Agriculture Reference
In-Depth Information
If the environment in which a population lived was totally
optimal and never changed, genetic change would occur,
but there would be no natural selection to direct it. How-
ever, since environmental conditions are always changing
and never optimal for very long, natural selection is always
occurring at some level. In addition to long-term changes
in factors such as climate, natural selection is driven by
such environmental changes as population growth of other
species, the appearance of new species through migration,
the evolution of predators and herbivores, and changes in
microhabitats due to erosion, sedimentation, succession,
and other processes.
Natural selection acts on populations, not whole species.
If a population of a species becomes reproductively isolated
from the rest of the species — that is, if physical barriers
prevent its members from interbreeding with members of
other populations — that population can undergo genetic
change in a unique way. Because the environment is never
homogenous over space and time, the isolated population
will be subjected to somewhat different selective pressures
than other populations of the species. The tendency, there-
fore, is for different populations to evolve somewhat differ-
ently. Biogeographically, the species becomes a mosaic of
populations, each of which has unique genetically based
physiological and morphological characteristics. Each dis-
tinct population is referred to as an
ecotype
. Through evo-
lutionary time, an ecotype can become distinct enough from
other ecotypes of the species that it becomes a distinct
species in its own right.
The evolutionary processes that cause the development
of ecotypes and drive speciation are constantly diversifying
the genetic basis of earth's biota. Although species go
extinct, new species are always evolving, and the genomes
of many existing species are becoming more varied over
time. One of our great fears today, however, is that human
activity, including agriculture, is fundamentally altering this
process. Our destruction, alteration, and simplification of
natural habitats is greatly increasing rates of extinction and
eliminating ecotypes, thus eroding natural genetic diversity
and the potential for its renewal (Wilson, 1992).
Today's agricultural species — both plants and
animals — were domesticated by gradually shifting their
context from natural systems dominated by natural selec-
tion to human-controlled systems in which directed selec-
tion operated. Some 10,000 to 12,000 yr ago, humans did
not create strictly controlled agricultural environments
like farmers do today. In the case of plants, they cared for
certain naturally-occurring species by modifying their
habitats, facilitating their reproduction, controlling their
competitors, and occasionally moving them to more con-
venient places. In the case of animals, they followed herds
of herbivores more closely, began to protect them from
predators, and often provided them with feed. Natural
selection still had an important role in such systems,
because the human intervention was not sufficient to over-
come the fact that the useful species still had to survive
the rigors of the natural environment.
The process of
domestication
began as humans
became better able to alter and control the environment in
which useful plants and animals occurred, and to manage
the reproduction of these species to such an extent that
they began to unintentionally select specific useful traits.
As domestication progressed, selection became more
intentional, with early agriculturalists choosing seed from
the plants with higher and more predictable yields, and
early pastoralists choosing, for example, to breed the goats
that produced the most milk. Throughout the process of
domestication, the screening effect of the natural environ-
ment became less important and directed selection took
on a greater role. Eventually, crop and livestock species
reached a point where their genetic makeup had been
altered to such an extent that they could no longer survive
outside of an agroecosystem.
A domesticated species is dependent on human inter-
vention, and the human species is now dependent on
domesticated plants and animals. In ecological terms, this
interdependency can be considered an obligate mutualism.
It has come about through a process of mutual change:
human cultures have both caused changes in the genetic
makeup of certain useful species and been transformed
themselves as a result of those changes.
DIRECTED SELECTION AND
DOMESTICATION
T
RAITS
S
ELECTED
IN
C
ROP
P
LANTS
AND
L
IVESTOCK
A
NIMALS
Genetic change in an agricultural context differs greatly
from genetic change in naturally occurring populations.
Humans construct and manipulate the environments in
which agricultural species live, grow, and reproduce,
thereby creating an entirely different set of selective pres-
sures for them. Humans determine which traits are most
desirable, and select these traits in the way they cultivate
and propagate the species. Because humans “direct” genetic
change in agricultural populations, the process by which
this genetic change occurs is called
directed selection
.
Today's crop plants and stock animals have been subjected
to many selection pressures over thousands of years. In
plants, humans have selected optimized yield, appealing
taste and appearance, and ease of harvest, and more
recently, fast response to fertilizer and water application,
ease of processing, resistance to shipping damage, longer
shelf life, and genetic uniformity. In animals, we have
selected docility, more easily manageable reproductive
cycles, and rapid growth and maximal production of
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