If we have a fast-changing bacteria, say 100 alleged "mutational events" per day, would 4 billion years be long enough to see some unused code get filtered out?
If we have a fast-changing bacteria, say 100 alleged "mutational events" per day, would 4 billion years be long enough to see some unused code get filtered out?
We have a bug problem here.
As I mentioned earlier, it is possible for extra DNA to be a drain on a bug, in certain high risk and tricky environments. However, in a vast majority of cases that extra energy would not be a factor in survival.
However, in lab bugs it won't happen since there is no competition, just plenty of food and great growing conditions.
Probably not, no, since organisms have more of a vested interest in making sure they faithfully copy their DNA (which necessitates accurately copying even the "fluff", since it can't "tell" the difference), than they do in error-prone "snippage" which under most circumstances provides a very negligible advantage.
In short, the risks of "losing" parts of the DNA outweigh the possible benefits, so organisms have an evolutionary incentive to maximize their reliable DNA preservation and copying (even when that includes copying garbage as well as the critical genes).
Short form: Yeah, there may be a *slight* evolutionary pressure to "clean" the genome, but there are *other*, *stronger* evolutionary pressures to leave a working genome alone and intact as much as possible.
Furthermore, while some junk DNA will (and does) drop out by "lucky" random mutation, more keeps getting made via other random mutation, so there will be no long-term trend towards "genome cleaning".
If the code really is completely unused, and the genetics are rapidly changing, you might expect it to get lost in the shuffle, yes. But DNA that isn't out-and-out harmful is hard to get rid of, and there's always some new "junk DNA" to take the place of the old stuff. Junk DNA just doesn't put a high enough cost on the cell for it to be selected out quickly. At least, that's one possible explanation. I haven't studied it, so I couldn't tell you for sure.
There are more ways for junk DNA to get into a cell than for it to be lost. For instance, some viruses will pick up other bacterial DNA instead of virus DNA by accident before leaving a host cell; when they attach to some other cell, instead of infecting it with viral DNA, they add in part of the DNA from the cell they just left. There's all sorts of crazy ways for bacteria to pick up junk DNA (or useful DNA).