The responses we've seen to human-imposed stress and chemicals constitute natural selection in any meaningful sense. Although the selective agents are devised by humans, the response is purely natural and, as we've seen, can be quite complex. But perhaps it would be even more convincing to see the whole process in action in nature—without human intervention. That is, we want to see a natural population meet a natural challenge, we want to know what that challenge is, and we want to see the population evolve to meet it before our eyes.
We can't expect this circumstance to be common. For one thing, natural selection in the wild is often incredibly slow. The evolution of feathers, for example, probably took hundreds of thousands of years. Even if feathers were evolving today, it would simply be impossible to watch this happening in real time, much less to measure whatever type of selection was acting to make feathers larger. If we are to see natural selection at all, it must be strong selection, causing rapid change, and we'd best look at animals or plants having short generation times so that the evolutionary change can be seen over several generations. And we have to do better than bacteria: people want to see selection in so-called "higher" plants and animals.
Further, we shouldn't expect to see more than small changes in one or a few features of a species—what is known as microevolutionary change. Given the gradual pace of evolution, it's unreasonable to expect to see selection transforming one "type" of plant or animal into another—so-called macroevolution—within a human lifetime. Though macroevolu-tion is occurring today, we simply won't be around long enough to see it. Remember that the issue is not whether macroevolutionary change happens—we already know from the fossil record that it does—but whether it was caused by natural selection, and whether natural selection can build complex features and organisms.
Another factor making it hard to see real-time selection is that a very common type of natural selection doesn't cause species to change. Every species is pretty well adapted, which means that selection has already brought it into sync with its environment. Episodes of change that occur when a species meets a new environmental challenge are probably rare compared to periods when there's nothing new to adapt to. But that doesn't mean that selection is not occurring. If a species of birds, for example, has evolved the optimum body size for its environment, and that environment doesn't change, selection will act only to cull birds that are larger or smaller than the optimum. But this kind of selection, called stabilizing selection, won't change the average body size: if you look at the population from one generation to the next, nothing much will have changed (although genes for both large and small size will have been eliminated). We can see this, for example, for birth weight in human babies. Hospital statistics consistently show that babies having average birth weights, around 7.5 pounds in the United States and Europe, survive better than either lighter babies (born prematurely, or from malnourished mothers) or heavier babies (who have difficulties being born).
If we want to see selection in action, then, we should look in species that have short generation times and are adapting to a new environment. This is most likely to happen when species either invade a new habitat or experience severe environmental change. And indeed, that is where the examples lie.
The most famous of these, which I won't belabor as it's been described in detail elsewhere (see, for example, Jonathan Weiner's superb book The Beak of the Finch: A Story of Evolution in Our Time), is the adaptation of a bird to an anomalous change in climate. The medium ground finch of the Galápagos Islands has been studied for several decades by Peter and Rosemary Grant of Princeton University and their colleagues. In 1977, a severe drought in the Galápagos drastically reduced the supply of seeds on the island of Daphne Major. This finch, which normally prefers small soft seeds, was forced to turn to larger and harder ones. Experiments showed that hard seeds are easily cracked only by larger birds, which have bigger and stouter beaks. The upshot was that only big-beaked individuals got adequate food, while those with smaller beaks starved to death or were too malnourished to reproduce. The large-beaked survivors left more offspring, and by the next generation natural selection had increased the average beak size by 10 percent (body size increased as well). This is a staggering rate of evolutionary change—far larger than anything we see in the fossil record. In comparison, brain size in the human lineage increased on average about 0.001 percent per generation. Everything we require of evolution by natural selection was amply documented by the Grants in other studies: individuals in the original population varied in beak depth, a large proportion of that variation was genetic, and individuals with different beaks left different numbers of offspring in the predicted direction.
Given the importance of food to survival, the ability to gather, eat, and digest it efficiently is a strong selective force. Many insects are host-specific: they feed and lay their eggs on only one or a few species of plants.
In such cases the insect needs adaptations for using the plants, including the right feeding apparatus to tap the plant's nutrients, a metabolism that detoxifies any plant poisons, and a reproductive cycle that produces young when there is available food (the plant's fruiting period). Since there are many closely related pairs of insects that use different host plants, there must have been many switches from one plant to another over evolutionary time. These switches, equivalent to colonizing a very different habitat, must have been accompanied by strong selection.
We have, in fact, seen this happen over the last few decades in the soapberry bug (Jadera haematoloma) of the New World. Jadera lives on two native plants in different parts of the United States: the soapberry bush in the south-central US and the perennial balloon vine in southern Florida. With its long, needle-like beak, the bug penetrates the fruits of these plants and consumes the seeds within, liquefying their contents and sucking them up. But within the last fifty years, the bug has colonized three other plants introduced into its range. The fruits of these plants are very different in size from those of its native host: two are much larger and one much smaller.
Scott Carroll and his colleagues predicted that this host switch would cause natural selection for changes in beak size. Bugs colonizing the larger-fruited species should evolve larger beaks to penetrate the fruits and reach the seeds, while bugs colonizing the smaller-fruited species would evolve in the opposite direction. This is exactly what happened, with beak length changing by up to 25 percent in a few decades. This may not seem like much, but it is enormous by evolutionary standards, particularly over the short span of 100 generations.30 To put it in perspective, if this rate of beak evolution was sustained over only 10,000 generations (5,000 years), the beaks would increase in size by a factor of roughly five billion, becoming about 1,800 miles long, and able to skewer a fruit the size of the Moon! This ludicrous and unrealistic figure is, of course, meant only to show the cumulative power of seemingly small changes.
Here's another prediction: under prolonged drought, natural selection will lead to the evolution of plants that flower earlier than their ancestors.
This is because, during a drought, soils dry out quickly after the rains. If you're a plant that doesn't flower and produce seeds quickly in a drought, you leave no descendants. Under normal weather conditions, on the other hand, it pays to delay flowering so that you can grow larger and produce even more seeds.
This prediction was tested in a natural experiment involving the wild mustard plant (Brassica rapa), introduced to California about 300 years ago. Beginning in 2000, Southern California suffered a severe five-year drought. Arthur Weis and his colleagues at the University of California measured the flowering time of mustards at the beginning and end of this period. Sure enough, natural selection had changed flowering time in precisely the predicted way: after the drought, plants began to flower a week earlier than their ancestors did.
There are many more examples, but they all demonstrate the same thing: we can directly witness natural selection leading to better adaptation. Natural Selection in the Wild, a book by the biologist John Endler, documents over 150 cases of observed evolution, and in roughly a third of these we have a good idea about how natural selection was acting. We see fruit flies adapting to extreme temperature, honeybees adapting to competitors, and guppies becoming less colorful to escape the notice of predators. How many more examples do we need?
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