[gort]

> Um. If this gene was the result of a mutation, isn't it possible, at least in theory, that mutation happened more than once? At two different times? At two different branches on the species tree?

Genes are long sequences of information. You could easily have quite different genes doing similar things, but if you have the very same gene (sans minor differences) it's just too unlikely to have originated completely independently twice.

The closest thing I can imagine to your scenario is where, because of shared ancestry, 2 species have a certain gene, and then some minor mutation hits both of them, and now they both have some other gene. I suppose that's possible. But they already shared the original gene to start with.

[chiefalchemist]

> " it's just too unlikely to have originated completely independently twice."

Unlikely, but not absolutely impossible. Correct?

Editorial: This is where Science / science loses me. It makes absolute statements that aren't in fact truly absolute.

The point being, __if__ there was a chance identical mutation that changes (a lot?) of things. Truth be told, life coming into being has to be a couple orders of magnitude coincidental than some gene mutation. In that context, "too unlikely" starts to feel much less so, yes?

[matte_black]

It is possible for every atom in a quarter to suddenly move in the exact same direction at the same instant, temporarily lifting the quarter off a table. But it won’t happen. It has likely never happened in the history of the universe.

[lisper]

> Unlikely, but not absolutely impossible. Correct?

Correct but irrelevant. Winning the Powerball 100 times in a row is also not absolutely impossible. Nonetheless, it won't happen.

[throwaway2048]

Science is fundamentally empirical, it serves no purpose to say "with extremely high likelyhood" behind every statement, its implicit in any scientific context.

The only thing this encourages is crap like creationism and flat earth theory because there is no way science is ACTUALLY SURE.

[xcvbxzas]

I don't think these statements are as absolute as you take them. I'm also not sure where you go if "almost certainly did(n't)" is no longer sufficient. At that point you basically have to give up on saying anything about anything.

Science doesn't (intentionally) forget about unlikely alternatives. That doesn't mean there is any good reason to treat highly unlikely scenarios as the equals of the vastly more likely cases.

And arguing anything about chances based on things that have already happened is kind of bullshit. They already happened, however unlikely. We know unlikely things can and do happen occasionally. That does not mean other unlikely things are more likely to happen.

And if you really insist on doing that, keep in mind the timespans here also differ by orders of magnitude.

[candiodari]

I know that this is true given the pure combinatorics of the situation. You think that it's a chance of 1/4^250 (or so), because it's a gene BUT that very much depends on the error surface in that high-dimensional space, and not so much on the exact combination.

In other words, if you need to hit haemoglobin exactly, odds are absurdly against that. But if there are 1e30 different genes resulting in substances that all increase oxygen saturation of blood, that then "coalesce" on haemoglobin as a result of optimization. In other words, I'm asking "haemoglobin" (and variants) are the bottom of a valley of an optimization process. But for evolution to "find" haemoglobin, it doesn't need to hit the bottom, it only needs to hit the valley. So it matters a great deal how big the valley is, and such a valley can be quite big. And I get it: we have no hope in hell of figuring out how big the valley is, so we just take this as answer.

So "What good is half a wing ?". Well if 1/1e30th of a correct gene already works, then "half a wing" can be quite bad and yet result in a wing. DNA is an optimization process, and if that was the defining change being selected against, is it that hard to imagine that it would converge on haemoglobin given 1e30 starting positions.

If you look at online evolution simulators, the ones with the wheel racing [1], then the "spikes for and aft, small wheel forward, big wheel aft, with a tail spike to prevent tipping over" could be haemoglobin in this example. The valley surrounding that optimal outcome is huge : it's essentially the whole universe in that case, which is a "gene" with 14 float32's and 2 integers. How many combinations is that ? Quadrillions, at least. And yet, all roads lead to Rome, or at least to the tailed bigwheel.

Of course this doesn't even seem to apply to haemoglobin. Haemoglobin and chlorophyll[2] aren't that different (in fact the gene is identical, or at least there are genes that code for chlorophyll and genes that code for haemoglobin that are identical, so ignoring variations, they're actually identical. The difference is not so much in the gene itself but what happens to the molecule after it's created, it's in the "meta" information in the gene, not in the transcription part). So what really needed to happen is a screwup in the haemoglobin gene animals inherited from plants, followed by a few hundred generation of fine tuning. (in fact, that molecule does other stuff too, animal blood, plant photosynthesis, and (most) plant colors, as well as some aspects ATP generation (and I'm sure there's more, we just haven't found those functions yet) have a very similar chemical basis, and therefore are likely regulated by very similar genes).

That could have happened 1000 times. Easily.

[1] http://rednuht.org/genetic_cars_2/

[2] https://patch.com/georgia/cascade/bp--hemoglobin-vs-chloroph...

[kryptiskt]

For most amino acids, there are several DNA triplets that code for it (there are 64 triplets and only 20 amino acids). Even if there is one magical optimized protein that the species converges to, there is no evolutionary pressure for the triplets to be the same in the two converged genes.

[sulam]

The word you're looking for is "converge", not "coalesce", as in convergent evolution, which is well known. Here's a quick find using the googles: https://www.nature.com/articles/nrg3483

[lkrubner]

If a mutation in a given gene leads to death 99.9999% of the time, but leads to oxygen transport the rest of the time, and the mutation happens many millions of times, then it is reasonable to think it could have been invented twice. Most of the time the mutation leads to death, but every once in a million times it leads to a happy result. Over the course of a few million years, the mutation happens many millions of times. In that scenario you would expect to happen at least twice, and probably much more often.

As an example, write a simple program that randomly generates strings of letters from 1 to 20 characters in length. Then include an English dictionary in the program. Compare the randomly generated strings with the words in the dictionary. How often does the random process generate actual words? Not often, but sometimes, and that is all that is needed.

[Sharlin]

If genetics were that fickle life would have never happened. The vast majority of mutations are neutral or affect the functionality of the resulting protein only by a matter of degree.

Your example totally ignores how selection works. Life does not generate genes randomly and see what sticks. Prebiotic chemistry might have worked like that to some degree. But at the point oxygen transport was invented every gene in an organism would already been subjected to billions of years of selection pressure and the gene or genes that eventually came to code for hemoglobin would already have had some other, related purpose.

[rlpb]

What if there are a million different possible genes that produce hemoglobin? In that case, if two different species were found to produce hemoglobin but use the same gene, then what would that imply? I think this is what grandparent meant by "You could easily have quite different genes doing similar things".

[gort]

Sure, but this is only possible if the original (shared) gene was really quite close to being able to transport oxygen.