Agriculture Reference
In-Depth Information
FIGURE 14.1
Diversity of beans for sale in an Oaxaca city market, Mexico
. Traditional varieties reflect local ecological and cultural
diversity.
•
modify local environments to the benefit of the
organism; or
exists both at the level of the
genotype
— the genetic
information carried by an individual — and at the level
of the
phenotype
— the physical and behavioral expres-
sion of the genotype.
An examination of a number of individuals of any
population quickly demonstrates the existence of pheno-
typic variability. Any characteristic, from number of
leaves on a plant to the length of the tail of an animal,
shows a range of variability. An average value or mode
for each characteristic occurs, and if variation in each trait
were graphed as a frequency distribution, it would tend to
follow a normal curve of probability (a bell-shaped curve).
Some populations show a very narrow range of variation,
while others show much more. Although phenotypic varia-
tion does not correlate directly with genotypic variation,
it usually has a significant genotypic basis (Figure 14.2).
The genetic variability within a species is due mainly
to the nature of DNA replication: DNA does not always
replicate itself perfectly; errors of different types, called
mutations, always occur at some frequency. Since DNA
replication is a prerequisite to reproduction, new individu-
als are constantly coming into existence with mutations.
Although some mutations are fatal, some detrimental,
•
facilitate reproduction
Any organism existing in nature must have a great
many adaptations in order for it to survive; in theory,
nearly all behaviors and physical characteristics of an
organism are adaptations. Another way of saying this is
that at any point in time a naturally existing organism as
a whole is always adapted
to its environment.
The adaptations possessed by a particular species,
however, do not remain the same over long periods of
time, because the environment is always changing and
organisms are continuously adapting. The process by
which adaptations change over time is also called adapta-
tion, and is understood in terms of natural selection.
V
ARIATION
AND
N
ATURAL
S
ELECTION
Individual members of sexually reproducing species are
not identical to each other. The variation that exists among
humans is mirrored in other species, even though we may
not always be able to discern it. This natural variability
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