Variation Within Species

Even if rescued from extinction at the last perilous minute, a species that has been reduced to a fraction of its original population loses much of what once made it unique. In all organisms that reproduce sexually, each individual plant or animal contains a different mix of genes. Known as genetic diversity, this incredible variation within species is what allows populations to adapt to changes in climate and other local environmental conditions. When a species--be it bison, goldfish, or daisies--loses too many individuals, it becomes genetically uniform and far less adaptable.

Problems of Genetic Uniformity

World agriculture will be hard hit of the genes lodged in forests and other wild ecosystems are lost. Time and again, wild tropical species have come to the rescue of their domesticated relatives. Genetic diversity boosts the total crop values on American farms by some $520 million a year, and, without a constant infusion of new hardy genes into our crop species, pests and diseases could quickly get out of hand. Already more than 400 species of crop pests have developed resistance to one or more of the pesticides used to control them.

Genetic diversity can also be critical in controlling disease. In 1970, for example, the United States lost 15 percent of its corn crop, work about $1 billion, when a fungus spread rapidly across the Midwest. In this case, genetic uniformity allowed a disease that is always present at low levels, even in the best of years, to run wild. Individual plants were virtually identical and thus genetically defenseless. Only introducing new corn varieties containing new genes stopped the fungus in its tracks.

In another instance in the United States, the South's sugarcane industry was saved from near collapse by a gene from a wild Asian species. Genes from a single species of wild rice found in India protects Asian crops from the four major rice diseases. And a barley plant from Ethiopia has provided genes that protect California's $160 million barley crop from the lethal yellow dwarf virus. Wild relatives of crop plants have also helped boost the yields and market appeal of our most important crops.

Understanding Genetic Diversity

The true boundlessness of the genetic diversity that plant breeders work with tasks the imagination.

"Each species is the repository of an immense amount of genetic information," writes E. O. Wilson in Biodiversity (1988. Washington, DC: National Academy Press). "The number of genes range from about 1,000 in bacteria and 10,000 in some fungi to 700,000 or more in many flowering plants and a few animals. A typical mammal such as the house mouse has about 100,000 genes. ...If stretched out fully, the DNA would be roughly one meter long. But this molecule is invisible to the naked eye. ...The full information contained therein, if translated into ordinary-size letter of printed text, would just about fill all 15 editions of the Encyclopedia Britannica published since 1768."

Tropical biologist Daniel Janzen of the University of Pennsylvania carries this bookish analogy a little bit further: carelessly destroying tropical forests and the species and genes they contain, he says, "is like pulping the Library of Congress to get newsprint."