I understand the basic concept of natural selection due to a mutation. So a giraffe is born with an unusually long neck by random chance. It is able to eat leaves from the top part of trees during a famine which kills all short-necked giraffes. Only the long-necked giraffes survive and breed and have long-necked offspring.
How does that work for something complex like an eye or an ear? Obviously we can see why an eye would be a benefit, but a whole eye doesn’t mutate in one generation. So, one individual has a mutation that starts the formation of an eye. That doesn’t give that organism any extra benefit to be like our giraffe friend. That mutation should die out with that individual as it is recessive.
So why did complex structures like eyes come about?
This is one of the oldest chestnuts in evolutionary theory, going back to Darwin himself: evolution of the eye.
The short answer is: no, complex structures like the eye do not evolve all at once, in a leap, but bit by tiny bit, with each little change conferring some selective advantage (or at least, no significant disadvantage). Eyes began as eye spots, small light sensitive patches on the surface of an organism, and continued to evolve from there. Many animals still around today still have just eye spots. (I hope, with that last sentence, I haven’t set myself up for the “Why are there still monkeys?” question.)
You need to read Climbing Mount Improbable. The idea that evolution will occur via large leaps is known as the Saltation theory It’s possible that there are incidences of evolution happening upon jumps like that but mostly evolution works by gradual change over long stretches of time.
An eye could start out as a small patch of photosensitive cells that can detect differing amounts of light and allow the amoeba or similar organism to orient itself in particular directions to find food. Then its descendants perhaps evolve a small niche for the cells to sit in to protect them from outside elements. Over time, various simple structures are created, their roles expanded or changed as circumstances demand and the eye develops in complexity. It’s not necessary for an animal to be able to see perfectly, just that it can see better than its predators and mating competition.
Actually, we understand the development of the eye better than the giraffe, really. To start with, you get a bit of photosensitive area on the surface of an organism. It’s only enough to tell the organism if it’s currently in light or not, but that’s enough to set up stuff like circadian rhythms. From there, it’s pretty obvious how incremental changes that improve the accuracy of the light sense are an advantage to later generations.
In fact, you can pretty much pick ANY stage of the evolution of the eye, and find an organism either currently alive or at least well documented that has that kind of proto-eye.
I’ll fully admit that I have only a cursory (if that) knowledge of evolution. So a “light sensitive patch” was enough for those organisms to survive in great enough numbers, mate with others who had exactly the same light sensitive patches, who then had offspring which expanded on that patch?
There’s no trick here, and I’m not a creationist looking for holes. I just can’t imagine that there was not one step in the process that didn’t produce a non-benefit. If a car was evolving, for example, surely the first rubber patch that began a tire wouldn’t do jack, and that mutation would just die out.
It’s like the story of the hikers and the bear. “I don’t need to outrun the bear, I just need to outrun you.” And they don’t have to mate with others who have light sensitive patches. (Actually, and I am not a biologist, are there many organisms with only light-sensitive patches that don’t reproduce asexually?) If the lightpatch proves a benefit in finding food or avoiding predators, the organism can’t help but flood the ecosystem with more like it.
IOW, let’s say that I had a child who has Superman hearing. He could hear sounds from miles away that would help him avoid danger. I can see how that would help him survive things that regular hearing humans would be killed by.
However, for that to continue to the next generation, it would have to be of such a sufficient step that vast majorities of regular hearing humans are killed by such a thing. Further, at least one other person, of the opposite sex, would have to have the same billion to one mutation that he did and they would have to meet and have children. And then hope that the child inherited this super hearing gene. And then that child would have to meet someone else who had the super hearing gene, etc.
It seems like such an outlandish thing wouldn’t happen but in extreme circumstances.
I don’t know about only light-sensitive patches, but the tuatara has a “third eye”, a light-sensitive patch in its forehead in addition to its two fully-developed eyes.
