The Evidence for Evolution

Alan R. Rogers

• Publisher: University of Chicago Press
• Year: 2011
• ISBN:
  9780226723853
• Topic: Biological Sciences
• Subtopic: Science General
• Index: Biological Sciences

According to polling data, most Americans doubt that evolution is a real phenomenon. And it's no wonder that so many are skeptical: many of today's biology courses and textbooks dwell on the mechanisms of evolution—natural selection, genetic drift, and gene flow—but say little about the evidence that evolution happens at all. How do we know that species change? Has there really been enough time for evolution to operate?

With The Evidence for Evolution, Alan R. Rogers provides an elegant, straightforward text that details the evidence for evolution. Rogers covers different levels of evolution, from within-species changes, which are much less challenging to see and believe, to much larger ones, say, from fish to amphibian, or from land mammal to whale. For each case, he supplies numerous lines of evidence to illustrate the changes, including fossils, DNA, and radioactive isotopes. His comprehensive treatment stresses recent advances in knowledge but also recounts the give and take between skeptical scientists who first asked "how can we be sure" and then marshaled scientific evidence to attain certainty. The Evidence for Evolution is a valuable addition to the literature on evolution and will be essential to introductory courses in the life sciences.

1

DARWIN’S MOCKINGBIRD

The mockingbird hopped out of the bright sunlight and into the shade, then onto the rim of a tortoise-shell cup. It lowered its beak to the water and began very calmly to drink. The cup, as it happened, was resting in the hand of a young naturalist named Charles Darwin, but the bird didn’t seem to care. It continued drinking even as Darwin raised the cup for a better look.

Darwin’s eyes must have widened with astonishment, but not as much as they might have. These events took place on the Galapagos Islands in 1835. At that time, Darwin was still a creationist and had no way of anticipating the revolution this bird would cause in his own thinking, let alone that of the entire world.

The bird on the cup looked much like the other mockingbirds on the island. Yet these mockingbirds did not look quite like those on a nearby island that Darwin had just visited. And those mockingbirds differed from the ones on the next island over. Each island seemed to have its own distinctive mockingbirds. Darwin found this astonishing. The environments offered by these islands were indistinguishable, and the islands were inmost cases within sight of each other. Why, Darwin wondered, had the creator made a different mockingbird on each island?

Furthermore, why was the bird on his cup a mockingbird? Mockingbirds are found only in the Americas, and Darwin’s bird was similar to the ones he had seen in Chile. Yet Darwin was 600 miles from the American mainland. He wondered why the creator had chosen to populate these remote islands with birds that looked so American.

This question was broader than mockingbirds, for the same pattern held for finches and other types of bird. Nor was it just about birds. Each island had its own distinctive tortoises, insects, lizards, and even plants. With only a few exceptions, these were most closely allied to species found in South America.

During his stay in the Galapagos, Darwin was able to explain these questions away. He seems to have assumed that the different populations of mockingbird were mere varieties of a single species. This sort of geographic variation is found in many widespread species and would not have challenged Darwin’s creationist views. Furthermore, the Galapagos species of mockingbird might have been created in South America and then immigrated to the Galapagos. For all Darwin knew, that species still lived somewhere in South America. Yet within 18 months, this hypothesis came crashing down [104, p. 351]. The difficulties arose after Darwin returned to England, where there were experts on birds, reptiles, insects, plants, and all the other forms of life that Darwin had collected. These experts were eager to examine collections from the far-off Galapagos. In case after case, they assured him that entire species of plant and animal were confined to individual Galapagos islands. Yet the species on different islands were similar to each other and also (to a lesser degree) to South American species. As Darwin put it in 1845,

one is astonished at the amount of creative force, if such an expression may be used, displayed on these small, barren, and rocky islands; and still more so, at its diverse yet analogous action on points so near each other. I have said that the Galapagos Archipelago might be called a satellite attached to America, but it should rather be called a group of satellites, physically similar, organically distinct, yet intimately related to each other, and all related in a marked, though much lesser degree, to the great American continent. [30, p. 398]

Darwin’s solution to this puzzle was subtle. It involved thinking not about the plants and animals that lived on the Galapagos, but about those that did not. There were bats and birds but no native land mammals, reptiles but no amphibians, herbaceous plants but no trees. In each case, the forms that were present were those that seemed best able to survive a long journey across several hundred miles of ocean. Bats and birds can fly, but land mammals cannot. Reptiles and their eggs are resistant to salt water and might have arrived alive on logs after weeks at sea. Amphibians die in salt water and could not survive such a journey. Herbaceous plants have small seeds, which can be carried by wind and in mud on the feet of birds. Trees have larger seeds that cannot travel in this fashion. The Galapagos, it seemed, were populated solely by travelers. This suggested that those plants and animals were not created on the Galapagos, but traveled there.

