Over at BioLogos, Dennis Venema provides the conventional view of transposons through a non-teleological prism. Like many others, he thinks of transposons as parasites:
They are the perfect parasites: using their host to provide resources so they can replicate themselves, and with a “lifestyle” so simple that replication is essentially its only feature.
Despite their parasitic nature, sometimes the host species can exploit transposons as a source of genetic novelty.
Yet it is not really accurate to describe transposons as parasites.
Parasitism is just one form of symbiosis. Symbiosis exists when two different species enter a relationship. If both benefit, it is called mutualism. If one benefits and the other is unharmed, it is called commensalism. If one benefits and the other is harmed, it is called parasitism.
Now, it is hard to argue that transposons are parasites given something else Dennis provides:
Despite their somewhat mysterious nature, transposon sequences make up a staggering 45% or more of our genome. That’s about 1.4 billion DNA base pairs of our genetic material that is recognizable as functional transposons or their mutated, fragmentary remains. Not surprisingly, nearly all transposon sequences in the human genome are not under selection – they are free to accumulate mutations. These mutations have no effect on us since they do not alter any function we require.
If 45% of the human genome was loaded with parasites, you would think they could carry out some serious hurt. Yet that “nearly all transposon sequences in the human genome are not under selection” tells us they, as a whole, are not doing any harm.
So I think that if one wants to draw from symbiotic relationships to employ metaphors about the genome, a far more accurate choice would be to refer to transposons as commensals. And sometimes, over time, particular commensals can evolve into relationships that are either parasitic or mutualistic.
Anyway, when it comes to metaphors about transposons, my preferences differ. That is, instead of drawing from ecology, I would draw from information technology and envision transposons as reformatting elements that facilitate the reconfiguration of genomes for evolutionary purposes. That 45% of the human genome contains transposons speaks of the inherent robustness of the eukaryotic genome, which was probably key in allowing it to unfold increasingly complex body plans over time. And that takes us to another point Dennis makes:
So, transposons are at once a good example of non-functional DNA in genomes (indeed, nearly half of our own genome is made up of them), and an example of how evolutionary processes can convert non-functional DNA into functional DNA through mutation and selection.
But here Dennis seems to be thinking of function in terms of the individual organism over short time scales. Otherwise, he might have recognized that the functional aspect of transposons is tied up with the very evolution he mentions. I would argue that transposons, as a group, function to facilitate the evolutionary process and that without them, “mutation and selection” would not have and could not have produced an organism like us. In other words, if transposons did not exist, yet other mutations and selection still existed, nothing like a human being (with its brain, immune system, etc.) would have ever evolved.
For anyone interested, I discussed transposons from a telic perspective back in 2009: