identified for all traits of interest. The perfect variety might require recombining alleles
from hundreds of different germplasm sources. The second answer is that the number of
progeny required to generate a segregating population that contained the perfect combi-
nation of alleles would be prohibitive. So the breeder who set out to make a perfect
variety would be busy for many decades, while his/her colleagues were busy releasing
very good varieties.
Whether working in a cross-pollinating species or a self-pollinating species, the breeder
needs to alternate between crossing and selection. Selection is done between crossing gen-
erations in order to increase the probability of success, and to release interim varieties.
Crossing is done following selection, either to introduce new material, or to recombine
existing material. The pedigree of most modern varieties shows a history of crosses that
have been made (e.g., see Fig. 3.6). What may not be obvious in Figure 3.6 is that each
cross is followed by selection, such that the final outcome is not a random result of the
crosses that have been made. Whether it is done intentionally in a systematic process, or
ad hoc in an ongoing breeding program, this iterative process of crossing and selection is
calledrecurrent selection. Importantly, plant breeders use material from other breeding pro-
grams in their crosses. Legal and ethical principles allow most released plant varieties to be
used for crossing purposes in any breeding program. Furthermore, many breeders actively
exchange unreleased germplasm with each other, knowing that reciprocal exchange of
germplasm has the net result of increasing the scale of their own program. Therefore, it
is very rare to find a pedigree such as that shown in Figure 3.6 that does not contain material
from many different breeding programs, and often from different countries.
3.2.9 Diversity, Adaptation, and Ideotypes
Why does natural or artificial selection not favor a single genotype? Where does genetic
variation come from, and why does genetic diversity remain in the presence of intense
natural or artificial selection? It is quite clear that genetic diversity originates through
mutations in DNA sequence, but when, and how often, do these mutations occur? Why
has it been possible, for example, to continually select for higher oil in a population of
oats without ever exhausting the genetic diversity (see Fig. 3.7)? Is it because of new
Figure 3.5.Yield of hybrid corn varieties versus year of release. Data were obtained from Duvick
and Cassman (1999), based on field experiments conducted at a plant density of 79,000 plants per
hectare at three locations in central Iowa in 1994.
58 PLANT BREEDING