The Dominant Animal

Human Evolution and the Environment

Protected: Chapter 1. Darwin’s Legacy and Mendel’s Mechanism

Chapter Summary

In a world in flux, organisms are constantly changing genetically, altering their hereditary characteristics and adapting to new conditions. The principal mechanism of adaptation is natural selection—the differential reproduction of genetic types—which was originally proposed simultaneously in 1859 by two British scientists, Charles Darwin and Alfred Russel Wallace. The basic notion is disarmingly simple. Influenced by the ideas of Thomas Malthus, Darwin and Wallace noted that organisms produced many more offspring than could survive, and they concluded that those members of a population that were best able to survive and reproduce—”favourable variations”—were selected by nature to pass on their characteristics to the next generation. With ever more rapid environmental change due to human activities, we can now see signs of evolutionary change itself speeding up as plants, animals, and microbes respond to powerful, shifting selection pressures.

A key part of the evolutionary story was filled in by the Austrian monk Gregor Mendel in 1865. His work was not discovered until the early twentieth century, but he showed that biological inheritance was fundamentally particulate and not blending. He showed in basic outline how the particles—what we now know as “genes”—are transmitted, and he made it clear that biological evolution resulted from changes in the genetic makeup of populations through time.

Evolution by natural selection was an idea that changed a world in which most people believed that the diversity of living beings had been created simultaneously. Darwin and the armies of biologists who followed him have amassed gigantic amounts of evidence of natural selection at work. Some of the most persuasive evidence has come from studies of island organisms, most famously small finches that invaded and have diversified in the Galápagos islands. First studied by Darwin himself, Galápagos (Darwin’s) finches have proven a superb subject of study in which it has been shown that their bill sizes evolve in response to drought-induced changes in food supply.

Evolution by natural selection is easily observed on continents too. The most famous case is the evolution in British peppered moths of melanism (darkening) as protective coloration. When soot from nearby factories began to coat the trees on which the moths rested, killing the lichens that normally grew there, the moths that were darkest were well camouflaged and were the most likely to survive and pass on their genes for dark coloration to the next generation. The evolution of industrial melanism was especially instructive—the frequency of melanic moths increased with industrialization, but the frequency of the normal black-and-white (peppered) individuals resurged when the pollution was cleaned up and the lichens returned.

Much of what has been learned about evolution has been not in nature but on farms and in laboratories where human beings have deliberately applied selection pressures by breeding what they saw as the most desirable organisms in a population. This process has supplied people for millennia with more productive and less poisonous crops, fatter pigs, faster horses, and cows that give enormous amounts of milk. In laboratories, artificial selection experiments with fruit flies, insects with very rapid life cycles, have taught scientists a great deal, for example, about how rapidly selection can produce flies resistant to DDT.

Human beings, of course, are also subject to natural selection. One of the best-studied cases involves sickle-cell anemia—a disease that afflicts mostly people in certain parts of Africa where the most virulent form of malaria (caused by the parasite Plasmodium falciparum) occurs. Individuals with one allele (a form of a gene) that codes for a special kind of hemoglobin are relatively resistant to P. falciparum. Those in whom both alleles code for “normal” hemoglobin are often killed by the malaria; those carrying both special alleles suffer distortion of the red blood cells (sickling) and often are sick or die because of it. Thus selection favors individuals with one allele of each kind, in this case holding a population constant rather than changing it.

A variety of genetic mechanisms influence how selection focused on one attribute of an organism can affect other attributes. Genes carry the information to determine the structure of proteins (including enzymes), and one protein may have several functions. Or genes that are very closely situated on a chromosome tend to be inherited together. Genes also serve to regulate the activities of other genes, and thus a gene under selection may change more than one attribute as its frequency in a population increases. Genes are actually segments of DNA, we now know, but their actual definition has become increasingly difficult as scientists learn more and more about the regulatory networks within which they function. The more is learned about genomes—the entire complex of DNA in an organism—the more complexity is uncovered; understanding the evolution of genomes themselves is a major challenge to evolutionary biologists today.

When studying evolution, one must never lose sight of the fact that the environment and an organism’s heredity work together both to produce a functioning organism and to adapt populations to their changing circumstances. Genes can function only in complicated cellular environments—DNA can’t reproduce itself any more than this printed page can. But put the page in a copying machine, or the DNA in a cell, and the situation changes dramatically. Expose a population of fruit flies to DDT, or a population of bacteria to penicillin, and the frequency of DNA with certain characteristics will increase in the population, making it more resistant to the pesticide or antibiotic.

Changes in DNA can occur in other ways besides selection. Accidents of DNA copying or changes caused by radiation or toxins can cause mutations—alterations in the genetic code that are not systematic responses to the environment in which the organism is living. The vagaries of reproduction or accidental deaths are always changing the frequencies of kinds of DNA in populations, a phenomenon called genetic drift. In addition, migrating individuals can carry kinds of DNA in them that are thereby reduced in the source population and increased in the recipient population. One must always be aware of mutation, drift, and migration, but, overall, selection is the creative force in evolution and is generally the main reason that the genetic composition of populations changes through time.

Although the “modern synthesis,” the union of natural history and genetics in the mid-twentieth century, produced the first fully coherent picture of the process of evolution, work in the area, especially in genome evolution, has been so active and exciting recently that in 2005 understanding of evolution in action was declared by Science magazine the “breakthrough of the year.”

Key Terms

  • Adaptation
  • Allele
  • Artificial selection
  • Chromosome
  • Charles Darwin
  • DNA
  • Enzymes
  • Evolution
  • Galápagos finches
  • Gene
  • Genetic drift
  • Genome
  • Hemoglobin
  • Heredity
  • Melanism
  • Gregor Mendel
  • Migration
  • Modern synthesis
  • Mutation
  • Natural selection
  • Peppered moths
  • Population
  • RNA
  • Sickle-cell anemia
  • Alfred Russel Wallace

Discussion Questions

  1. What are the principal unanswered questions in evolutionary theory today?
  2. Can science “prove” something? Explain.
  3. What is natural selection, and how does it occur?