The Dominant Animal

Human Evolution and the Environment

Protected: Chapter 2. The Entangled Bank

Chapter Summary

Changing populations within a species through time is now relatively well understood, but how do we explain the evolution of organic diversity—the “origin of species”? Actually, the answer is quite simple—each location on Earth is different from every other and always has been. Climate, soil type, salinity, wave action, other organisms, and many other factors vary geographically. Because of this, populations of organisms in different places are living in different environments and thus are subjected to different selection pressures. This leads to geographic variation in the characteristics of populations, which usually increases as each population responds to a different, changing environment. This population differentiation tends to increase if there are few (or no) migrants moving between populations and reproducing, that is, if they are more or less isolated from one another. When a lot of migrants are transferring DNA from one population to another (known as “gene flow”), there will be less differentiation. Thousands of examples of geographic variation in organisms have been documented, showing everything from statistically barely perceptible gradients of change to dramatic differences such as those between the Audubon’s and myrtle warblers, which are so different that, despite intermediate individuals where their ranges overlap, many people consider them separate species. Taxonomists traditionally consider similar populations that live in the same locality and do not significantly interbreed to be separate species, and they do the same for isolated populations that have developed a similar level of differentiation. For that reason, and following terminology that traces to Darwin, the process of population differentiation is usually referred to as “speciation.”

Other species living in the same place are often important parts of the environment of an organism and frequently put selection pressures on its population. Quite naturally, the favor is often returned; while organism A is changing the environment of organism B, B is also changing that of A. The result is what scientists call “coevolution.” Plants, animals, and microbes thus interact evolutionarily, and we see, for example, host-parasite, predator-prey, and plant-herbivore coevolution, in addition to evolution in response to changes in the physical and chemical environments. Each organism, including each of us, is thus embedded in a web of physical, chemical, and biological relationships called an ecosystem.

At present, Homo sapiens is so substantially altering the global environment that, through climate change and the spread of toxic substances, for example, it is changing selection pressures on many, if not most, organisms. At the same time, people are also subject to selection—prominently by pathogenic microbes such as Plasmodium falciparum, which has led to sickle-cell anemia. In turn, human beings are culturally evolving chemicals to kill Plasmodium, and the microbes, in their turn, evolve resistance to them. Much of the investigation of coevolution started with the study of plants and herbivores and the discovery that many of the complex chemicals produced by plants are poisons evolved to keep them from being eaten. That has many consequences for humanity today, since plant-eating insects are our biggest competitors for food. For example, insect herbivores have been evolving for many millions of years in response to the plants’ attempts to poison them. Small wonder they can evolve resistance more rapidly than insect predators to human-made poisons such as DDT. This partly explains why chemical control of insect pests is so difficult, since pests tend to become resistant at the same time as their insect enemies are being wiped out.

Key Terms

  • Coevolution
  • Ecosystem
  • Gene flow
  • Geographic variation
  • Herbivores
  • Isolation (of populations of organisms)
  • Migration (of organisms)
  • Selection pressure
  • Speciation
  • Toxic substances

Discussion Questions

  1. What are the similarities or differences between the mechanisms that generate the diversity of species and those that cause genetic changes within populations of a single species?
  2. What are your principal personal coevolutionary interactions?
  3. What are the most important ways that an understanding of coevolution can help people to control insect pests?
  4. What does “speciation” mean, and how does it occur?
  5. Explain how antibiotic resistance might develop. What practices could help preserve the medicinal power of antibiotics?