It would take a much longer book than the one I’m writing to examine all of the evidence concerning evolution. No individual person could do so much as read about (much less evaluate) all of the relevant research in genetics, paleontology, geology, and so on. Nevertheless, it will be worthwhile to consider four major types of evidence that are frequently cited in support of the theory of evolution, and also to consider some of the objections critics have raised against each type of evidence.
Although I have my own opinions on these matters, I will do my best to present the arguments on both sides of the issue as impartially as I can, so that you may weigh the evidence and judge for yourself what is true. I’ll focus on the scientific arguments here; arguments concerning the interpretation of Genesis and other relevant biblical passages will be considered in the next chapter.
With that disclaimer and promissory note out of the way, here are four types of evidence often cited to support the theory of evolution, along with a brief summary of common objections to each type of evidence:
Evidence from observed cases
There are documented historical cases of biological evolution occurring, in small degrees, even within the past few centuries. Many types of bacteria have developed resistance to antibiotics over the past several decades, for example.
The story of the peppered moth, mentioned earlier, is frequently cited as a case of evolutionary adaptation. However, it isn’t really an example of organisms developing new traits, because both dark and light varieties of peppered moth existed before the industrial revolution.
In response to this type of evidence, critics point out that all such examples are cases of microevolution (small variations in pre-existing organs or structures) as opposed to macroevolution (the emergence of entirely new organs and structures). Many opponents of the theory of evolution, including some young-earth creationists, happily concede that microevolution occurs; but they deny that this fact is compelling evidence that macroevolution has also occurred.
One reason why microevolution might not be persuasive evidence for macroevolution is that the latter requires the production of new genes through mutation, whereas microevolution can often be explained by the elimination or recombination of pre-existing genetic information. Consider the case of the Galápagos finches, for instance. Perhaps the first birds who migrated to the islands already had the genes needed to produce all of the beak shapes that Darwin observed, but most of those possible traits were not yet manifested, because the corresponding alleles were recessive. When natural selection eliminated some of the dominant traits, new traits emerged as more and more recessive alleles were expressed.
Some examples of microevolution cannot be explained by recombination. For example, bacteria reproduce asexually, so their recent adaptations—developing immunity to antibiotics—presumably resulted from mutation and natural selection. However, critics of evolution have argued that these adaptations could be explained by the loss of genetic information. Mutations are usually destructive in the sense that they impair or disable genes rather than introducing functional new genes. Destructive mutations are sometimes beneficial, and this may explain how bacteria develop resistance to antibiotics: a mutation might disable a gene required for a bacterium to ingest the poison used in an antibiotic, for example. If all mutations are destructive, then the occurrence of microevolution would not support the possibility of macroevolution. In light of this, some argue that it is unreasonable to extrapolate from microevolution to macroevolution.
However, evolutionary biologists think that mutations are occasionally constructive, introducing new genes that are actually useful. This brings us to a second type of evidence for evolution:
Evidence for evolutionary mechanisms
Genetic recombination, mutation, and natural selection are plausible mechanisms by which life could evolve, and there is abundant evidence that all three of these processes do occur. Natural selection clearly occurs, as seen in the case of the peppered moth and innumerable other examples. Genetic recombination obviously occurs too, as demonstrated in Mendel’s experiments. Moreover, all organisms pass their characteristics on to their offspring via DNA, which can mutate. DNA mutation is a plausible mechanism by which new genetic information—new genes and alleles—could be added to the genome.
In response to these points, critics note that finding plausible mechanisms by which life could evolve doesn’t show that life did evolve. Moreover, it is far from obvious whether mutation and recombination could produce new varieties of life quickly enough to account for some of the discontinuities, or “sudden jumps,” in the fossil record. (We’ll return to this point about fossils in a moment.) Furthermore, some features of living organisms might be irreducibly complex, in the sense that the transitional forms of these features would not have provided any advantage to the organism until the features had evolved to a high level of complexity. Arguably, the transitional forms should have been eliminated by natural selection, so the proposed mechanisms of evolution cannot explain why irreducibly complex features exist—if they do indeed exist. It is controversial whether any organisms truly do have irreducibly complex features. We’ll consider some purported examples of irreducible complexity in chapter 10, when we discuss intelligent design theory.
Evidence from genetics
An organism’s DNA is usually similar to that of its supposed ancestors (in the rare cases when DNA from the remains of those ancestors can be extracted and analyzed) and also to its evolutionary “cousins”—organisms believed to have descended from a relatively recent common ancestor. For example, chimpanzees and bonobos are regarded as the closest living relatives to human beings.Our most recent common ancestor is believed to have lived 5 to 10 million years ago. However, fossils of this common ancestor have not been discovered, so it is difficult to determine how long ago it lived. Estimates are based on DNA analysis and studies of other related fossils. According to recent estimates, chimps and humans share around 94% of the same genes; some older estimates suggested that the similarity was as high as 98.5%.
