586 CHAPTER 22Agronomists should heed what evolutionary biologists have long known: that
genetic diversity is essential for a population’s long-term success. For example, an
epidemic of the southern corn leaf blight fungus in 1970 destroyed much of the
U.S. corn crop because so much of the corn being grown was a single genetic strain
that carried an allele that increased susceptibility (see Chapter 13) [114]. Experi-
ments have shown that genetically diverse plots of rice suffer much less disease
than do single-genotype plots [124]. The corporations that dominate much of
modern agriculture may profit from propagating a single strain across broad land-
scapes, but this approach courts disaster.
Many wild plants have characteristics that can improve crop species. Before
modern genetic technology, the genes underlying these traits were typically intro-
duced into the crop by hybridization. For example, at least 20 genes for resistance
to various diseases have been crossed into commercial tomato stocks from wild
species of tomatoes. Today, molecular methods of genetic engineering can move
a gene from any organism into a crop plant, once a useful gene is identified. For
example, genes for tolerating salt can be transferred from plant species that are
naturally adapted to saline conditions. Genes that confer resistance to insects or
other crop pests are already in use; widely planted strains of corn and cotton carry
genes derived from the soil bacterium Bacillus thuringiensis (Bt), which produces Bt
toxin that kills the larvae of Lepidoptera such as the corn earworm. Many other
genes in wild plants and microbes that confer resistance to pests will surely be
isolated and introduced into crops in the future.
Evolutionary biology aids this revolution in agronomics by contributing to gene
mapping methods, by identifying likely sources of genes for useful characteris-
tics, and by evaluating possible risks posed by transgenic organisms (often called
genetically modified organisms, or GMOs). Food crops obviously must be tested
for safety for humans, but there are also potential ecological risks; for example,
Bt toxin and other natural insecticides might kill nontarget species, and there is
concern that transgenes may spread from crop plants to wild species, which then
could become more vigorous weeds. Phylogenetic studies can identify wild spe-
cies that might hybridize with crop plants, and population genetic methods can
estimate the fitness effects of transgenes and the chances of gene flow into natural
plant populations [40]. “Darwinian agriculture” may provide other guidelines for
crop improvement as well [33, 34]. For example, crop yield may be improved by
understanding allocation trade-offs among growth, survival, and reproduction
(see Chapter 11), and by selecting characteristics that reduce the competitive ability
of individual plants but enhance the productivity of the group. Individual plant fit-
ness is often enhanced when plants grow taller than their neighboring plants and
shade them; artificial selection for reduced height in cereal crops may reallocate
energy from growth to seed production.
Insects, weeds, and other organisms cause billions of dollars’ worth of crop
losses. Much of this loss is caused when crop pests evolve resistance to chemical
insecticides and herbicides (see Chapter 3). This resistance not only increases the
costs of agriculture, but also results in a steady increase in the amount of toxic
chemicals sprayed on the landscape (some of which find their way up the food
chain, affecting humans and other consumers). In some places, regulations on the
use of pesticides follow recommendations made by evolutionary biologists about
how to manage pest populations in order to keep them susceptible to pesticides
[53, 54]. Biological control of pests also benefits from evolutionary analysis. When a
new pest species suddenly appears, phylogenetic systematics is the first approach
to identifying the pest and determining where in the world it has come from. That
is where entomologists will search for natural enemies, scrutinizing in particular
those that are related to known enemies of species that are related to the new22_EVOL4E_CH22.indd 586 3/22/17 1:49 PM