Is Evolution Reversible

It would often be qute hard for evolution to go back the way it came.It is true that for any series of mutations that will get you from A to B, there's an exactly opposite set of mutations that will get you from B back to A. But then there's natural selection to be taken into account.Consider birds for example. They evolved from things that couldn't fly into things that could fly, into things that could fly really well --- and then some of them evolved to be flightless. Now, you might call that last step "evolution going backwards" in a loose sense, but it cannot go back the way it came.For flightless birds evolve when the benefits of being large outweigh the disadvantage of being too large to fly, and this natural selection permits. But for evolution to go back the way it came, flighted birds would have to evolve to a flightless form via an intermediate stage like Archaeopteryx. But this would require them to evolve from birds that fly to birds that still fly but not so well. And this natural selection will not permit.---An interesting example of a similar phenomenon can be seen today in the laboratory. If you infect a culture of single-celled Chlorella algae with single-celled flagellate predators, then the algae evolve into a colonial form which is too big to eat. The way in which they do this is interesting.The first step is a mutation which makes the normal process of cell division incomplete. The result is that they go straight from single-celled organisms to colonial organisms consisting of hundreds of cells. While this is favored by natural selection, it has its drawbacks: most obviously, the cells on the inside are going to be deprived of nutrients.So what happens next is that selection favors progressively smaller and smaller colonies until it reaches a stable form of eight cells, all of which are at least partly on the outside of the organism: this ameliorates the nutrient problem but is still too large to be eaten.Now, what do you suppose happens when you take the predators away again? Does evolution go back the way it came? No: it can't. For to do that, the colonies would have to get bigger and bigger, which would exacerbate the nutrient problem but carries no benefits whatsoever, so natural selection will not permit evolution in this direction. And indeed experimenters observe that the eight-celled form remains stable in the absence of predators.Edited by Dr Adequate, : No reason given.

It would be completely impossible: hoses do not reproduce with variation.

Well, it depends on how you interpret the question. I don't think he means: "could evolution go against natural selection?"

I think that you, too, are misunderstanding the question.By "evolution has no direction", he means that there is no innate tendency in in evolution to produce increased size or complexity or what-have-you.So if some lineage got larger, than smaller, the period over which it got smaller would be "evolution going backwards".

A chromosome loss would be almost impossible to survive unless it was a duplicate of another chromosome.

Not likely: given the mutation rate, you yourself have many more single nucleotide substitutions than that, in the order of about 100.See A.S. Kondrashov, Direct estimates of human per nucleotide mutation rates at 20 loci causing Mendelian diseases.Edited by Dr Adequate, : No reason given.

That's not a loss, that's a fusion. Chromosome 2. An article so profound in its intellectual brilliance that I can only say ... I wrote it.Edited by Dr Adequate, : No reason given.