IX.

Human Impact

Extinction is not exclusively a natural phenomenon. For thousands of years, as the human species has grown in number and technological sophistication, we have demonstrated our power to cause extinction and to upset the world’s ecological balance. In North America alone, for example, about 40 species of birds and more than 35 species of mammals have become extinct in the last few hundred years—mostly as a result of human activity. Humans drive plants and animals to extinction by relentlessly hunting or harvesting them, by destroying and replacing their habitat with farms and other forms of development, by introducing foreign species that hunt or compete with local species, and by poisoning them with chemicals and other pollutants.

The rain forests of South America and other tropical regions offer a particularly troubling scenario. Upwards of 50 million acres of rain forest disappear every year as humans raze trees to make room for agriculture and livestock. Given that a single acre of rain forest may contain thousands of irreplaceable species of plant and animal life, the threat to biodiversity is severe. The conservation of wildlife is now an international concern—as evidenced by treaties and agreements enacted at the 1992 Earth Summit in Rio De Janeiro, Brazil. In the United States, federal laws protect endangered species. But the problem of dwindling biodiversity seems certain to worsen as the human population continues to expand, and no one knows for sure how it will affect evolution.

Advances in medical technology may also affect natural selection. The study from the mid-20th century showing that babies of medium birth weights were more likely to survive than their heavier or lighter counterparts would be difficult to reproduce today. Advances in neonatal medical technology have made it possible for small or premature babies to survive in much higher numbers.

Recent genetic analysis shows the human population contains harmful mutations in unprecedented levels. Researchers attribute this to genetic drift acting on small human populations throughout history. They also expect that improved medical technology may exacerbate the problem. Better medicine enables more people to survive to reproductive age, even if they carry mutations that in past generations would have caused their early death. The genetic repercussions of this are still unknown, but biologists speculate that many minor problems, such as poor eyesight, headaches, and stomach upsets may be attributable to our collection of harmful mutations.

Humans have also developed the potential to affect evolution at the most basic level—the genes. The techniques of genetic engineering have become commonplace. Scientists can extract genes from living things, alter them by combining them with another segment of DNA, and then place this recombinant DNA back inside the organism. Genetic engineering has produced pest-resistant crops as well as larger cows and other livestock. To an increasing extent, genetic engineers fight human disease, such as cancer and heart disease. The investigation of gene therapy, in which scientists substitute functioning copies of a given gene for a defective gene, is an active field of research. The way this tinkering with genetic material will affect evolution remains to be determined.