No species exists in a vacuum; every form of life on Earth interacts over time with other organisms, as well as with its physical environment. For that reason, the evolution of one species influences the evolution of species with which it coexists by changing the natural selection pressures those species face. The classic examples of this sort of evolution, called coevolution, are predator-prey and host-parasite relationships. One such predator-prey relationship exists between garter snakes and a species of salamander-like amphibian called a rough-skinned newt. In parts of the midwestern United States, garter snakes prey on newts, and probably have for thousands of years. In that time, over countless generations, the newts have evolved a powerful defense: a toxic chemical that they secrete through their skin. Where garter snakes are concerned, however, this defense mechanism has only been marginally successful. Generation after generation, as the newts became more poisonous, the snakes also evolved, developing greater tolerance to the newt’s toxin. The result of this coevolutionary process, played out over countless snake and newt generations, is a chemical more toxic than almost any other natural substance on Earth, and a population of snakes that are seemingly immune to the toxin’s effects.
2. What does it mean when some scientists say humans have stopped evolving?
Technology and culture have protected us to a great extent from the selective pressures that drive evolution, allowing many people, especially those in developed nations who would otherwise not live to reproductive age, to pass their genes on to the next generation. In addition, human groups are no longer isolated; they travel the globe. Without genetic isolation, there is no further opportunity for speciation among humans. On the other hand, natural selection is a function of environmental change, and our physical, biological, and cultural environments have changed tremendously. Humans face, for example, new diseases like HIV/AIDS that can greatly impact survival and reproduction. Human populations may also be undergoing evolutionary changes of which we’re not yet aware.
3. If extinction is a natural part of life on Earth, why should we care about protecting endangered species?
If the mass extinction most scientists agree we’re experiencing now is allowed to continue, it will be the first time in the history of life on Earth that a single species would be responsible for such a catastrophe. And although extinction is a natural process that has occurred many times in the distant past, it’s a process that would be in our best interest to avoid. The extinction of just one species can dramatically impact many others, and like all creatures, humans rely heavily on other species. It is therefore impossible to predict how we might ultimately be affected by a mass extinction.
4. How does evolution affect me in my daily life?
We can and do experience the indirect effects of evolution nearly every day. One of the more important evolutionary concerns facing humans today is the continual evolution of antibiotic-resistance in bacteria. The successful medical battle we have waged against bacteria for the last 50 years is now an even race, according to some scientists. Similarly, the use of pesticides in agriculture has driven the evolution of resistant insects, requiring the use of harsher chemicals in greater quantity to kill them.