CHAPTER of 5.7 billion . L. R. Brown(1994). In



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As we head into the next millennium, the world faces a
greater demand on agricultural output than at any time in history. Despite
efforts to curb birthrates, the Earth’s human population is expected to rise to
8.9 billion by the year 2030, corresponding to more than a 50% increase from
the current population of 5.7 billion . L. R. Brown(1994). In the past, we have
met the demand for increased agricultural productivity by a combination of
genetic improvements, greater farming inputs (fertilizers, pesticides, and
water), and cultivation of more land. With dwindling freshwater reserves and
petroleum resources (on which fertilizers and pesticides are based) and
increased problems caused by agricultural pollution, we can hardly expect to
increase or even maintain our current levels of agricultural inputs. Similarly,
much existing farmland is falling victim to urban expansion, and it is unlikely
that new farmland will become available in the near future. That leaves the
genetic improvement of crops as the most viable approach by which food
production can attempt to keep pace with the anticipated growth of the human
population. For the genetic approach to succeed, we must harness the wealth of
genetic variation provided by nature and currently warehoused in our seed
repository. Until now we have been only modestly successful in utilizing these
resources for plant improvement. New findings from genome research indicate
that there is tremendous genetic potential locked up in seed banks that can be
released only by shifting the paradigm from searching for phenotypes to
searching for superior genes with the aid of molecular linkage maps.


2.1 The
Narrow Genetic Base of Crop Plants

Today, modern agriculture and, for that matter, human
existence is dependent on the cultivation of a few highly productive crop
species. These food crops were first domesticated from wild species about
10,000 years ago during the transition from nomadic hunter-gatherers to life in
agrarian societies. Considering that flowering plants first evolved over 150
million years ago, crop plants as we know them have existed for the mere blink
of an evolutionary eye.

Although the exact series of steps by which plants were
domesticated is unknown, it is likely that strong selection pressure exerted by
humans on the genetic diversity found in the wild resulted in rapid and radical
changes in plant species N. I. Vavilov (1940). Certain traits, such as non shattering
of seeds, compact growth habit, or loss of germination inhibition, would have
been selected by early agriculturists J. R. Harlan, (1975). Selective
propagation of lines containing these favorable mutations would have resulted
in a progressive narrowing of the genetic base of subsequent populations (Fig.

Following domestication, the genetic variation in crop plants
has continued to be reduced by another force modern plant breeding. Over the
past century, the development and successful application of plant- breeding
methodologies has produced the high-yielding crop varieties on which modern
agriculture is based. Yet, ironically, it is the plant-breeding process itself
that threatens the genetic base on which breeding depends. Because new
varieties are usually derived from crosses among genetically related modern
varieties, genetically more