140 • Impacts of Applied Genetics— Micro-Organisms, Plants, and Animals 
Photo credit: U.S. Department of Agriculture 
Bundles of wheat showing variance in height 
was recognized by the National Research Coun- 
cil’s Committee on Genetic Vulnerability of Ma- 
jor Crops:® 
If we bear in mind the fairly recent origin of 
modern potato varieties and that they are, for 
the most part, derived from the survivors of the 
late blight epidemics of the 1840’s in Europe and 
North America, it seems likely that the genetic 
•^National Academy of Sciences, Genetic Vulnerability of Major 
Crops, Washington, D.C., 1972. 
base was already somewhat narrow by the time 
modern potato breeding got under way. The 
five-fold increase in yield resulting from selec- 
tion during the last 100 years of potato improve- 
ment has produced a group of varieties that are 
genetically similar and unlikely to respond to 
further selection for yield. In the long run re- 
sponse to selection for other characteristics is 
also likely to be limited. 
As these examples indicate, the le\ el of genetic 
homogeneity of some crops may make selection 
for higher yields in general more difficult. 
Nevertheless, while the genetic basis for o\ erall 
crop improvement is poorly undei'stood, refine- 
ments in plant breeding techni(|ues may in- 
crease the potential for greater efficiency in the 
transfer of genetic information for more precise 
selection methods, and as a new source of ge- 
netic variation. 
Besides these two constraints, othei' |)i'cs- 
sLires and limitations may also affect crop pro- 
ductivity; some are biological (see Itudi. Not(’ B, 
p. 162.), requiring technological breakthroughs, 
while others are related to environmental, 
social, and political factors. (See Tech. Note 7, p. 
162.)— e.g., it has been argued that the agri- 
cultural rate of growth is declining: In 1976, tin? 
U.S. Department of Agriculture (USDA) esti- 
mated that the total-factor [yrothictivity of U.S. 
agriculture increased by 2 percent per yeai' 
from 1939 to I960, hut by only 0.9 pei’cenl fiom 
the period of 1960 to 1970.^ 
'U.S. Department of .Agricultun'. I•.('()n()mi(>, Statistics and ( «)■ 
operation Ser\ ic:es, Agricultural Product ivity: I'.t^pamllnfi, the l.imils. 
Agriculture Information Bulletin .4:11 . Washington. I)( I!I79 
Genetic technologies as breeding tools 
The new technologies may provide potential- 
ly useful tools, but they must be used in com- 
bination with classical plant breeding tech- 
niques to be effective. The technologies devel- 
oped for classical plant breeding and those of 
the new genetics are not mutually exclusive, 
they are both tools for effectively manipulating 
genetic information through methods that have 
been adapted from genetic recombination ob- 
served in nature. Plant breeders have many 
techniques for artificially controlling pollina- 
tion-some are capable of o\ (Mcoming natural 
harriers such as incompatibility. Net (>\en 
though one new technology— proto[)last lusion 
—allows breeders to o\(M’come incom[)atihilit\ . 
the new plant must still he selected, regener- 
ated from single-c(?ll culture, and evaluated 
under field conditions to ensure that the genetic 
change is stable and the atti ihutes ot the new 
variety meet ('ommereial re(|uirements I v.ilu.i 
lion is still the most expi'iisiv c and time-eonsum 
ing step. 