Comparing the genomes of different organisms is far from straightforward, and estimates of genetic similarity rely on a number of arbitrary choices. For example, there isn’t any standard way of deciding whether two similar sequences of genetic information are similar enough to count as the “same gene.” Moreover, a genome typically contains multiple copies of certain genes, and researchers have to make an arbitrary choice whether to count them as a single gene or several genes.
Genetic similarities are also found in pseudogenes—segments of DNA that resemble genes but do not produce functional proteins. Some pseudogenes have other functions besides producing proteins, but many appear to serve no purpose at all. The prevailing theory is that non-functional pseudogenes are relics of evolution: they may have served important functions for the organism’s ancestors, but they are no longer useful. (In many cases, a pseudogene differs only slightly from a functional gene, suggesting that it originated as a functional copy of that gene but was damaged by mutation.) The existence of identical pseudogenes in different species is considered strong evidence that the two species descended from a common ancestor.
In rebuttal, critics point out that similar DNA doesn’t necessarily imply common descent. It may instead point to a common designer. When we find cars with similar engines and body styles, we don’t conclude that the cars descended from each other; we conclude that they were manufactured according to a similar blueprint, perhaps designed by the same team of engineers. Analogously, someone who believes that God designed all varieties of life need not be surprised to learn that He used a similar design plan for many of His creations. The degree of genetic similarity is still less surprising when we consider that DNA contains the instructions for producing many cell parts that are common to all living organisms, regardless of how closely related those organisms may be. The fact that similar organisms share a large proportion of the same genes is to be expected, whether they descended from a common ancestor or not. This alternative explanation doesn’t apply to non-functional pseudogenes, but there may be other ways of accounting for those similarities too. For example, bacteria and other microbes sometimes take segments of DNA from one organism and transplant it into another. This process, called horizontal gene transfer, might explain why humans and chimps have some identical pseudogenes even if we didn’t have a common ancestor.
Another objection points out that geneticists rely on DNA analysis to construct theories of evolutionary genealogy: they look for genetic similarities to determine which organisms are closely related to each other. If these genetic similarities are then cited as evidence of common descent, there is a danger of circular reasoning here. For example, if geneticists conclude that humans and chimps are closely related because our DNA is similar, and then cite this similarity as evidence that closely-related species have similar DNA, they would be reasoning in a circle. Presumably no individual scientist would reason in such a patently fallacious way, but errors of circular reasoning might occur when scientists working in different fields of study rely on each other’s conclusions without examining the evidence that led to those conclusions. One may hope that other scientists would soon discover and correct these errors, but opponents of the theory of evolution often lack such faith in the scientific community.
Evidence from paleontology
Paleontology is the science of studying fossils—the remains of ancient organisms, often preserved in sedimentary rocks. The word ‘fossil,’ like any word in English, is somewhat flexible; but in paleontology the word is usually defined to include only remains and traces of organisms from a prior geological age. According to most paleontologists, fossils provide substantial evidence that life has evolved and diversified over the past several billion years. Moreover, the oldest fossils (estimated to be roughly 3.5 billion years old)We’ll talk about the methods used to date fossils on the next page. were formed by colonies of bacteria. This supports the claim that life originated with single-celled organisms and gradually evolved to the diverse and complex varieties of life we see today.
The set of all fossils, together with geological evidence used to interpret the history recorded in fossils, is known as the fossil record. Although there are many “gaps” (missing transitionary forms) in the fossil record, there are also countless examples of apparent evolutionary lineage: gradual transitions from one type of organism to another, sometimes leading all the way up to species living today.
Opponents of the theory of evolution have responded to this type of evidence in a variety of ways. Young-earth creationists propose an alternative explanation for the fossil record, contending that most fossils are the remains of organisms destroyed during the flood of Noah described in Genesis 7. In defense of this idea, they also argue that the methods used to date fossils are unreliable. (We’ll examine some of those methods on the next page.)
Other critics of evolutionary paleontology, including some who believe God created life billions of years ago, argue that the fossil record is actually inconsistent with the prevailing theory of evolution. In fact, some evolutionary paleontologists themselves have suggested that there may be problems with the standard account of how evolution occurs. According to the prevailing theory, evolutionary transitions from one species to another occur gradually over long periods of time, because genetic recombination is limited and significant mutations are rarely beneficial. (Major mutations are usually harmful, and are quickly eliminated by natural selection.) However, evolutionary paleontologists Niles Eldredge and Stephen Jay Gould have argued that the fossil record tells a different story. We’ll consider their view later in this chapter. On the next several pages, we’ll take a closer look at the evidence from genetics and the fossil record.