March 2006, updated August 2012

The tests confirm, beyond a shadow of doubt, that Roger Keith Coleman did it, but Alan Crotzer did not. In 1992, Coleman was executed for the rape and murder of his sister-in-law. In 1981, Crotzer was sentenced to 130 years in prison for a robbery and pair of rapes. Though the crimes themselves are old, judgments long since rendered, and punishments already meted out, for many observers, the actual guilt or innocence of these two defendants for two different crimes was only just settled by an increasingly important test: the DNA fingerprint. Recent DNA tests revealed that it was, indeed, Coleman’s semen in the body of his victim, and that he had actually committed the crime for which he was executed more than 10 years ago. And recent DNA tests revealed that Crotzer is not a rapist and has spent 24 years in prison for crimes he did not commit.

Where's the evolution?

DNA fingerprinting allows forensic scientists to determine whether the DNA found at a crime scene came from a particular individual. But how does this technique work and what does it have to do with evolution? Answering this question depends upon understanding the genetic variation in human populations and the rates at which different parts of the genome evolve.

Humans are 96% genetically identical to our closest living relative, the chimpanzee. Obviously, we are even more similar to each other: two randomly chosen people from anywhere on Earth are expected be 99.9% genetically identical. So how can one person’s DNA be unique enough to identify him or her as the perpetrator of a particular crime? The answer, it turns out, is volume: the human genome is composed of three billion base pairs! Even at 99.9% similarity, any two people will still differ at about three million base pairs. In fact, no two people on Earth have exactly the same genetic sequence, except identical twins.

Some of these genetic differences influence the unique set of characteristics that makes you you: eye color, hair color, height, tendency towards heart disease, and numerous other traits. But most of these genetic differences have no discernable effect on your phenotype, or set of physical features, at all. And it is these genetic differences that biologists focus on when they are trying to identify or exonerate a suspect using DNA fingerprinting.

Different parts of the genome evolve at different rates. DNA that encodes important traits tends to evolve slowly. This is because most (though not all) mutations in critical regions of the genome are likely to cause detrimental effects and be selected out of the population rapidly. If a stretch of DNA evolves slowly, few changes in its sequence will occur and many people in the population will likely carry identical sequences. Though important, these regions of the genome will not be very helpful for identification.

On the other hand, some regions of the genome don’t seem to do anything in particular. Because variation in these regions has little effect on the characteristics of the organism, variants are largely “invisible” to natural selection. Here, mutations accumulate without much consequence and gene frequencies change via genetic drift. These regions evolve quickly, and as a consequence, different individuals in the population carry different sequences in these regions.

Even within fast-evolving regions of the genome, there may be particular mutation “hotspots,” which are unusually variable in sequence. Many of these regions contain DNA that repeats the same sequence of bases over and over again (e.g., ATGGATGGATGGATGG…). Biologists think that cells frequently make mistakes copying these regions, accidentally producing more or fewer repeats than in the original DNA sequence and, hence, causing a new mutation. Because they evolve so quickly and vary so much in the number of repeats, these hotspots are ideal targets for DNA fingerprinting.

In DNA fingerprinting, scientists collect samples of DNA from different sources — for example, from a hair left behind at the crime scene and from the blood of victims and suspects (see diagram below). They then narrow in on the stretches of repetitive DNA scattered throughout these samples. The profile of repetitive regions in a particular sample represents its DNA fingerprint, which ends up looking a bit like a barcode. Each bar in the barcode represents one particular stretch of repetitive DNA. Since these repetitive regions are common in the genome and highly variable from individual to individual, no two people (except identical twins) will have exactly the same set of repetitive regions and, hence, the same DNA fingerprint.

The importance of DNA fingerprinting for figuring out who was involved in a particular crime is clear: since the advent of the technique, DNA evidence has exonerated more than 150 wrongly convicted people and has become an accepted and expected line of evidence in many thousands of trials. The approach has other applications as well, including determining family relations (such as in paternity suits) and helping biologists study mating habits in the wild. However, it’s important to keep in mind that the technique only works because evolution does: the human genome is constantly evolving, acquiring new mutations over time — and it is the variation generated by this evolution that forensic scientists leverage to help solve crimes.