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Comment by tieTYT | Hacker News Reader

> When a mutation came along that made the receptor not work, that animal was just as likely to survive as one that didn’t have the mutation.”

Then shouldn't there still be a chance of having the mutation because nothing is selecting for or against it? Instead it seems the entire species can't taste sweetness so the gene was removed. Why would that happen? I don't know of any other explanation than, "there is an advantage to not being able to taste sweetness".

EDIT: I removed "50/50" from my question. That wouldn't be the right percentage.

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It's likely that there are many ways to turn it off, and without selection pressure they all randomize.

If there's 10 codons needed to make it work, and there's no selection pressure, then there's only a one in a million chance it's working by chance. Even when it gets close, there's a 9:1 chance that the next mutation will make things worse instead of fixing the issue. Mostly it would stay around the noise floor with ~2.5 of the codons set correctly.

That being said, I doubt it's as simple as "these 10 codons must have these values for this taste receptor to work".

If the mutation only occurred once, yes. But if there is no selection pressure against a broken gene, every time it mutates it will enter the gene pool until the working version eventually disappears completely. (After many years, of course.)

But wouldn't there be an equal chance of the mutation reversing itself? This is true in digital transmissions, why not in DNA?

A gene isn't one bit.

For starters, there are four nucleotides in DNA (A, C, G, and T). Simplifying our model of mutation to "a flipped bit" where mutation takes takes a nucleotide to another one with equal likelihood, and only one nucleotide in that position makes it functional, there is a 3/3 chance that a mutation breaks it but only a 1/3 chance that a subsequent mutation fixes it.

Next, there's the fact that every gene is a sequence of many nucleotides. The odds that the next mutation of the same gene would happen in the same place is low.

Finally, many mutations don't take the "flipped bit" form we assumed above. Repetitions, insertions and deletions also happen, and may be even less reversible (deletion of a meaningful portion would be exceedingly unlikely to be recreated by chance - what provides the information about what should have been there?).

No, this is not true, even in digital transmissions.

Consider this digital signal:

0000000000

Let's say there's a 1% chance of a bit flipping. But any one of these 10 bits could flip. So for, say, the third bit to flip there's a 0.1% chance to become 0010000000.

Now, to go back there's still a 0.1% chance - but there's a 0.9% chance that _another_ bit will flip. At which point you need two bit flips to go back to your original value.

As you can see, since errors accumulate, the probability of wandering back to a working version is much lower than that of producing increasingly broken versions instead.

Because there are so many more ways for things to be wrong than for them to be right.

It's also possible that there's some subtle form of selection against the trait, that we're just unaware of.

Something else occurred that removed those that didn't have that mutation.

Maybe it's a dominant genetic trait?

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