[thaumasiotes]

> It seems unlikely that, say, vertebrates are in the future going to "undo" the major evolutionary developments of the past and lose their skeleton, body symmetry, number of limbs, lungs, alimentary canal, nervous system, brain, etc. We see things like these developing in the evolutionary tree and mostly staying in place once created.

Try looking at whale skeletons over time. What isn't beneficial gets undone.

[knodi123]

Try looking at the giraffe's recurrent laryngeal nerve. What isn't beneficial is sometimes retained as long as the cost isn't bad enough to impair reproduction.

https://en.wikipedia.org/wiki/Recurrent_laryngeal_nerve#Evid...

[heavyset_go]

What doesn't affect fitness, or has relatively little cost, can and does propagate over time. By definition, nothing is selecting against them.

Similarly, beneficial and complex traits, like eyes, can "regress" if nothing selects against that trait. Plenty of species have lost their sight, making them less generally fit for many environments, because in a certain place and time those species could reproduce even without perfect vision, or just as the result of genetic drift.

[HarHarVeryFunny]

Yes, but it still seems that large scale structure tends to be preserved and it's more localized things like limbs/eyes/ears/teeth that may adapt. A chicken may have no teeth, but it's still basically a therapod.

I'm guessing there may be at least a couple of reasons for this:

1) Large scale structures evolved over long periods of time, involving layer upon layer of genetic change. This isn't going to be undone quickly or by any localized change, and those rare cases where a genetic change/defect does impact some fundamental aspect of the body plan (e.g. a frog with six legs) are very unlikely to be successful.

2) It seems possible that evolution acts to preserve large scale structure that has proved itself over time, and changes to which tend to be detrimental. In the same way that sexual reproduction seems like an evolution hack to evolve faster, then perhaps animals have also evolved genetic hacks to preserve/stabilize large scale structures that are critical to survival.

[thaumasiotes]

>>> It seems unlikely that, say, vertebrates are in the future going to "undo" the major evolutionary developments of the past and lose their skeleton, body symmetry, number of limbs

> but it still seems that large scale structure tends to be preserved and it's more localized things like limbs/eyes/ears/teeth that may adapt

Make up your mind.

[HarHarVeryFunny]

Large scale vs localized. They are not the same thing.