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Most of those mutations occur in parts of the genome that are not essential, and therefore they have little or no consequence. The ones that fall in the more vulnerable parts of the genome are generally harmful, and are thus rapidly culled out of the population because they reduce reproductive fitness. But on rare occasions, a mutation will arise by chance that offers a slight degree of selective advantage. That new DNA "spelling" will have a slightly higher likelihood of being passed on to future offspring. Over the course of a very long period of time, such favorable rare events can become widespread in all members of the species, ultimately resulting in major changes in biological function.
In some instances, scientists are even catching evolution in the act, now that we have the tools to track these events. Some critics of Darwinism like to argue that there is no evidence of "macroevolution" (that is, major change in species) in the fossil record, only of "microevolution" (incremental change within a species). We have seen finch beaks change shape over time, they argue, depending upon changing food sources, but we haven't seen new species arise. This distinction is increasingly seen to be artificial. For example, a group at Stanford University is engaged in an intense effort to understand the wide diversity of body armor in stickleback fish. Sticklebacks that live in salt water typically have a continuous row of three dozen armor plates extending from head to tail, but freshwater populations from many different parts of the world, where predators are fewer, have lost most of these plates.
The freshwater sticklebacks apparently arrived in their current locations ten to twenty thousand years ago after widespread melting of glaciers at the end of the last Ice Age. A careful comparison of the genomes of the freshwater fish has identified a specific gene, EDA, whose variants have repeatedly and independently appeared in a freshwater situation, resulting in loss of the plates. Interestingly, humans also have an EDA gene, and spontaneous mutations in that gene result in defects in hair, teeth, sweat glands, and bone. It is not hard to see how the difference between freshwater and saltwater sticklebacks could be extended, to generate all kinds of fish. The distinction between macroevolution and microevolution is therefore seen to be rather arbitrary; larger changes that result in new species are a result of a succession of smaller incremental steps.
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This evidence alone does not, of course, prove a common ancestor; from a creationist perspective, such similarities could simply demonstrate that God used successful design principles over and over again. As we shall see, however, and as was foreshadowed above by the discussion of "silent" mutations in protein-coding regions, the detailed study of genomes has rendered that interpretation virtually untenable—not only about all other living things, but also about ourselves.
Some of these may have been lost in one species or the other, but many of them remain in a position that is most
consistent with their having arrived in the genome of a common mammalian ancestor, and having been carried along
ever since. Of course, some might argue that these are actually functional elements placed there by the Creator for a
good reason, and our discounting of them as "junk DNA" just betrays our current level of ignorance. And indeed, some
small fraction of them may play important regulatory roles.
But certain examples severely strain the credulity of that explanation. The process of transposition often damages the
jumping gene. There are AREs throughout the human and mouse genomes that were truncated when they landed, removing any possibility of their functioning. In many instances, one can identify a decapitated and utterly defunct
ARE in parallel positions in the human and the mouse genome (Figure 5.2).
Unless one is willing to take the position that God has placed these decapitated AREs in these precise positions to
confuse and mislead us, the conclusion of a common ancestor for humans and mice is virtually inescapable. This kind of recent genome data thus presents an overwhelming challenge to those who hold to the idea that all species were created ex nihilo.
The placement of humans in the evolutionary tree of life is only further strengthened by a comparison with our closest living relative, the chimpanzee. The chimpanzee genome sequence has now been unveiled, and it reveals that humans and chimps are 96 percent identical at the DNA level. A further example of this close relationship stems from examination of the anatomy of human and chimpanzee chromosomes. Chromosomes are the visible manifestation of the DNA genome, apparent in the light microscope at the time that a cell divides. Each chromosome contains hundreds of genes. Figure 5.3 shows a comparison of the chromosomes between a human and a chimpanzee. The human has twenty-three pairs of chromosomes, but the chimpanzee has twenty-four.
The difference in the chromosome number appears to be a consequence of two ancestral chromosomes having fused together to generate human chromosome 2. That the human must be a fusion is further suggested by studying the gorilla and orangutan—they each have twenty-four pairs of chromosomes, looking much like the chimp.
Recently, with the determination of the complete sequence of the human genome, it has become possible to look at the
precise location where this proposed chromosomal fusion must have happened. The sequence at that location—along the long arm of chromosome 2—is truly remarkable. Without getting into the technical details, let me just say that special sequences occur at the tips of all primate chromosomes. Those sequences generally do not occur elsewhere. But they are found right where evolution would have predicted, in the middle of our fused second chromosome. The fusion that occurred as we evolved from the apes has left its DNA imprint here. It is very difficult to understand this observation without postulating a common ancestor.
Yet another argument for the common ancestry of chimps and humans comes from the peculiar observation of what are
called pseudogenes. Those are genes that have almost all of the properties of a functional DNA instruction packet, but are afflicted by one or more glitches that turn their script into gibberish. When one compares chimp and human, occasional genes appear that are clearly functional in one species but not in the other, because they have acquired one or more deleterious mutations.
The human gene known as caspase-12, for instance, has sustained several knockout blows, though it is found in the identical relative location in the chimp. The chimp caspase-12 gene works just fine, as does the similar gene in nearly all
mammals, including mice. If humans arose as a consequence of a supernatural act of special creation, why would God have gone to the trouble of inserting such a nonfunctional gene in this precise location?
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From The language of God, by Francis Collins. chapter DECIPHERING GOD'S INSTRUCTION BOOK, pp. 132-139
Perhaps some here who are proponents of common design materials, may want to affirm or reject his points, especially the last question?