Glossary Plant Breeding

Speciation. The formation of a new species including the development of species
diversity. It refers to the origin of a group of individuals capable of living in a new
way and at the same time acquiring some barriers to genetic exchange with the
species from which it arose. Once speciation gets completed, each newly formed
species will take an independent evolutionary course; inevitably the species will
become increasingly different as time passes. It occurs when gene flow within the
common gene pool is interrupted by an isolating mechanism. When isolation occurs
through geographical separation of populations descended from a common ancestor,
allopatric (different fatherland) speciation may result that is more common in birds. If
isolation occurs through ecological or genetic means within the same area, sympatric
(joint fatherland) speciation is a possibility as occurs more commonly in parasites. In
plants and in a few group of animals, a different form of speciation is effected through
polyploidy (instantaneous speciation), the doubling of chromosomes, because
polyploid individuals are immediately isolated reproductively from their parents.
Species. A natural biological unit tied together by the sharing of a common gene pool. In
sexually reproducing organisms, a species is a group of potentially interbreeding
populations that are reproductively isolated from any other such groups. Species are
often subdivided into subspecies, races, ecotypes, varieties, or other subspecific
categories.
Specific Combining Ability. The deviation in performance of a cross from average of
all crosses possible among n number of parents. For an accurate measure of SCA, the
tests must be conducted in more seasons and more locations compared to GCA.
Specific Locus Test. A test for detecting recessive mutations in diploids. Normal
individuals treated with a mutagen are mated to testers that are homozygous for the
recessive alleles at a number of specific loci. If markers are related to the seed,
recessive phenotype (in case of mutation) can be visible in the mating-generation
itself [white endosperm colour (ccc) in Indian corn; cc × CC (mutagenised)].
Otherwise, screening is done for recessive phenotypes in the next generation.
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