Principles of Plant Genetics and Breeding

18 CHAPTER 2
discovered. As will be discussed in detail later in the
book, scientists collect, process, and store this natural
variation in germplasm banks for use by breeders in their
breeding programs.
The process of evolution has parallels in plant breeding.
Darwin’s theory of evolution through natural selection
can be summed up in three principles that are at the
core of plant breeding. These are the principles of:
1 Variation. Variation in morphology, physiology,
and behavior exist among individuals in a natural
population.
2 Heredity. Offspring resemble their parents more
than they resemble unrelated individuals.
3 Selection. Some individuals in a group are more
capable of surviving and reproducing than others
(i.e., more fit).
A key factor in evolution is time. The changes in
evolution occur over extremely long periods of time.
Plant breeders depend on biological variation as
a source of desired alleles. Induced mutation and
hybridization for recombination are major sources of
variation. Once variation has been assembled, the
breeder imposes a selection pressure (artificial selection
in this case) to discriminate among the variation to
advance only desired plants. Plant breeding may be
described as directed or targeted and accelerated evolution,
because the plant breeder, with a breeding objective
in mind, deliberately and genetically manipulates
plants (wild or domesticated) to achieve a stated goal,
but in a very short time. Conceptually, breeding and
evolution are the same, a key difference being the
duration of the processes. Plant breeding has been
described as evolution directed by humans. Compared
to evolution, a plant breeding process is completed in a
twinkle of an eye! Also, unlike evolution, plant breeders
do not deal with closed populations. They introgress
new variability from different genotypes of interest, and,
for practical and economic purposes, deal with limited
population sizes.
Domestication
Domestication is the process by which genetic changes
(or shifts) in wild plants are brought about through
a selection process imposed by humans. It is an evolutionary
process in which selection (both natural and
artificial) operates to change plants genetically, morphologically,
and physiologically. The results of domestica-
tion are plants that are adapted to supervised cultural
conditions, and possessing characteristics that are preferred
by producers and consumers. In some ways, a
domesticated plant may be likened to a tamed wild
animal that has become a pet.
There are degrees of domestication. Species that
become completely domesticated often are unable to
survive when reintroduced into the wild. This is so
because the selection process that drives domestication
strips plants of natural adaptive features and mechanisms
that are critical for survival in the wild, but undesirable
according to the needs of humans.
Like evolution, domestication is also a process of
genetic change in which a population of plants can experience
a shift in its genetic structure in the direction of
selection imposed by the domesticator. New plant types
are continually selected for as domesticates as new
demands are imposed, thereby gradually moving the
selected individuals farther away (genetically, morphologically,
and physiologically) from their wild progenitors.
Both wild and domesticated populations are
subject to evolution.
Patterns of plant domestication
Domestication has been conducted for over 10,000
years, and ever since agriculture was invented. Archeological
and historical records provide some indications
as to the period certain crops may have been domesticated,
even though such data are not precise. Archeological
records from arid regions are better preserved
than those from the humid regions of the world.
Concepts of domestication
As G. Ladizinsky points out in discussing patterns of
domestication, the challenge is to determine whether
the domesticate evolved under wild conditions, or was
discovered and then cultivated by humans, or whether
cultivation preceded the selection of domesticates. This
is a subject of debate. For example, seed dormancy is a
problem in wild legumes, and hence would have hindered
their use in cultivation. It is likely that the domesticates
evolved in the wild before being used in cultivation.
However, in most cereal species, most experts believe
that domestication occurred after cultivation. In wheat
and barley, for example, a tough rachis, which is resistant
to natural seed dispersal, and characterizes domesticates,
would have been selected for during cultivation.
Two categories of crop plants are identified, with
respect to domestication, as primary crops or secondary