Genetic Resources in Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 185

FIGURE 14.2 Squash fruit variability from a farmer's field in Tabasco, Mexico. Seed from one fruit were used to
plant the field.

some neutral, and only a few advantageous, all mutations
represent genetic difference and thus genetic variability.
Most mutations are simply single changes in the nucleo-
tide sequence of DNA molecules; by themselves they may
have no significant effect, but added together over time
they can result in fundamental changes, such as bigger
fruit, resistance to frost, or the addition of tendrils for
climbing.
Variability is also produced by sexual reproduction.
When two individuals reproduce sexually, the genes of
each are distributed differently into different gametes (sex
cells), and the genetic material carried in the gametes is
mixed in novel ways when the gametes combine during
fertilization. Variation is also introduced during meiosis
(the formation of gametes) when chromosomes are deleted
or translocated, or when homologous chromosomes fail
to separate at the first meiotic division.
This latter kind of “error” creates gametes that have
two copies of each chromosome (diploid) instead of the
usual one (haploid). If one of these diploid gametes fuses
with a normal haploid gamete, a zygote with three times
the haploid number of chromosomes can result, and when
one fuses with another unreduced diploid gamete, a zygote

with four times the haploid number can be formed. Such
increases in the number of chromosomes represent another
source of genetic variety, particularly important in plants.
Plants with more than the diploid number of chromo-
somes, called polyploid , typically have different charac-
teristics than their diploid forebears, and occur relatively
commonly in nature.
Because of natural genetic variation, some individuals
of a population will have traits not possessed by others,
or will express a certain trait to a greater or lesser degree
than others. These traits may give the individuals who
possess them certain advantages in living. These individ-
uals may grow more rapidly, survive in greater numbers,
or have some reproductive advantage. Due to such factors,
they may leave more offspring than other individuals, thus
increasing the representation of their genetic material in
the population as a whole. It is through such a process of
differential reproductive success that a species undergoes
genetic change over time.
The direction and manner of this change is determined
by natural selection — the process by which environmen-
tal conditions determine which traits confer an advantage
and therefore increase in frequency in the population.