Originally posted by rickymouse
Evolution can be reversed but certain conditions need to be in place. I read an article on that somewhere, it is why they think they can change a
chicken back into a little T-rex or something.
Let me tell you why this is considered a never-will-happen sort of thing.
Let's hypothesize an animal with a five base DNA strand. The basal form of that strand is ATTGA, for all members of the species right now. Let's
assume reproduction by budding (sex would work the same but will only muddle the argument). Everything's going swimmingly for our friends; they're
well positioned in their environment and functioning prosperously. Now, the chances of any particular change to the DNA through a single substitution
is 1:1024, because there are 1024 possible expressions of the DNA..
Suddenly, an egg cell in one individual suffers a mutation. It might have been from a cosmic ray, or it may have been a chemical hiccup (something he
ate), or a mitosis error. There are a lot of sorts of errors that could have happened: a duplication, a deletion, a swap, or inversion. We'll limit
ourselves to simple substitutions. The cell's DNA now reads ACTGA. The egg cell goes on to bud and become an individual.
Several things could happen. The second spot on the DNA might be left over from an old virus attack thousands of years ago, and today have no
function. In that case the new bud will look and function just like the old ones; no one will know there's been a change, because functionally there
has not been one. Another possibility is that that base may have been part of the code for a vital protein, and is smack in the middle of the
protein's active spot. The protein no longer works, or works differently. The chances are that the new bud probably dies, and the mutation dies
with him. Since it only affected the bud, even his own mother won't have any greater probability to create such a mutant as any other species member
would. The chance would be 1:1024 that the same mutation would happen in any new bud, including hers.
Or the protein created by the mutation might change the animal's function so that it tolerates fresh water better than it used to. The branch of the
species living in fresh water can invade the estuary, lives a better life, buds prolifically and passes down the change, while the others can't
bother them. We have the beginning of speciation.
Now, what was the chances of what happened happening? The chance of that particular change was 1:1024, but no one was looking for that particular
change to happen before it did. Assuming some mutation happens, then the mutation occurring is just 1 - some change will happen if mutations are
possible.It might have been ATTTA, but it actually was ACTGA, it worked, and the animal has expanded into a new niche. Whether that was good or not
depends on the fresh water preditors, the existence of fresh water food, etc, etc.
Now, the fresh water preditors, small and disinterested in the new animal, themselves mutate to double their size. Suddenly the fresh water is not as
good an environment for exploiting as it was before. What will happen?
Well, the mutant fresh water buds could fail, get eaten up, and they and the mutation die off. Or they could randomly develop a new mutation which
might be neutral, detrimental or advantageous - say, they too double their size. Like the mutations before these will happen willy-nilly, and only
natural selection will judge the results.
But there is your possibility - the mutation they had before that encouraged them to go fresh could reverse, and they would move back to the salt,
evading the preditor. But what are the chances?
The chances of any change is 100%; mutations happen at a more or less fixed rate all the time. That could be any change at all to the DNA. But what
you want is one particular change that would revert the DNA to ATTGA; that's a 1:1024 chance. It isn't likely that that change would happen; no
more likely than ATTGT, or any other combination of the 1024 combinations.
The difference is that when going forward, mutations are bland, and any one will do as well as any other, from Mother Nature's disinterested
viewpoint. But going backward requires an exact event to occur, only one of the 1024 possibilities. A low probability.
Finally, consider that a real genome has billions of base pairs, not five. And that most mutations visible in a species physiology or behavior
require a long, multi-step process of individual changes, brought to the species by multiple members. You should be able to see that backtracking is
hugely, hugely unlikely. Much more likely is that a different random mutation will in effect, remove their fresh water abilities since their
environment doesn't favor going there, in a way completely independent of the way they got there.
No, reversal doesn't occur,not at the DNA level at least.