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Why would it be advantageous to have lighter skin at high latitudes?
Why would it be advantageous to have lighter skin at high latitudes?
The most simple reason would be vitamin D production. When you live in climates where you get less sun, have more overcast days and times of the year where they days are very short, your body can cease or have lowered natural vitamin D production leading to rickets.
Darker skin is better able to protect against the harmful effects of sunlight, but with lighter skin more D vitamin can be generated with less sun exposure. Since there's less sun exposure at higher latitudes, due to the angle of the sun and increased use of clothing, the skin will have to be lighter to generate enough D vitamin.
The exception is in populations where vitmain D isn't limited in the diet, for instance eskimos, whose diet consist mainly of fish and other D vitamin rich sources, tend to have darker skin.
Peter
InterestingThe gist here is correct, although the details are a bit more complicated. There are multiple genes that control the production of melanin, and many have them have had mutations under positive selection in human populations outside Africa. For example, a mutation in the gene SLC24A5 is present in all Europeans but not at all in sub-Saharan Africans. The European version clearly started as a single, fairly recent mutation, which spread rapidly because it was advantageous to have lighter skin at high latitudes. In Africa, however, the gene shows every sign of not having recently come from a single copy, but instead of being been around in a large population for a very long time. (Basically, the European version resides on a single DNA background that looks identical in everyone in the population, while the African versions reside on many different backgrounds, showing a long history of mutation and recombination.)
They reproduce asexually, so no inbreeding. Two sexually reproducing organisms will quickly develop severe inbreeding. This is what happens in humans:Why can bacteria maintain a viable long-term population from two 'individuals', whereas, e.g., two dogs cannot?
For a single dog to have all the genetic variety of all dog breeds, it would probably need extra copies of many genes. I don't know any details about dog genetics, but considering the number of different breeds we've been able to coax out of them, there must be a lot of variation involved. A pair of ancestral dogs could have at most four varieties of each gene between them. (Dogs are diploid.) For every gene that exists in more than four versions in the full gene pool of living dogs, you need either extra copies in the ancestors that were then lost, or new variation that arose since the common ancestors. Whether lots of extra genes are a biological problem or not is hard to guess without knowing the details, but it doesn't sound likely.Thanks everyone for your interesting replies. I feel as though I have learnt something.
I have a very different question this time. It is related to DNA but a somewhat different concept to natural selection, as far as I can understand it. Namely, taking the example of the dogs in that chart above. I think it would be fair to say that all dogs have one common first ancestor of dog. Suppose the first ancestor of dog was created with all the DNA of the dogs included in that chart. However the first generation puppies, though having approx. the same amount of variety of DNA, would have no choice but to mate amongst themselves, which in itself would pose no problems with birth defects because they still have a large variety of DNA. However, after a number of generations, these dogs divided up into different families, and each family moved further and further apart from each other. After more and more generations, these families became different 'breeds' of dogs. However, now, none of these breeds have anywhere near the amount of variety of DNA that the first ancestral dog had.
Technically, losing genetic variation WOULD be evolution. We may even be able to reconstruct what really happened, since the gene losses would run in families. So... it wouldn't be an illusion of evolution, but bona fide evolution.Now for my question, if someone were to study the DNA of these various modern dog breeds, would their DNA give the illusion of being evolved via natural selection? Richard Dawkins sometimes mentions the phrase "illusion of design", could an illusion of evolution be possible?
<pet peeve> Not unless dogs can give birth to bacteria or something... Genetic code - Wikipedia, the free encyclopedia </pet peeve>Each puppy would have a distinct genetic code.
It appears that bacteria hasn't really evolved much at all, all these millions and millions of years. Their 'branch' in the evolutionary hierarchy appears to be very short, twig-like, perhaps.
Well, if bacteria has not been left here intentionally
what, would you image, would the world be like without bacteria in the case where bacteria evolved into little bunny rabbits instead, for example?
No question is silly!Hi again, I'm back in this thread with more silly questions.
