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How Predictable Is Evolution?
sciencehabit (1205606) writes "If the clock rewound, would organisms evolve the same way they did before? Humble stick insects may hold the answer to that long-running question in biology. Through studies of these bugs, whose bodies match the leaves the insects live on, researchers have found that although groups of the bug have evolved similar appearances, they achieved that mostly via different changes in their DNA. 'I think it says that repeatability of evolution is very low,' says Andrew Hendry, an evolutionary biologist at McGill University in Montreal, Canada, who was not involved with the work."
Convergent evolution suggests it is somewhat predictable, unrelated species having evolved similar solutions to similar problems. If a solution is clearly better nature will tend to go there given sufficient time and experimentation (mutation).
The fact that a trait may be expressed by different DNA sequences doesn't really seem to undermine this. The DNA sequences are implementation details. Evolution is about solutions and environments not DNA sequences.
This tells us that getting a sensor is repeatable. There are high-level design details of eyes that are divergent across species. The "blind spot" is a flaw in the eye design that is shared by all vertebrates, but cephalopods don't have it. Either it's very hard to mutate our way out of the flaw, or the flaw is by itself not important enough for the extraordinarily rare mutants who evolve their way past it to gain any ground on non-mutant populations.
It's easy to think of that as an accident of fate, and eventually such accidents are bound to build up into going a different direction in response to strong selection pressures.
I think sharks and dolphins is better than sharks and whales. That demonstrates convergent evolution -- but note that dolphins still have lungs, and sharks still have gills. They got to similar body plans but they are not fundamentally the same.
The degree of molecular similarity in the DNA changes to achieve a particular result will depend strongly on the type of change one is looking at.
For the case of toxin-resistance, which is much closer to the molecular level, the odds of similar changes to the DNA are much higher than for complex morphological changes.
Molecular changes like toxin-resistance are more likely to involve a single gene that codes for a single enzyme, changing the enzyme so that the toxin is no longer metabolized in a harmful way. There are going to be a very limited number of ways to do this because it's pretty close to a one-gene/one-enzyme mapping in many cases.
Morphological changes, on the other hand, involve a whole network of genes that are turned on over the course of development, and the network can be altered in many different ways to get to the same result. Think about it like a road network where you're used to taking a particular route to get from A to B. If a bridge goes out on your your usual route, you may choose different alternatives depending on time of day, the kind of vehicle you drive, etc. Networks create choices.
Even then it will depend on the kind of morphological change we are talking about.
For example, there is a lizard in Mexico, which was studied in the '80's or '90s. There were several related species living inland, and a couple of isolated species on the coast near the Yucatan peninsula. Both the coastal species had an extra cervical (neck) vertebra, and it had been assumed on the basis of this similar morphology that their evolutionary history had been a general migration to the coast, an adaptation to coastal environments that involved having a longer neck, followed by a general die-back that resulted in the two existing but separate populations.
It turns out based on their genes the two coastal species hadn't had a common ancestor for millions or tens of millions of years, and the adaptation to coastal living had happened independently but fairly recently. In this case, because certain aspects of body plan are controlled by a highly conserved and relatively simple set of genes, the additional vertebra were the result of similar sets of genetic changes.
Things like body width, which is what TFA is talking about, are a lot more complicated in their regulation, so more likely to be achieved via different genetic changes that have the same morphological outcome.
I'm going to throw in a shameless plug here because it seems relevant to the topic at hand. I've just published a hard SF novel that's premised on a what-if about the role of mathematics and law-like descriptions in evolution. If you're interested in that sort of thing you should check it out: http://www.amazon.com/Darwins-...
Blasphemy is a human right. Blasphemophobia kills.
If I'm reading this wrong, and I hope I am, please let me know.
...researchers have found that although groups of the bug have evolved similar appearances, they achieved that mostly via different changes in their DNA. 'I think it says that repeatability of evolution is very low...
I read this as "Stick bugs have reached similar appearances through different means thus the same change probably won't make the same result".
Is this equivalent to "People can change their appearance to include a hole in the abdomen through different means (bullets and knives). Thus shooting or stabbing people are unlikely to produce holes in people"?
