Glossary Plant Breeding

Selection Pressure. The intensity of selection usually measured by the change of gene
frequency per generation under the influence of selection. Selection pressure is
intense if the intensity of selection is the highest.
Selection Progress. See selection response.
Selection Response. The difference between the mean of the progeny of the selected
individuals and the mean of the original population. Quantitatively, it is expressed as:
Rs = Sd×h 2 = k×σ P ×h 2 . Numerically, it is equal to the genetic gain. It is also called
selection progress.
Selective Advantage. The relative advantage of a genotype when it competes for survival
and reproduction against another in the same population. In artificial selection, s.a. is
conferred by the breeder on some genotypes because they contain some desired
characteristics. For example, modern semi-dwarf varieties of wheat and rice are poor
competitors in mixture, but have been propagated by careful nurturing since they are
high yielders in isolation.
Selective Neutrality. A situation in which different alleles of a gene are adaptively
neutral, that is, each allele confers equal fitness to the corresponding genotypes. For
example, under situation of selective neutrality, both homozygotes (AA and aa) and
the heterozygote (Aa) will have equal fitness. This condition gives rise to a passive or
neutral equilibrium instead of a stable one such that an allele frequency p is as good
as any other q. Therefore, the presence of a widespread polymorphism is not
necessarily the evidence for superior heterozygotes.
Selective System. An experimental technique that enhances the recovery of specific but
usually rare genotypes.
Selective Value. The relative survival value and the reproductive ability of a genotype in
a population in any given environment; also called adaptive value, fitness, biological
fitness or Darwinian fitness.
Self Assembly. The ability of certain multimeric biological structures to assemble from
their component parts through random movements of the molecules and formation of
weak chemical bonds between surfaces with complementary shapes.
Self-fertile. Capable of setting seed following fertilization after self-pollination.
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