It seemed plausible that these travelers might have come from South America, since that is the closest continent. This accounted nicely for the observation that Galapagos plants and animals were similar to those of South America. But it also raised immediate problems: If they were immigrants from South America, why was it impossible to find any Galapagos species in South America? And why were there different species on different islands?

The only explanation, it seemed, was that the immigrants had changed after their arrival in the Galapagos. And not only that, the immigrants to each island must have changed once again. This hypothesis would account for all the facts, but it flew in the face of conventional wisdom. For at that time, each species was held to be separately created and unchanging. Darwin’s hypothesis was so radical that he did not dare publish it for many years.

During those years, Darwin was hard at work. If you are a good skeptic, you may have noticed some of the same problems that bothered him. Is it really true that the seeds can travel on the feet of birds? How long can the seed of a tree survive in salt water? If Darwin’s explanation holds for the Galapagos, then we should find the same pattern in other island chains. Do we? Darwin found ways to answer all these questions and many more. In some cases, his approach was direct and experimental. If we had visited his home during these years, we would have found rows and rows of jars in which seeds soaked in sea water. One wall was hung with ducks’ feet, on each of which (if we looked closely) we would have found seeds embedded in dried mud. To answer other questions, he collated information gleaned from the literature and from an extensive correspondence with other scientists. Only after 20 years did he dare to publish. The resulting book—On the Origin of Species—is one of the most famous in all of science [27]. In it, Darwin argued not only that evolution happens, but also that the mechanism of evolution is a process he called “natural selection.”

Darwin’s contemporaries found the first of these arguments more persuasive than the second. During Darwin’s lifetime, most working scientists came around to the view that evolution is a fact, but they argued about the importance of natural selection. One hundred and fifty years later, it has turned out that Darwin was essentially right on both counts, but his theory of natural selection left out a lot of details. Those details are still a subject of active research. There is no research, however, about whether evolution happens. That issue was settled over a century ago and is no longer an interesting scientific question.

This has led to a bias in the way we scientists teach courses and write textbooks. We tend to emphasize what we find interesting and to gloss over the rest. For this reason, students learn a lot about the mechanisms of evolution but only a little about the evidence that evolution really happens. (Perhaps this contributes to the fact that most Americans view evolution with skepticism and suspicion.) This book will reverse the traditional emphasis. It will focus on the evidence that evolution happens, while saying as little as possible about how it happens.

The general structure of the argument is much as it was in 1859. Like Darwin, we must ask: Do species change? Do they split into new species? Does evolution make big changes? Can evolution account for adaptation? These questions form the outline of the book. In every case, however, the answers will involve evidence that Darwin did not have. The case for evolution is stronger today than it has ever been.

2

DO SPECIES CHANGE?

The opinion amongst naturalists that species were independently created, and have not been transmitted one from the other, has been hitherto so general that we might almost call it an axiom.

Thomas VernonWollaston, 1860 [56, p. 128]

Breeding experiments on [Drosophila melanogaster] have been going on continuously since 1910; and, as twenty-five successive generations can be bred in the year in the laboratory, some nine hundred successive generations of this fly have been bred. Yet, although many varieties of the fly have been produced, they are all clearly Drosophila melanogaster, and are all capable of breeding with the parent form—unless so defective as to be incapable of breeding at all.

Thus, in so far as it is possible to prove a negative, experimental evidence shows that evolutionary theory is not true.

Douglas Dewar, 1949 [36, p. 11]

One hundred and fifty years ago, Thomas VernonWollaston found it easy to argue that species did not change (see above). For one thing, no one had ever seen one change in the wild. There was plenty of evidence of change in domestic species, but when these escaped into the wild, they seemed to revert to the wild form in a few generations. And judging from the mummified animals found in Egyptian tombs, cats and ibises had remained unchanged for 3,000 years. The fossil record was also disappointing to Darwinists. It did suggest that species occasionally went extinct and were replaced by new species, but it seemed to lack the intermediate forms that (according to Darwin) should link the old to the new. After reviewing all this in 1860, Bishop Samuel Wilberforce concluded that Darwin’s new theory was based “on the merest hypothesis, supported by the most unbounded assumptions” [121, p. 248]. The case for fixity of species seemed so strong that Wollaston viewed it as an axiom.

Nearly 90 years later, Douglas Dewar (also quoted above) argued the same position in a published debate with J.B.S. Haldane. In his view, decades of genetic research had produced no evidence that species ever change. But Dewar was on weak ground, for he was ignoring several studies suggesting that species do change, as Haldane was quick to point out [36, p. 21].