No on both counts. The first organism with some new gene needs to be somewhat lucky, since the gene isn’t likely to do anything with only a single copy, and even if it does, you’re still probably in a population that already does reasonably well, so surviving a little better probably isn’t too big a deal. But let’s suppose that that first individual is lucky, and just by chance happens to breed and produce descendants, a few more of them per generation more than average. Eventually, two of those descendants who happen to both have that novel gene will mate with each other, and some of their offspring will finally express the new trait. They won’t need as much luck to do better than their peers, since they’ve got this new advantage, and so the gene for that trait becomes a little more widespread, and gradually becomes prevalent in a large population.
Now, you might object, what if the progenitor isn’t quite so lucky? Well, then, that particular mutated gene just happens not to catch on, even if it might be beneficial. It happens. In fact, genes that would be good but happen to lose the dice roll before they have a chance are probably a lot more common than the ones that win out. But give it enough time, and it happens eventually.
Why do vast majorities of regular humans have to die off? If the “super human” has children, he’s passing along his genes to the next generation regardless of what happens to everyone else.
Not at all. His children get half their genes from the super-human and half from his normal wife. On average, half the kids will have super-hearing and half won’t. Over multiple generations the super-hearing gene will spread through the population. If it provides some survival advantage it will spread faster, but it will spread even if it doesn’t provide any advantage at all.
So to continue the hypo of my son with super human hearing. He must be lucky enough to do so much fucking that he has a gaggle of kids. Then, say, a hundred years later, some of those kids mate with each other producing more super human hearing people. Then, over time, the odds favor the super human hearing so that in millions of years, the super human hearers are now dominant to the point that the whole population has it?
Again, from my limited knowledge, I assumed that such a mutation would be recessive so that the kids from him and his normal wife wouldn’t have such super human hearing. Then those children would have no special advantage and would die off at the same rate as everyone else.
Or is it as mentioned above that it is just a random chance that these people with mutations also reproduce more than average so that pretty soon, there will be some distant incest going on?
Even if the mutation is recessive it will gradually spread through the population. After many generations there are enough people with the recessive gene that you start seeing situations where both parents have it.
You are substantially underestimating how fast a gene with a slight advantage can spread. Genghis Khan probably didn’t have much of a genetic advantage over other men of his time, but in only 750 years, his Y-chromosome signature is found in 1 in 200 men - and since that genetic signature would only exist in men who are sons of sons of sons of sons… of Genghis, that means that it is very likely that most of the Earth’s population is a descendant of Khan.
Similarly, the few people with a gene that allowed them to tolerate lactose as adults (a gene that apparently appeared only a few thousand years ago) have spread that gene (which provides a much smaller advantage than the advantage that you are postulating) throughout a large proportion of the world’s population.
The evolutionary origin of eyes appears to be in our distant photosynthetic ancestors. As single cells we had chemicals that allowed us to turn light energy into chemical energy. We retained those chemicals even when abandoned photosynthesis because they allowed us to detect where in the water column was most suited to photosynthesis, and thus where we would be most likely to bump into prey. Note that at this stage we couldn’t *see *our prey, we just tumbled around aimlessly until we bumped into it, then we grabbed it. But the ability to detect light was gave a huge advantage int hat it kept as int he same general area as our prey and increased our odds of bumping into it. From that unicellular beginning we moved into a multicellullar form that was basically a floating ball. We retained the ability to detect light for the same reason.
We then settled down on the ocean bottom as a creeping sheet. At that stage we started to develop a head. That was the only sensible thing to do, because by having a preferred direction of travel we could concentrate our attacking organs in the one part of the body rather than having them scattered all over. That meant that when we did encounter prey, we had a much better chance of killing it. Once we had a head, it also made sense to concentrate all the light sensitive chemicals in that region, so that when we crawled out from under a rock and were open to attack from above, we knew about it. So then we were basically a worm: a sheet of cells with a distinct head.
Having the light sensitive cells in the head was an improvement over having them scattered over the whole body, since it let use know when we had left cover, but it still only gave general data about the light levels on the head. It couldn’t detect edges or direction, so we couldn’t know wether we were hiding under a leaf that could be lifted off us or under a rock. And we couldn’t tell if it was dark because we were totally hinder under a rock on bright day, or because we were half hidden on a cloudy day. We could detect relative levels of light and dark, but we had no resolving power.
