Chapter 4: GENETIC RESOURCES IN AGRICULTURE
Centers of Origin and Diversity
The terms ‘center of origin’ and ‘center of diversity’ have
been used interchangeably. Though the two concepts are related and highly
intertwined, there is a distinction between the two. One, center of diversity,
is frequently used to identify the other.
Though the principle behind centers of origin and diversity
applies to all organisms, they are most often used in relation to plants,
particularly in plant breeding and studies of crop domestication.
Definitions
The center of diversity of a plant is defined as the
geographic area wherein the plant exhibits the highest degree of variation.
This variation manifests itself both at the population and genetic levels. That
is, the center of diversity is where the highest number of cultivated types and
wild relatives, as well as gene variants (alleles), exist. Based on the premise
that it is only over time that genetic variation can be accumulated, the center
of diversity often corresponds to the area where the plant has existed the
longest, which is by definition the center of origin.
The center of origin of a plant is that location where it is
considered to have first appeared. The primary criterion in identifying a
center of origin is the presence of wild relatives.
Though centers of origin and diversity are highly correlated,
they do occasionally diverge. This happens when there is a high variation in
cultivated crops, but no or few wild relatives. The variation occurs due to
environmental forces and human intervention that may have conspired to increase
a plant’s diversity away from its site of origin. A plant species may also have
more than one center of origin or diversity.
Germplasm
Germplasm is living tissue from which
new plants can be grown. It can be a seed or another plant part – a leaf, a
piece of stem, pollen or even just a few cells that can be turned into a whole
plant. Germplasm contains the information for a species’ genetic makeup, a
valuable natural resource of plant diversity.
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Genome:
the complete set of chromosomes carried by a cell.
Genotype:
the total of all genetic information contained in an individual organism.
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THREE LEVELS OF DIVERSITY
Genetic diversity is the variety present at the level of
genes. Genes, made of DNA (right), are the building blocks that determine how
an organism will develop and what its traits and abilities will be. This level
of diversity can differ by alleles (different variants of the same gene, such
as blue or brown eyes), by entire genes (which determine traits, such as the
ability to metabolize a particular substance), or by units larger than genes
such as chromosomal structure.
Genetic diversity can be measured at many different levels,
including population, species, community, and biome. Which level is used
depends upon what is being examined and why, but genetic diversity is important
at each of these levels.
Biodiversity studies typically focus on species. They do so
not because species diversity is more important than the other two types, but
because species diversity is easier to work with.
Species are relatively easy to identify by eye in the field, whereas genetic
diversity (above) requires laboratories, time and resources to identify and
ecosystem diversity (see below) needs many complex measurements to be taken
over a long period of time. Species are also easier to conceptualize and have
been the basis of much of the evolutionary and ecological research that
biodiversity draws on.
Species are well known and are distinct units of diversity.
Each species can be considered to have a particular "role" in the
ecosystem, so the addition or loss of single species may have consequences for
the system as a whole. Conservation efforts often begin with the recognition
that a species is endangered in some way, and a change in the number of species
in an ecosystem is a readily obtainable and easily comprehensible measure of
how healthy the ecosystem is.
Ecosystem
diversity
Ecosystem diversity is the variety of ecosystems in a given
place. An ecosystem is a community of organisms and their physical environment
interacting together. An ecosystem can cover a large area, such as a whole
forest, or a small area, such as a pond.
An ecosystem is a community of organisms and their physical
environment interacting together. An ecosystem may be as large as the Great
Barrier Reef or as small as the back of a spider crab's shell, which provides a
home for plants and other animals, such as sponges, algae and worms.
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