Today, we know a lot that none of these authors knew. The question of change within species has become especially easy to answer. But this is not the only relevant kind of change. Evolutionists argue that all modern species descend from a single ancestor, and this requires something more: it requires that species sometimes split to form new species. In what follows we take these issues one at a time.

Do species change at all?

It would take a lot of ink to discuss this question in full. We might begin with the fossil record, turn next to variation among domesticated animals, then to results from selection experiments, and finally to direct observation of change within species. But we will discuss only two of these categories of evidence, and those only briefly.

First, there are the fossils. There is evidence of change in any sequence of fossiliferous rocks, but that evidence is often confusing. Most fossiliferous rock accumulates in shallow water near the shore. These deposits are pushed alternately up and down by the same geological forces that cause continental drift (see Chapter 6). As they subside, they accumulate sediment. When they are uplifted, some of that sediment erodes away. The sediment that survives usually represents only a fraction of that originally deposited. The rock record is mostly gaps.

There is one exception to this rule: the sediment of the ocean floor, far from land and far below the surface. Here the sediment accumulates slowly but seldom erodes away. These deposits are studied from core samples, and at first glance they appear devoid of fossils. To find fossils in this rock, the paleontologist works with a microscope.

Core samples from deep-sea sediment often contain the tiny, intricate shells of radiolarians. These creatures float near the surface in life, but as they die their shells rain down onto the ocean floor. The shells are pure glass and do not decay. They are common in deep-sea sediment and provide an unusually detailed and continuous fossil record.

Figure 2.1: Evolutionary change in two radiolarians over about 3 million years [60].

Davida Kellogg and James Hays studied these fossils in the 1970s, and one of their graphs is reproduced in Figure 2.1 [60]. The graph shows change through time in the width of the shell. Early on, the shells of Eucyrtidium calvertense (the filled circles) are about a tenth of a millimeter across. They get gradually narrower over the entire three-million-year period. About four meters down, a new species (E. matuyamai) suddenly appears. It is possible that we are witnessing here the formation of a new species, as calvertense splits in two. This seems plausible because the two species are at first barely distinguishable. On the other hand, the new species may simply have immigrated from another region. However matuyami got there, its arrival seems to have provoked rapid evolutionary change in both species. E. matuyamai widens rapidly, and calvertense narrows even faster than before. As the difference between them grows, the two species are soon easy to tell apart. In cases such as this, there is little room for doubt about the fact of evolutionary change.

Yet some people find such evidence unconvincing. I learned this as a child during a visit to the home of one of my father’s brothers in north Texas. That area is full of fossils, and I soon had the driveway covered with them. From a book of Texas fossils I learned that mine all lived in the upper Cretaceous period, between 65 and 100 million years ago. My uncle was skeptical. “Couldn’t God have created those rocks all at once,” he asked, “with the fossils right in them?” I was at a loss to reply. Years later, I learned that Philip Henry Gosse had made the same argument in the 1850s, and it is easy to find in creationist literature today [47]. For skeptics of this sort, fossils provide no evidence of anything. We’ll return to Gosse’s argument in Chapter 10. For the moment, let’s turn to evidence that’s harder to dismiss.

The bacterium Staphylococcus aureus usually lives harmlessly in human noses. It can be lethal, however, when it gets under the skin during surgery or as a result of injury. In 1928 Alexander Fleming was searching for a way to fight these infections. His laboratory contained colonies of S. aureus, each in a small glass dish. At the end of a vacation, Fleming returned to find his bacterial colonies contaminated with mold. As he surveyed the ruins of his experiments, he noticed something surprising: the mold kept the bacteria from growing. Fleming had discovered penicillin, the first really successful antibiotic.

Penicillin was not used on a large scale until 1943. At that time, resistance was unknown. The first strains of penicillin-resistant S. aureus were reported in 1944, and by 1950 the resistant fraction of hospital infections had risen to 40%. This number reached 80% by 1960 and stands at 98% today. As the effectiveness of penicillin declined, doctors turned increasingly to other antibiotics. One of these, methicillin, showed great promise when introduced in 1961. However, the first strain of methicillin-resistant S. aureus appeared within a year. This new strain of bacteria, resistant both to penicillin and methicillin, is now common in hospitals. To treat these doubly-resistant bacteria, hospitals now rely on antibiotics such as vancomycin. Resistance to this drug was slow to evolve, presumably because it was less-often used. Recently, however, some strains of S. aureus have evolved resistance to it as wel...