Saturday, September 28, 2013

The Land Bridges of Solution Space

Once upon a time, before the scientific community as a whole really started taking continental drift seriously, doctrine was that flora and fauna distribution was brought about by some sort of land bridge network that spanned the oceans.  There was no evidence for these land bridges other than the terrible need for them to have existed, the explanandum itself.

Now, in sequence space, there is also a terrible need for bridges.  To a certain extant bridges can be made through sequence space, "wormholes" that connect remote regions of the space, but it can be very tricky to make the ends of the wormhole open at useful locations.

One kind of such wormhole, maybe Kenneth Miller's favorite kind, is the frame shift mutation.  Where these wormholes open up is determined by the genetic code itself, but the chances of a useful mutation on one part of sequence space being bridged to a much more useful  (for a completely different purpose) mutation in a remote part of solution space does not seem like something that can be relied upon for a pervasive mechanism.

Crossover is another, more plausible source of wormholes through sequence space.  Again, there is a precipitous element of contingency about this, not about what is predetermined by the genetic code, but in the chances of a useful part of gene P being spliced onto a useful part of gene Q so that gene P'Q' does something that is of significant value to the organism.

As with the land bridges that were presumed ubiquitous (and explanatorily sufficient) before continental drift, the chance in a quintillion bridges between islands of measurable utility in sequence space are evidenced to occur in the right locations and frequency by whatever explanatory gap calls for them to exist.  It is not clear that hyperadaptationists necessarily depend on them (it's all smooth fitness functions like Methinks It Is Like A Weasel), but if they do, you can be sure that all sorts of enzymes can be produced as easily as "nylonase."

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