recap

20.1 recap

Evolution, or changes in gene frequencies in biological populations over time, is directly observable. Natural selection occurs when specific alleles increase the rates of survival or reproduction of individuals in a population relative to individuals that do not possess those alleles. Under such conditions, the frequency of the favored allele increases in the population from one generation to the next.

learning outcomes

You should be able to:

  • Describe an example in which evolution by natural selection can be observed directly within a human lifetime.

  • Differentiate between the everyday use of the word “theory” and a scientific theory.

  • Apply the principles of selection to predict and explain evolutionary outcomes.

  • Explain how evolution produces diversity and provides evidence of common ancestry.

Question 1

Why do biologists speak of “evolutionary theory” if the facts of evolution are not in doubt?

In science, the word “theory” does not mean just a guess or an untested idea. Instead, theory refers to a well-tested body of knowledge that explains the facts that we observe in the natural world. The millions of observations biologists make of living and fossil organisms every year demonstrate the factual basis of evolution. We can observe changes over time in the extensive fossil record of the Earth, just as we can observe the process of evolution at work in living natural populations and in controlled laboratory experiments. Observing that evolution occurs, however, does not by itself explain how evolution occurs. “Evolutionary theory” refers to the body of knowledge about the processes of evolution and our models of how those processes work. For example, using mathematical modeling, a biologist can show that we expect all biological populations of a finite size to evolve by genetic drift. That does not mean, however, that other processes are not resulting in evolutionary change as well. Charles Darwin’s major contribution was to argue that the process of natural selection was a major factor in the evolution of populations of living organisms over time. That idea has been tested many thousands of times by many thousands of biologists since Darwin’s time, and natural selection has been repeatedly shown to be important in the evolutionary change that biologists have observed, and continue to observe, across life on Earth.

Question 2

Antibiotics are drugs that kill most bacteria, but genetic mutations can allow some individual bacteria to survive short-term exposure to these drugs. How would you expect the frequency of resistant bacteria to change over time in populations of bacteria that were exposed to an antibiotic drug on a regular basis? Why do you think antibiotics come with a warning to take the full course of the treatment, rather than stopping after you begin to feel better?

Although most individual bacteria would die upon exposure to an antibiotic, bacteria that could survive short-term exposure would multiply rapidly after the antibiotic treatment ended. Over time, the population of bacteria would evolve resistance to the antibiotic, as any mutations that allowed survival would increase in frequency. The full treatment is judged to be effective against virtually all the bacteria in the population. If no bacteria survive the full course of the antibiotic, then the population cannot evolve resistance. If the treatment is stopped short, then there is an increased likelihood that some of the bacteria (those with the greatest antibiotic resistance) will survive, and the population of bacteria will evolve increased antibiotic resistance.

Question 3

In what ways does selection by humans in developing agricultural crops differ from natural selection? Can you give an example of a trait that might be favored by artificial selection in agriculture, but selected against by natural selection in a wild population?

Humans select traits in domestic plant and animal populations based on our interest in the trait, rather than on how it affects the natural reproductive rate or survivorship of the organisms. Many of the traits selected by humans would not be advantageous in wild populations. For example, humans have selected many cattle breeds for high body fat and high body weight. These traits result in large calves, which in turn result in calving difficulties for cows. Ranchers often have to assist in the birth of such calves, because the calf (and likely its mother) would often die without such assistance. In a natural population, there would be selection for smaller calf size and birth weight, which would increase the successful reproductive rate and survivorship.

Question 4

Natural selection cannot adapt populations to conditions they have not experienced. Yet many organisms appear to respond to natural events before they happen. For example, many mammals go into hibernation while it is still quite warm. Similarly, many birds leave the temperate zone for their southern wintering grounds long before winter has arrived. How do you think such “anticipatory” behaviors evolve?.

Behaviors can respond to environmental cues that are predictive of future conditions, and these behaviors can be selected for if they are under genetic control. For example, day length becomes shorter as we move closer to winter, so individual mammals have a survival advantage if they respond to shortening days by going into hibernation. In this case, the environmental cue (day length) is predictive of future environmental conditions (the cold of winter). The traits exist in the present because these associations (as between shortening day length and the approach of winter) have existed for a long time.

Question 5

As more humans live longer, many people face degenerative conditions such as Alzheimer’s disease that (in most cases) are linked to advancing age. Assuming that some individuals may be genetically predisposed to successfully combat these conditions, is it likely that natural selection alone would act to favor such a predisposition in human populations? Why or why not?

Natural selection cannot act when there is no effect on the effective reproductive rate of the organism. Diseases such as Alzheimer’s usually occur long after the reproductive years have passed. As long as the disease does not affect the relative likelihood of the survival of the affected person’s offspring (as a result of reduced parental care, for example), we would not expect natural selection to lead to any reduction in Alzheimer’s disease in human populations.

Although the importance of natural selection to evolution has been confirmed in many thousands of scientific studies, it is not the only process that drives evolution. In the next section we’ll consider a more complete view of evolutionary processes and how they operate.