Actually, they're one of the most diverse (about 10[sup]7[/sup] species) and populous (total biomass exceeds that of plants and animals) groups on Earth.It appears that bacteria hasn't really evolved much at all, all these millions and millions of years. Their 'branch' in the evolutionary hierarchy appears to be very short, twig-like, perhaps.
It's a good question. For whatever reason, bacteria didn't evolve into large, complex, multicellular life - they do form slime colonies that are eerily clever, though.Well, if bacteria has not been left here intentionally, what, would you image, would the world be like without bacteria in the case where bacteria evolved into little bunny rabbits instead, for example?
For whatever reason, bacteria didn't evolve into large, complex, multicellular life -
You mean the bacteria that exists today, right?
Weren't all the animals and us suppose to have evolved from bacteria?
Ah, no. Life is split into three large kingdoms (unless you adopt the 2004 6-kingdom system):You mean the bacteria that exists today, right?
Weren't all the animals and us suppose to have evolved from bacteria?
No, your questions are fun!Hi again, I'm back in this thread with more silly questions.
Bacteria are kind of weird when it comes to evolving. Some of them have looked and lived the same way for literally hundreds of millions of years. Things like the bacteria discussed here are the closest things we have to a true living fossil. On the other hand, they come up with nylonase, evolve to feed on other things they couldn't eat before, become resistant to antibiotics etc. in mere decades. So clearly a lot of evolution goes on in those simple little cells - it just doesn't tend to be as obvious as a new kind of limbIt appears that bacteria hasn't really evolved much at all, all these millions and millions of years. Their 'branch' in the evolutionary hierarchy appears to be very short, twig-like, perhaps.
Not to mention that bunny wabbits power their bodies with oxygen, and guess who makes that? (In fact, guess who uses that oxygen to power the bunny?)Bunny wabbits couldn't evolve without bacteria, because the existence of bacteria is a necessary condition for the existence of bunny wabbits. For instance bacteria fix nitrogen, and nitrogen is part of amino acids, the building blocks of proteins. Also, wabbits require bacteria in their digestive tract in order to digest food properly.
Haha, you managed to stumble on one of the toughest questions in evolutionary biology. Life is nowadays divided into three great domains: bacteria, archaea and eukaryotes. Bacteria and archaea are prokaryotes: they have relatively small and simple cells with no nucleus and little in the way of organelles. Eukaryotes, which include animals, are made of bigger cells that are much more complex inside. The problem is no one is really sure how the three domains are related, and where eukaryotes come from.You mean the bacteria that exists today, right?
Weren't all the animals and us suppose to have evolved from bacteria?
Bacteria are kind of weird when it comes to evolving. Some of them have looked and lived the same way for literally hundreds of millions of years.
Archeopteryx didn't evolve feathers, it inherited them. Likely feathers initially evolved for thermal insulation, but has since been co-opted for display, gliding and flight.
Peter
Whichever species evolved them first. Archaeopteryx couldn't fly - it only used them for their original purpose, which was thermal insulation. Later, one of its arboreal (tree-dwelling) cousins found that these feathers, as well as providing thermal insulation, also aided jumping - you can jump further with feathers.so who did it inherit them from and how were they co-opted into something they were not original intended for?
Whichever species evolved them first. Archaeopteryx couldn't fly - it only used them for their original purpose, which was thermal insulation. Later, one of its arboreal (tree-dwelling) cousins found that these feathers, as well as providing thermal insulation, also aided jumping - you can jump further with feathers.
There was the first species who evolved feathers for thermal insulation. It split into numerous other species. One was Archaeopteryx. Another was a tree-dwelling species that jumped from tree to tree. Its feathers provided a distinct advantage, to the point that natural selection could coop the feathers and make them more about gliding and less about thermal insulation - the trade-off was worth it.
From there, gliding can become better and better with lighter bones, bigger and more aerodynamic feathers, etc. And from there, you get true flight.
Note: I glossed over the exact origin of the feather. Over the generations, speciation occurred, and new species evolved different scale structures, some of which approached what we now know to be feathers. Just when these filament structures constitute true feathers is somewhat subjective. Wikipedia, as ever, has more.
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