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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."
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-...
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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?
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.
we fixed that in software a long time ago...
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.
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