resis-
tance and requiring the application of more pesticide or
the continual development of new pesticide types.
A fundamental problem is that traits of environmental
resistance have been lost not just from the genetic makeup
of individual species and varieties, but from the structure
and organization of the entire agroecosystem (Chapter 16).
Attempts to reincorporate environmental resistance into
domesticated species' genomes, therefore, must work at
the agroecosystem level, not just at the level of individual
species, breeds, and varieties.

Selected seeds
are sown

Influence of the
local environment

Seeds selected from
most vigorous or
highest-yielding plants

Mature plants are
variable in vigor and
yield

Variable distribution of pollen
among plants (if plants are
cross pollinated)

FIGURE 14.4 The mass selection process. This method of
selecting desirable characteristics maintains adaptations to local
conditions and allows for maximum genetic variability.

selection , can produce a gradual shift in the relative fre-
quency of a trait or traits in the population (Figure 14.4).
Through mass selection methods, farmers all over the world
have developed varieties called landraces . Landraces are
adapted to local conditions, and although a landrace, as a
whole, is genetically distinct, its members are genetically
diverse (Figure 14.5).
Mass selection works similarly for both self-pollinated
and cross-pollinated plants. When cross-pollinated plants are
involved, mass selection allows open pollination to occur.
Also known as out-crossing, this natural mixing of pollen
among the members of a population results in high genotypic
variability. With self-pollinated plants, mass selection also
allows the maintenance of relatively high variability.
This older, more traditional method of directed
selection involves the whole organism and field-based
selection; despite being a relatively slow process and more
variable in its results, it has the advantage of being more
like natural selection in natural ecosystems. Traits involv-
ing adaptation to local conditions are retained along with
the more directly desirable aspects of yield or
performance, and genotypic variability is maintained as
well. Such characteristics are very important especially
for small farm systems with limited resources and more
variability in production conditions. All other methods of
directed selection tend to increase genetic uniformity,
and most greatly reduce or eliminate the role of local
environmental conditions in the selection process.

M ETHODS OF D IRECTED S ELECTION
IN P LANTS

Farmers and crop breeders change the genetic makeup of
crop species and varieties in a number of ways, ranging
from indirect means that resemble natural selection to high
technology means that work directly on the plant genome.
These latter methods are not selection per se, but they are
discussed here since they have the same results as directed
selection methods.
The methods that can be used on a particular species
depend on its manner of reproduction. Some plant species
(more annuals than perennials) reproduce primarily by
self-pollination — the female parts of a plant's flowers
are fertilized by pollen from the same plant, and often
from the same flower. Other plant species (more perenni-
als than annuals) reproduce mainly by cross-pollination .
Such plants typically have some kind of morphological,
chemical, or behavioral adaptation to assure that an indi-
vidual's female flower parts are fertilized only by pollen
from other plants.

Pure Line Selection

In self-pollinating plants, a common method of selection
is to choose several superior-appearing plants from a
variable population and then subject the progeny of each
to extensive testing over many generations. At the end of
the testing period, any line sufficiently distinct from and
superior to existing varieties is released as a new variety.
Because the plants are self-pollinating, the selected
genotype stays relatively stable over time.

Mass Selection

Until relatively recently, the only method of directed selec-
tion was to collect seed from those individuals in a pop-
ulation that showed one or more desirable traits, such as
high yielding ability or disease resistance, and to use that
seed for planting the next crop. This method, called mass