THE LIVING WORLD

Unit Four. The Evolution and Diversity of Life

 

14. Evolution and Natural Selection

 

14.7. Evolution’s Critics

 

Of all the major ideas of biology, evolution is perhaps the best known to the general public because many people mistakenly believe that evolution represents a challenge to their religious beliefs. A person can have a spiritual faith in God and still be an excellent scientist—and evolutionist. Because Darwin’s theory of evolution is the subject of often-bitter public controversy, we will examine the objections of evolution’s critics in detail to see why there is such a disconnect between science and public opinion.

 

History of the Controversy

An Old Conflict Immediately after publication of The Origin of Species, English clergymen attacked Darwin’s book as heretical; Gladstone, England’s prime minister and a famous statesman, condemned it. The book was defended by Thomas Huxley and other scientists, who gradually won over the scientific establishment, and by the turn of the century evolution was generally accepted by the world’s scientific community.

The Fundamentalist Movement. By the 1920s the teaching of evolution had become frequent enough in American public schools to alarm conservative critics of evolution who saw Darwinism as a threat to their Christian beliefs. Between 1921 and 1929, fundamentalists introduced bills outlawing the teaching of evolution in 37 state legislatures. Four passed: Tennessee, Mississippi, Arkansas, and Texas.

Civil rights groups used the case of high school teacher John Scopes to challenge the Tennessee law within months of its being passed in 1925. The trial attracted national attention— you might have seen it portrayed in the film Inherit the Wind. Scopes, who had indeed violated the new law, lost.

After the 1920s, there were few other attempts to pass state laws preventing the teaching of evolution. Only one bill was introduced between 1930 and 1963. Why? Because Darwin’s fundamentalist critics had succeeded quietly in winning their way. Textbooks published throughout the 1930s ignored evolution, new editions of texts removing the words evolution and Darwin from their indices. In the 1920-29 period, for example, the average number of words about the evolution of humans in 93 secondary school texts was 1,339; in the 1930-39 period it had dropped to 439. As late as 1950-59 it was 614. To quote biologist Ernst Mayr, “The word EVOLUTION simply disappeared from American schoolbooks.”

These antievolution laws remained on the books for many years. Then in 1965 teacher Susan Epperson was convicted of teaching evolution under the 1928 Arkansas law. In 1968 the United States Supreme Court found the Arkansas antievolution law to be unconstitutional; the 1920s laws were soon repealed.

Russian advances in space in the early 1960s created a public outcry for better American science education. New biology textbooks reintroduced evolution, and gave it renewed emphasis. The average number of words per text devoted to the evolution of man, for example, rose from 614 in the period 1950-59 to 8,977 in the period 1960-69. By the 1970s, evolution again formed the core of most biology schoolbooks.

The Scientific Creationism Movement. Again alarmed by the prevalence of evolution in public school biology classes, Darwin’s critics took a new approach. It began with a proposal by the Institute for Creationism Research in 1964, which said, “Creationism is just as much a science as is evolution, and evolution is just as much a religion as is creation.” This proposal has become known as creationism science. It was soon followed by the introduction of legislation in state legislatures mandating that “all theories of origins be accorded equal time.” Creationism was represented as being as much a scientific theory as evolution, to which students had a right to be exposed.

In 1981 the state legislatures of Arkansas and Louisiana passed “equal-time” bills into law. The Louisiana equal-time law requiring “balanced treatment of creation-science and evolution-science in public schools” was struck down by the Supreme Court in 1987, which judged that creation science is not, in fact, science but rather a religious view that has no place in public science classrooms.

Local Action. In the following decades, Darwin’s critics began to substitute the school board for the legislature. Unlike most European countries, which set their school curricula through a central education ministry, U.S. education is highly decentralized, with elected education boards setting science standards at the state and local levels. These standards determine the content of statewide assessment tests, and have a major impact on what is taught in classrooms.

Critics of Darwin have run successfully for seats on local and state education boards across the United States, and from these positions have begun to alter standards to lessen the impact of evolution in the classroom. Great publicity followed the removal of evolution from the Kansas state standards in 1999 and again in 2005, but in many other states the same effect has been achieved more quietly. Only 22 states today mandate the teaching of natural selection, for example. Four states fail to mention evolution at all.

Intelligent Design. In recent years, critics of Darwin have begun new attempts to combat the teaching of evolution in the classroom, arguing before state and local school boards that life is too complex for natural selection and so must reflect intelligent design. They go on to argue that this “theory of intelligent design” (ID) should be presented in the science classroom as an alternative to the theory of evolution.

