8.1: Evolution is an ongoing process.

What big eyes you have! The glasswing butterfly, Cithaerias aurorina, from Peru.

What’s the longest that you’ve ever gone without food? Twenty-four hours? Thirty-six hours? If you’ve ever gone hungry that long, you probably felt as if you were going to die of starvation, but humans can survive days, even weeks, without food. In 1981, for example, 27-year-old Bobby Sands went on a hunger strike. Forsaking all food and consuming only water, he gradually deteriorated and ultimately died—after 66 days without food.

Each hour without food is even more dangerous when you’re tiny. A fruit fly, for example, can only last just under a day—20 hours, give or take a few (FIGURE 8-1). Fruit flies can’t live long without food because their tiny bodies don’t hold very large caloric reserves. But could you breed fruit flies that could live longer than 20 hours, on average? Yes.

Figure 8.1: How long can a fruit fly survive without food?

First, start with a population of 5,000 ordinary fruit flies. In biology, the term population means a group of organisms of the same species living in a particular geographic region. The region can be a small area such as a test tube or a large area such as a lake. In your experiment, your population occupies a cage in your laboratory.

From these 5,000 fruit flies, you will choose only the 20% of flies that can survive the longest without food:

• 1. Remove the food from the cage with the population of 5,000 flies.
• 2. Wait until 80% of the flies have starved to death, then put a container of food into the cage.
• 3. After the surviving flies eat, they’ll have the energy to reproduce. When they do, collect and transfer the eggs to a new cage.

You can now start a new generation consisting only of the offspring of those fruit flies able to survive without food for the longest amount of time. When these eggs hatch, the flies show increased resistance to starvation. The average fly in the new generation can live for about 23 hours without food. And again, some of the new flies survive for more than 23 hours, some for less (FIGURE 8-2).

Figure 8.2: Evolution in action: increasing fruit flies’ resistance to starvation.

What if you kept repeating these three steps? Start each new generation using eggs only from the fruit flies in the top 20% of starvation resistance. After five generations, would the average starvation-resistance time for a fly in your latest population be even higher? In fact, it would. With each new generation, you would see a slight increase, and with the fifth generation, the population’s average survival time would be about 32 hours.

After 60 generations of allowing only the flies that are best at surviving without food to reproduce, how has the population changed? Amazingly, the average fly in the resulting population can survive for more than 160 hours without food (see Figure 8-2). In 60 generations, the flies have gone from an average starvation resistance of less than a day to one of nearly a week! At the point where the food is removed, the flies in generation 60 are noticeably fatter than the flies in generation 0.

What happened? In a word: evolution. That is, there was a genetic change in the population of fruit flies living in the cage. In the generation 60 population, even the fly with the worst starvation resistance is more than seven times better at resisting starvation than the best fly in the original population. Later in this chapter we’ll discuss the mechanisms by which evolution can occur, but, in short, in this situation it is the consequence of certain individual organisms in a population being born with characteristics that enable them to be more likely to survive and to reproduce than other individuals in the population. In this experiment, the 20% of fruit flies that were the most starvation resistant had a huge reproductive advantage over less starvation-resistant flies because they were the only flies in the population that survived to reproduce.

This experiment answers a question that is sometimes perceived as complex or controversial: does evolution occur? The answer is an unambiguous yes. We can watch it happen in the lab whenever we want. Recall from our discussion of scientific thinking (see Chapter 1) that for us to have confidence in our hypotheses about anything, they must be testable and reproducible. To be certain of the results in the fruit fly study, researchers carried out the starvation-resistance experiment described above five separate times. The results were the same every time.

In this experiment we changed starvation resistance in fruit flies, but what if we used dogs instead of flies and, instead of allowing the most starvation-resistant individuals to reproduce, we allowed only the smallest dogs to reproduce? Would the average body size of individual dogs in subsequent generations decrease over time? Yes. What if the experiment were done in a natural habitat, in the absence of human intervention, on rabbits, and only the fastest rabbits escaped death from predation by foxes? Would the average running speed in rabbits increase over time? Yes. We know these answers because each of these experiments or observations has already been done. An unfortunate example of evolution in action is the evolution of resistance to antibiotics in numerous illness-causing bacteria, including some bacteria responsible for pneumonia and tuberculosis.

In this chapter, we examine how evolution takes place and the types of changes it can cause in a population. We also review the five primary lines of evidence indicating that evolution and natural selection are processes that help us clarify all other ideas and facts in biology. Let’s begin our investigation with a look at how the idea of evolution by natural selection was developed. This knowledge will help us better understand why evolution by natural selection is regarded as one of the most important ideas in human history, why it is sometimes considered a dangerous idea, and why it generates emotional debate.