The are several ideas about why this junk exists.
- We know that the junk consists largely of selfish DNA - e.g. LINEs
and SINEs. Perhaps this DNA is parasitic on its host and exists solely
to benefit itself.
- The junk could exist as a result of a random walk in genome
size. In this case, the junk could persist simply because the
resulting selection pressure to eliminate it is relatively weak.
- Evolution often proceeds by duplicating genes - or sometimes
entire chromosomes. Some parts of the duplicated structure are
then redundant, are under reduced selection pressure - and so decay.
Of course, evolution can delete things too. The prevalence of
junk could be as a result of duplication being a more useful
source of novel variation than deletion.
- Eucaryotes typically have much more junk than procaryotes do.
This difference could be explained by sexual recombination - which
acts to conserve genome lengths by selecting strongly against small
changes in genome length - which is the type of mutation which is
needed to gradually eliminate junk DNA.
- There are a bunch of ideas that claim that the sequence doesn't matter -
but that the junk still weakly performs some useful function. Larger genomes
take longer to divide and result in larger cells - these are traits
that could be selected positively for. The junk could act as an absorber
for randomly-inserted retroviruses - which would then not be expressed.
Or it could act as a harmless sink for chemical mutagens within the cell.
The gene-meme analogy suggests that we may find that a lot of culture is worthless junk too. This idea is strongly reminiscent of Sturgeon's law - named after Ted Sturgeon, who famously said:
Sure, 90 per cent of science fiction is crap. But then, 90 per cent of everything is crap.
Many superstitions certainly look like junk memes - in that they are behaviours that don't have much impact fitness, and where the details don't matter very much. Whether you touch wood or throw salt over your shoulder makes little difference.
In genetics, the idea of 'junk' refers to sequence being irrelevant - and not significantly impacting fitness.
I think fairly simple thought experiments show that if you randomly change 'sequence information' in the Bible, in The Origin of Species, in Harry Potter, in MP3s, or in videos, their fitnesses would be negatively affected. Junk memes seem to be less common than junk genes are.
Computer programs are another area to consider. Here, large programs often use multiple libraries - and sometimes there's a lot of library routines that aren't used. Random changes to these would then have little effect on the resulting phenotype. However, these large programs are more like symbiotic unions than individual creatures. They have many genomes - not one main genome. This weakens the comparison with junk genes.
Maybe libraries and organizations have their share of junk - but again, these are more like ecosystems than individual organisms - and the comparison with junk genes again becomes more strained.
If we provisionally grant the conclusion that junk memes seem to be less common than junk genes are, the next question that arises is: what feature of cultural evolution leads to this difference. Some ideas about that follow:
- Cultural evolution doesn't have recombination that acts to preserve genome size.
That could mean that small selection pressures to eliminate junk memes have a
chance to work - and small deletions are not strongly selected against by the
pressure to have the same genome size as everyone else in the population.
- The idea that the junk defends against retroviruses (and other endogenous mutagens)
doesn't work very well in cultural evolution either. There are sometimes parasites,
and even some memes that work like retroviruses (by inserting themselves into other texts) but
this isn't so much of a problem as it is in the organic realm.
- The memetic codes used in cultural evolution often lack the "start" and "stop"
codons that make junk genes possible in the first place. Where there are
such codes (e.g. inside computers) there's often more junk.
- Memes are often compared to viruses. Viruses have little junk DNA. Maybe
these memes also resemble viruses in that regard.
- Lastly, maybe there's just more selection pressure against junk in cultural