Scientists object strongly to dubbing ID a scientific theory. The essence of science is seeking explanations in what can be observed, tested, replicated by others, and possibly falsified. Explanations that cannot be tested and potentially rejected simply aren’t science. If someone invokes a nonnatural cause—a supernatural force—in their research, and you decide to test it, can you think of any way to do so such that it could be falsified? Supernatural causation is not science.

The source of sharp public controversy, intelligent design has been overwhelmingly rejected by the scientific community, which does not regard intelligent design to be science at all, but rather thinly disguised creationism, a religious view that has no place in the science classroom.

 

Arguments Advanced by Darwin's Critics

Critics of evolution have raised a variety of objections to Darwin’s theory of evolution by natural selection:

1. Evolution is not solidly demonstrated. “Evolution is just a theory,” critics point out, as if theory meant lack of knowledge, some kind of guess. Scientists, however, use the word theory in a very different sense than the general public does (see section 1.8). Theories are the solid ground of science, supported with much experimental evidence and that of which we are most certain. Few of us doubt the theory of gravity because it is “just a theory.”

2. The intelligent design argument. “The organs of living creatures are too complex for a random process to have produced.” This classic “argument from design” was first proposed nearly 200 years ago by William Paley in his book Natural Theology—the existence of a clock is evidence of the existence of a clockmaker, Paley argues. Similarly, Darwin’s critics argue that organs like the mammalian ear are too complex to be due to blind evolution. There must have been a designer. Biologists do not agree. The intermediates in the evolution of the mammalian ear are well documented in the fossil record. These intermediate forms were each favored by natural selection because they each had value—being able to amplify sound a little is better than not being able to amplify it at all. Complex structures like the mammalian ear evolved as a progression of slight improvements. Nor is the solution always optimal. The vertebrate eye, for example, is poorly designed.

 

 

The visual pigments in a vertebrate eye that are stimulated by light are embedded in the retinal tissue, facing backward to the direction of the light. The light has to pass through nerve fibers 1, nerve cells 2, and receptor cells 3, before reaching the pigments 4. No intelligent designer would design an eye backwards!

3. There are no fossil intermediates. “No one ever saw a fin on the way to becoming a leg,” critics claim, pointing to the many gaps in the fossil record in Darwin’s day. Since then, however, most fossil intermediates in vertebrate evolution have indeed been found. A clear line of fossils now traces the transition between fish and amphibians, between reptiles and mammals, and between apes and humans. The animal that produced the fossil shown below is an extinct lobe-finned fish (genus Tiktaalik) that lived approximately 375 million years ago. Coined by its discoverer as a “fishopod,” it clearly has some characteristics that are fishlike, similar to fish that lived about 380 million years ago, and others that are more like early tetrapods, which lived about 365 million years ago. Tiktaalik appears to be a transitional animal, between fish and amphibians.

 

 

4. Evolution violates the second law of thermodynamics. “A jumble of soda cans doesn’t by itself jump neatly into a stack—things become more disorganized due to random events, not more organized.” Biologists point out that this argument ignores what the second law really says: Disorder increases in a closed system, which the earth most certainly is not. Energy enters the biosphere from the sun, fueling life and all the processes that organize it.

5. Natural selection does not imply evolution. “No scientist has come up with an experiment where fish evolve into frogs and leap away from predators.” Is microevolution (evolution within a species) the mechanism that has produced macroevolution (evolution among species)? Most biologists that have studied the problem think so. The differences between breeds produced by artificial selection—such as chihuahuas, dachshunds, and greyhounds—are more distinctive than differences between wild canine species. Laboratory selection experiments with insects easily create forms that cannot interbreed and thus would in nature be considered different species. Thus, production of radically different forms has indeed been observed, repeatedly.

6. Life could not have evolved in water. “Because the peptide bond does not form spontaneously in water, amino acids could never have spontaneously linked together to form proteins; nor is there any chemical reason why biological proteins contain only the L-isomer and not the D-isomer.” Both of these contentions are valid, but do not require rejecting evolution. Rather, they suggest that the early evolution of life took place on a surface rather than in solution. Amino acids link up spontaneously on the surface of clays, for example, which can have a shape that selects the L-isomer.

 

The Irreducible Complexity Fallacy

The century-and-a-half-old “intelligent design” argument of William Paley has been recently articulated in a new molecular guise by Lehigh University biochemistry professor Michael Behe. In his 1996 book Darwin’s Black Box: The Biochemical Challenge to Evolution, Behe argues that the intricate molecular machinery of our cells is so elaborate, our body processes so interconnected, that they cannot be explained by evolution from simpler stages in the way that Darwinists explain the evolution of the mammalian ear. The molecular machinery of the cell is “irreducibly complex.” Behe defines an irreducibly complex system as “a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning.” Each part plays a vital role. Remove just one, Behe emphasizes, and cell molecular machinery cannot function.

As an example of such an irreducibly complex system, Behe describes the series of more than a dozen blood clotting proteins that act in our body to cause blood to clot around a wound. Take out any step in the complex cascade of reactions that leads to coagulation of blood, says Behe, and your body’s blood would leak out from a cut like water from a ruptured pipe. Remove a single enzyme from the complementary system that confines the clotting process to the immediate vicinity of the wound, and all your lifeblood would harden. Either condition would be fatal. The need for all the parts of such complex systems to work leads directly to Behe’s criticism of Darwin’s theory of evolution by natural selection. Behe writes that “irreducibly complex systems cannot evolve in a Darwinian fashion.” If dozens of different proteins all must work correctly to clot blood, how could natural selection act to fashion any one of the individual proteins? No one protein does anything on its own, just as a portion of a watch doesn’t tell time. Behe argues that, like Paley’s watch, the blood clotting system must have been designed all at once, as a single functioning machine.

What’s wrong with Behe’s argument, as evolutionary scientists have been quick to point out, is that each part of a complex molecular machine does not evolve by itself, despite Behe’s claim that it must. The several parts evolve together, in concert, precisely because evolution acts on the system, not its parts. That’s the fundamental fallacy in Behe’s argument. Natural selection can act on a complex system because at every stage of its evolution, the system functions. Parts that improve function are added, and, because of later changes, eventually become essential, in the same way that the second rung of a ladder becomes essential once you have added a third.

The mammalian blood clotting system, for example, has evolved in stages from much simpler systems. By comparing the amino acid sequences of the many proteins, biochemist Russell Doolittle has estimated how long it has been since each protein evolved. You can see what he has learned in figure 14.20. The core of the vertebrate clotting system, called the “common pathway” (highlighted in blue), evolved at the dawn of the vertebrates approximately 600 million years ago, and is found today in lampreys, the most primitive fish. As vertebrates evolved, proteins were added to the clotting system, improving its efficiency. The so-called extrinsic pathway (highlighted in pink), triggered by substances released from damaged tissues, was added 500 million years ago. Each step in the pathway amplifies what goes before, so adding the extrinsic pathway greatly increases the amplification and thus the sensitivity of the system. Fifty million years later, a third component was added, the so-called intrinsic pathway (highlighted in tan). It is triggered by contact with the jagged surfaces produced by injury. Again, amplification and sensitivity were increased to ultimately end up with blood clots formed by the cross linking of fibrin (highlighted in green). At each stage as the clotting system evolved to become more complex, its overall performance came to depend on the added elements. Mammalian clotting, which utilizes all three pathways, no longer functions if any one of them is disabled. Blood clotting has become “irreducibly complex”—as the result of Darwinian evolution. Behe’s claim that complex cellular and molecular processes can’t be explained by Darwinism is wrong. Indeed, examination of the human genome reveals that the cluster of blood clotting genes arose through duplication of genes, with increasing amounts of change. The evolution of the blood clotting system is an observation, not a surmise. Its irreducible complexity is a fallacy.

 

 

Figure 14.20. How blood clotting evolved.

The blood clotting system evolved in steps, with new proteins adding on to the preceding step.

 

Key Learning Outcome 14.7. Darwin's theory of evolution, while accepted overwhelmingly by scientists, has its objectors. Their criticisms are without scientific merit.

 

A Closer Look

Putting Intelligent Design to the Test

 

 

In the spring of 2006 the South Carolina Board of Education rejected a state panel's proposal to change high school standards by calling on students to critically analyze evolution. The Board stated it felt the proposal was a ploy to promote the avoidance of teaching evolution. Similar proposals to add a requirement that students critically analyze evolution had been rejected earlier in the year by the Utah and Ohio Boards of Education, and are currently under consideration in several other states.

What are we to make of this? Surely no scientist can object to critically analyzing any theory. That is what science is all about, seeking explanations for what can be observed, tested, replicated, and possibly falsified. Indeed, biologists claim that Darwin's theory of evolution has been subjected to as much critical analysis as any theory in the history of science.

