Comment on Open Thread Feb 22 - Feb 28, 2016

Wed, 24 Feb 2016 18:33:48 GMT

Recall that we are talking about evolution.

I'm well aware. If you don't think that evolution describes the changes in the population of mitochondria in a cell, then I think you're taking an overly narrow view of evolution!

Your mitochondrion might well live longer, but it still won't make it to the next generation.

I happen to be male; none of my mitochondria will make it to the next human generation anyway. (You... did know that mitochondrial lines have different DNA than their human hosts, right?)

But for the relevant population--mitochondria within a single cell--these mutants do actually win and take over the population of the cell, because they're reproductively favored over the previous strain. And if we go up a level to cells, if that cell divides, both of its descendants will have those new mitochondria along for the ride. (At this level, those cells are reproductively disfavored, and thus we wouldn't expect this to spread.)

That is, evolution on the lower level does work against evolution on the upper level, because the incentives of the two systems are misaligned. Since the lower level has much faster generations, you'll get many more cycles of evolution on the lower level, and thus we would naively expect the lower level to dominate. If a bacterial infection can go through a thousand generations, why can't it evolve past the defenses of a host going through a single generation? If the cell population of a tumor can go through a thousand generations, why can't it evolve past the defenses of a host going through a single generation?

The answer is twofold: 1) it can, and when it does that typically leads to the death of the host, and 2) because it can, the host puts in a lot of effort to make that not happen. (You can use evolution on the upper level to explain why these mechanisms exist, but not how they operate. That is, you can make statements like "I expect there to be an immune system" and some broad properties of it but may have difficulty predicting how those properties are achieved.)

(That is, the lower level gets both the forces leading to 'disorder' from the perspective of the upper system, and corrective forces leading to order. This can lead to spectacular booms and busts in ways that you don't see with normal selective gradients.)

Discuss