Back on track then: Polar water is cold.
It makes sense that animals living in polar water would have to come up with a strategy for coping with this cold, salty water. Unlike marine invertebrates, saltwater fish osmoregulate. That is, their internal salt composition is much like ours, and is hypotonic in relation to the water around it. Saltwater typically has over 1000mOsm/L, while blood has an osmolarity of 300 mOsm/L. This is important because at 1100mOsm/L, the freezing point of water is about -2C but at 300mOsm/L is -0.6C. This means that a fish can swim in water colder than the freezing point of its own blood. And for the liberal arts majors out there, having ice water in you veins is only good as a figure of speech. Never freeze your fish. NEVER. Ruins the texture.
Jellyfish, squid, and other marine invertebrates solve this problem quite simply: they keep their internal composition the same as the water around it. It's the marine equivalent to being cold-blooded. It's a straightforward solution, but it does have its tradeoffs. Nerves and muscles aren't quite as reliable, and they are very sensitive to changes in the sea around them.
But vertebrates want to have their cake and eat it, too. Fortunately, this is possible. Evolution to the rescue! Enter the antifreeze glycoproteins, or AFGPs of the common arctic cod. These are really cool little proteins dreamed up as an alternate use of some cleaveble precursor gene. Basically, it's this long chunk of protein that is nothing but a repeat of threonine-alanine-alanine over and over again, with an occasional proline thrown in. I'll get to the proline later.
The amino acids in this repeat are ripe for glycosylation, the attachment of long chains of carbohydrate to protein. Glycosylation is not an unusual thing, you find it damn near everywhere. Snot is a bunch of glycoproteins (glycosylated proteins). Antibodies are often glycosylated, many membrane proteins are. It is often even a regulatory event, telling a protein where to go or what to do. But in this case, the purpose is a little more ingenious.
The long chains of glycoprotein act as a type of antifreeze, disrupting the formation of tiny ice crystals in the blood, reducing the effective freezing point while keeping the osmolarity at an acceptable level. Those prolines? Cleavage points. Make one long protein, glycosylate it, and chop it up into three amino acid bits. Beautiful!
Let's head south. There are fish there, too. It's cold there, too. And these fish also osmoregulate. And they have been a separate evolutionary branch for a pretty damn long time...longer than the AFGP gene has been around. And they don't mix. Just like tropical fish don't fare well in cold water, arctic fish generally don't fare well in the tropics. So how does an antarctic fish solve this quandary? Exactly the same way
The antarctic icefish Chaenocephalus aceratus has an AFGP with the repeat thr-ala-ala, just like the arctic cod, Boreogadus saida. The cod repeats are glycosylated, just like the icefish.
The genes are not related, neither are the animals. See? But don't take my word for it, or theirs. A picture is worth a thousand words: this is an arctic cod; this is an antarctic icefish. Can you see the family resemblance? I think the milkman was involved there somewhere, and he was one ugly sonuvabitch.
The proteins are virtually identical, but the genes are not. The intron/exon setups are very different. Icefish use an arginine-glycine cleavage point instead of a lone proline. They are very different in origin, but identical in purpose and function. They do the same thing in the same way, but they got there by different routes.
This is a classic example of convergent evolution, two completely different organisms that develop similar characteristics. You see it in marsupials and eutheria (placentals). Compare a koala to a monkey, or a thylacine to a canid. Hedgehogs, porcupines, and echidnas have all developed very similar spiny protrusions as a defense, but their most recent common ancestor was a contemporary of the dinosaurs. Puffins (auks) and penguins are widely geographically separated. Their common ancestor could fly, and lived about 70MYA. Sharks and tuna have similar swimming strategies, using muscles in a bulge near the center of the body to swing the tail for propulsion, allowing a streamlined shape and the mechanical advantage of a lever. There are many examples, and they all follow the same general rule: The same or similar solution to the same problem, arrived at by different paths.
It doesn't always happen. Batwings and bird wings are different solutions to the same problem. If those aren't different enough for you, compare both to insect wings. And there are flying bugs as big as some birds, the goliath beetle can weigh 100g. Some hummingbirds weigh close to 1% of that.
So what's the point? You people should already know about convergent evolution. Well, sorry, that was all just a working up to the real point here. An undisclosed number of years ago, I took a bunch of evolution courses in college. Evolutionary genetics, comparative anatomy, evolutionary principles, and possibly a few others. The undisclosed number does have more than one digit, and I haven't looked at my college transcript in a while. We really dove into convergent evolution in some of those classes; it's a pretty cool thing and fun to ponder over. But to me it always flew in the face of one of the primary principles of evolutionary biology, that evolution has no purpose or plan. It is random, it has no destination or goal, it is simply a force of nature, like the wind.
But even that analogy falls flat on its face. The wind does have a "purpose" of sorts, it equalizes air pressure between two points. Has anyone ever seen wind blowing from low pressure to high? Evolution does have a purpose. To steal a phrase, it is to "be fruitful and multiply." And no, this is NOT a proposal of intelligent design. I do not subscribe to that bunk. This proposal neither invokes nor requires God, avian pasta, invisible pink unicorns, or anything else outside the realm of science. It invokes only the physical world.
There's not a complete design behind evolution. There was no blueprint 4.5BYA that had an Archean genome, biology, and structure spelled out. But the design requirements were there. It must be able to use one or more of the energy sources available, which are x, y, and z. It must be able to withstand temperature T, pressure p, chemicals ad through nauseum, and be able to make copies of itself. It must not be set to destroy its own environment. It must be able to adapt. Proto-organisms that did not meet these design requirements didn't make the cut. They were discarded, so to speak.
Same thing on down the line. The changes introduced must improve the ability of the organism to thrive. Too much oxygen? You need to cope with that. Some organisms just buried themselves and became anaerobes. They aren't heard from very much any more. Others decided to use this new ox-y-gen, and many of their descendants are now doing quite well. Food's in the trees? You'll need a way to get up there. Come up with one, there are several. Climb, fly, grow a long neck, it's up to you. But the best one gets the food, and therefore wins. The "coming up with designs" is random. There is no other source. However, the new designs (i.e., new genes) are just the fuel for evolution, they are not evolution itself. Evolution cannot be random, it is guided along certain paths by the nature of the environment. This includes not just the physical inanimate world, but other species as well. Evolution changes its own design requirements. Light => plants => herbivores => thorns => herbivores with thick skin => poisonous plants. Predators get spun off as well, as do pollinators, nitrogen fixers, parasites, and so on. A niche shows up with certain characteristics. The organism that comes along best able to exploit this niche is the winner and gets to modify the environment a little more to form another niche.
The source is random, but the destination is not. There may be an infinite number of designs, but there is a finite number of successful designs for a given niche. The success or failure is determined by the environment.
Now please, discuss.