Tandem Repeats

Posted on Tuesday 21 December 2004 to unknown

Purebred bull terrier snouts have changed dramatically over a short period. Photo shows specimens from 1931, 1950 and 1976 (by Fondon and M. Nussbaumer).

One problem with our current understanding about how evolution works is that the main mechanism we think of when it comes to genetic modification, the single-point mutation, is a comparatively rare and generally detrimental event. The changing of a single nucleotide only occurs at a rate of 1 in a 100 million nucleotides per generation so it is hard to see how species depending solely on this mechanism would be able to adapt very quickly to changes in their environments. And yet, as the fossil record demonstrates, new species emerge and diversify quite rapidly in geological terms and for species to be able adapt and evolve as quickly as they clearly do, other mechanisms for genetic modification must also be in play.

One such mechanism which has been suggested but only recently shown to be significant, at least in land animals, is a DNA copying error which causes tandem repeats. The term tandem repeat refers to a region of DNA which repeats a sequence over and over again. The repeat sequence might be a simple alternation of two nucleotides or it could be a pattern of dozens or even thousands of nucleotides repeated over and over. These mutations occur at a rate 100,000 times more frequently than single-point mutations do but, unlike the latter, their effect are generally be quite subtle and normally not detrimental the organism. Single-point mutations, on the other hand, are usually either neutral or fatal and only a tiny number of these mutations bestow any benefit at all on the organism.

Tandem repeats in DNA have recently been identified as the main mutation type responsible for astonishing variety of shapes and sizes in domesticated dogs. This variation in dogs, ranging from Chihuahuas to Great Danes, has emerged very rapidly - in the space of a only a few thousand years. Dog breeding is an extreme case when viewed in evolutionary terms because even dogs with particularly unfavourable mutations can be kept alive by their owners and this enables their genotypes to vary quite considerably in ways that would not be possible in the wild. Consequently, domesticated dogs have much higher incidences of tandem repeats in the genes that affect their shape and size than wild dog strains. In other words - and this should come as no surprise - they have been bred to be highly adaptable to human whim and fancy. Wild dogs, on the other hand, have had to face much stricter selection for fitness and this has, presumably, kept this kind of variability in check and ensured morphological stability for long periods of time.

It's interesting to ponder one of the implications of a tandem repeat driven theory of evolution, that our genomes may be considerably more adaptable and in a state of flux to a greater extent than we have previously recognised.