It may make it more difficult to guess which DNA change caused them to look like that (without an actual DNA test), but it in no way implies that those DNA changes won't necessarily cause them to look like that.
you are silly, vast differences in eyes in the animal kingdom. the spookfish eye has a side chamber with mirrors and a second retina, and works like a reflecting telescope. The Tarsier can't even move its eyes in the sockets, has to turn its head, besides night vision can see in ultraviolet but can't see color. The collosal squid has a built-in headlight, a photophore, in each eye to illuminate what it is focusing on, the dragonfly has 30,000 eyes that can see polarization of light as well as ultraviolet let, and moreover has 3 additional eyes of another type that are hypersensitive to extremely fast movements a human can't perceive. How about four-eyed fish with eyes to see in air and another pair for water?
The key question is whether the same results would come from different ends, again.
And the key evidence is that parallel evolution uses different changes from different genes to achieve the same end.
The question that I have to ask is, if different changes result in the same end, can the follow-on changes result? Or are they stopped?
Flippers turn into hands, but using different gene combinations - does that stop the thumb from differentiating? Or would evolutionary pressure still reward the mutant with the thumb?
I haven't read the whole thing, but I'm not swayed on any part of the question other than someone is now thinking about this. It is far from the foregone conclusion you think it is. In fact, in your statements, it stops at the interesting point. Will eyeballs that evolved differently be able to further evolve in similar ways? Or are they forever doomed, due to their makeup of different proteins, to be different? Or is it somewhere in the middle, which sounds plausible pending further research?
Looking at cows, dolphins and horses genetic proximity shows unexpected results, as cows and horses are not the closer in the trio, despite their similar features.
That suggests environment drives evolution in a predictable way, while the genetic evolution is not. This is the really amazing point: evolution find similar solutions to similar problems, but it does so through different ways.
I think the answer is self-evident: alternate reality results would be just as diverse as species are today, and while they would bear superficially similar results, they would be "different animals." Commenters above have noted that vastly diverse organisms in a common environment still successfully evolve common features: they may have similar means of locomotion, means of food detection, means of sexual partner selection, and on and on, yet the specifics for any given species will be completely different from the other species.
Would the appearance of an opposable thumb on a flipper cause the lengthening of the appendage into something more useful, like an arm? Maybe, because arms are a useful advantage for food gathering; or maybe not because arms aren't as hydrodynamic as flippers. Or maybe there'd be a fork with two successful species resulting. I don't think the follow on changes would stop, they just would be different changes.
But as to the original article, why would anyone think that if we rewound the clock that a chaotic process would repeat? It's not like the universe called rand() with a common seed when it started mutating DNA.
John
also could have mentioned some bird's eyes that can see the earth's magnetic field; and goats with their horizontal rectangular pupils, which combined with the eyes position on the skull gives them a 340 degree field of vision without even having to move their eye. they can see you coming up behind them!
Many people don't actually know they have blind spots so I'd say we don't need to fix it.
we fixed that in software a long time ago...
Can we get a Star Trek like movie but instead of meeting human looking weirdos in outer space, let's meet species that look really weird, yet make friends with us and we commnunicate. Like Octopuses, and Snake-people, bug-looking-people, birds with intellect, Koala bear looking chess players, etc.
It's not that simple. Something like a blind spot can't just be evolved away. There needs to be a pathway from "has blind spot" to "doesn't have blind spot" that doesn't go through "vastly decreased eyesight" along the way. Otherwise evolution will stick with what it has, and no amount of selection pressure can cause it to change.
We're vastly suboptimal in many ways. We're not perfectly tuned machines, we're cobbled-together from evolutionary scraps, and you can see it by looking at any part of our physiology. That's precisely the thing that makes intelligent design a stupid idea. Yet, we "work", and are capable of survival, and that's enough.
A fool and his hard drive are soon parted.
There's a great book by the artist Wayne D. Barlowe, called "Expedition". it shows the life forms of a fictional planet called Darwin 4 . With dense atmosphere and low gravity, Everything evolves big, and almost nothing has anything like eyes (sonar is both popular and often very advanced). Without giving too much away for those who still haven't run across this, there are several common body plans that tend to run through whole phyla, and which don't occur on Earth, but make really good sense on Darwin 4. The underlying science is generally sound - I base this on the way various people who have read it point to this or that creature as less probable than the others, but seem to pick out different ones. This book has become my standard for SF aliens.
Who is John Cabal?