So why the objection to this change in high school standards? Because many scientists and teachers, apparently including the South Carolina Board of Education, feel the change is simply intended to promote the teaching of a nonscientific alternative to evolution in classrooms.

This distinction between an assertion that can be tested and one that cannot goes to the very nature of science. Actually, nothing makes this difference more clear cut than the critical analysis so sought after by South Carolina's critics of evolution. So let's do it. Let's put Darwin to the test.

As explained earlier in the chapter, if Darwin's assertion is correct, that organisms evolved from ancestral species, then we should be able to track evolutionary changes in our DNA. The variation that we see between species reflects adaptations to environmental challenges, adaptations that result from changes in DNA. Therefore, a series of evolutionary changes should be reflected in an accumulation of genetic changes in the DNA. This hypothesis, that evolutionary changes reflect accumulated changes in DNA, leads to the following prediction: Two species that are more distantly related (for example, humans and mice) should have accumulated a greater number of evolutionary differences than two species that are more closely related (say, humans and chimpanzees).

So have they? Let's compare vertebrate species to see. The "family tree” above shows how biologists believe 18 different vertebrate species are related. Apes and monkeys, because they are in the same order (primates), are considered more closely related to each other than either are to members of another order, such as mice and rats (rodents).

The wealth of genomes (a genome is all the DNA that an organism possesses) that have been sequenced since completion of the human genome project allows us to directly compare the DNA of these 18 vertebrates. To reduce the size of the task, investigators at the National Human Genome Research Institute working at the University of California, Santa Cruz, focused on 44 so-called ENCODE regions scattered around the vertebrate genomes. These regions, corresponding to 30 Mb (megabase, or million bases) or roughly 1% of the total human genome, were selected to be representative of the genome as a whole, containing protein-encoding genes as well as noncoding DNA.

For each vertebrate species, the investigators determined the similarity of its DNA to that of humans—that is, the percent of the nucleotides in that organism's 44 ENCODE regions that match those of the human genome.

 

 

You can see the result in each instance presented as a number above the picture of each organism on the vertebrate family tree. As Darwin's theory predicts, the closer the relatives, the less the genomic difference we see.

The chimpanzee genome is more like the human genome (91%) than the monkey genomes are (72 to 79%). Furthermore, these five genomes, all in the primate order, are more like each other than any are to those of another order, such as rodents (mouse and rat).

In general, as you proceed through the taxonomic categories of the vertebrate family tree from very distant relatives on the right (some in the same class as humans) to very close ones on the left (in the same family), you can see clearly that genomic similarity increases as taxonomic distance decreases—just as Darwin's theory predicts. The prediction of evolutionary theory is solidly confirmed.

The analysis does not have to stop here. The evolutionary history of the vertebrates is quite well known from fossils, and because many of these fossils have been independently dated using tools such as radioisotope dating, it is possible to recast the analysis in terms of concrete intervals of time, and assess directly whether or not vertebrate genomes accumulate more differences over longer periods of time as Darwin's theory predicts.

For each of the 18 vertebrates being analyzed, the graph above plots genomic similarity—how alike the DNA sequence of the vertebrate's ENCODE regions are to those of the human genome—against divergence time (that is, how many millions of years have elapsed since that vertebrate and humans shared a common ancestor in the fossil record). Thus the last common ancestor shared by chickens and humans was an early reptile called a dicynodont that lived some 250 million years ago; since then the genomes of the two species have changed so much that only 7% of their ENCODE sequences are still the same.

The result seen in the graph is striking and very clear: Over their more than 300 million year history, vertebrates have accumulated more and more genetic change in their DNA. "Descent with modification” was Darwin's definition of evolution, and that is exactly what we see in the graph. The evolution of the vertebrate genome is not a theory, but an observation.

The wealth of data made available by the human genome project has allowed a powerful test of Darwin's prediction. The conclusion to which the test leads us is that evolution is an observed fact, clearly revealed in the DNA of vertebrates.

This is the sort of critical analysis that science requires, and that the theory of evolution has again passed. Anyone suggesting that a nonscientific alternative to evolution, such as Intelligent Design, offers an alternative scientific explanation to evolution is welcome to subject it to the same sort of critical analysis you have seen employed here. Can you think of a way to do so? It is precisely because the assertion of intelligent design cannot be critically analyzed—it does not make any testable prediction—that it is not science and has no place in science classrooms.