To do that, we needed to really concentrate all the light sensitive cells into a few points, so that we knew *exactly *where light was falling on us. If we move an inch and the light goes away, we then know to within an inch where the edge of our cover is. The best arrangement was to have one light sensitive spot on each side of the head, that way we would not only get resolving power, we would get the ability to detect direction. If we move left and it gets bright, we know that the rock ends on the left.
That’s a big improvement on diffuse light detecting abilities, but it still only allowed extremely close range resolution. It let us know that we were under a rock, but it didn’t help us find rocks to hide under when we got caught in the open because it could only resolve direction at very close range. In essence all it did was tell us where shadows started and stopped, which is useful, but limited.
However if you put the light detecting cells onto a hemispherical surface, you can now detect direction was well as edges. You can generate a hemisphere as either a convex “dome” or a concave “bowl”. Either works in theory, but by having a “bowl” recessed into the skin you protect the light sensitive tissue from getting covered in crap.
Because the edges of the bowl cast shadows, you know that when the top of the hemisphere is in shadow and the bottom in in light, the light is coming from directly above, but when the back only is illuminated, the light is coming from directly in front of you. Simply by making the light sensitive spots concave you can now see what direction shadows are coming from and move towards them. IOW you can use these spot not only to stay under cover, but to move towards it.
The only flaw with that is that it can’t be adjusted for light levels. If you optimise it to work at night, then the reflections of the noon sun will cause so much glare that you will no longer be able to detect the direction of the light. And if you optimise it for noon, then there will be so little light at night that you will be blind. The system still works great at dawn and dusk when you do most of your hunting, but it woudl be nice to have a little more flexibility. That is easily done by optimising it to night, and then “squinting” by folding the skin in around the edges of the bowl, so less light can get in during the day.
The next step is then to add a layer of transparent skin over the bowl to protect it further from damage and keep it clean.
And if the skin over the spot is just a little thicker in the middle than at the edges, the light will bend before it hits the tissue. By doing this you can get a really sharp image of objects at a specific distance. So rather than just knowing that *something *is casting a shadow in front of you and lightly to the left, you will now be able to tell that it is more or less than a foot in front of you. So you know whether it is worth running towards it if you are attacked. You will also be able to detect really sharp edges on objects within your focal distance, between 4 and six inches. That means that within that range you will be able to detect which way your prey or predators are facing, and react accordingly. That’s obviously a huge advantage.
This is great, but you have a fixed focal length. Objects more than a foot away are still just shadows, and objects that aren’t between 4 and 6 inches are blurry. But if you contract the muscles in your face, you can stretch the transparent covering over the eyes, making it change shape. By doing that, you can force it to focus on closer or further objects.
And at that point you basically have an eye. You have a lens with variable focal length, a retina, an iris and a cornea.
Note that all the steps along the way to the creation of this eye are perfectly useful all by themself. There are blind humans who would love top be able to just detect night and day or to be able to detect shadows, or be able to see objects sharply even if only at one distance and so forth. None of these “partly formed” eyes are anything less than spectacular advantages over organisms that don’t have them.
Starfish have eyespots. They also spend a lot of their time moving around (albeit slowly) and/or not firmly attached to the substrate. If they detect a shadow, they quickly activate their suction cups and stick firmly to the rocks. If the starfish in the next tide pool is blind and does not react to a shadow, it’s the one more likely to get eaten by a hungry gull.
Imagine a population of proto-starfish without eyespots and a single one with shadow-detecting capability. If the average “blind” starfish has two offspring that reach maturity, and the a erase “seeing” one has three, the “visionary” starfish will win eventually out over a number of generations. It certainly doesn’t have to happen in a single generation, nor is there any reason to assume the gene is recessive.
Most of the non-parasitic worms… and that covers about 12 phyla.
No. You are failing to appreciate that modern humanity is decidedly atypical.
In nature, populations are stable. That means the vast majority of all organism are killed. If a fish produces a million eggs a year for 50 years, then 49, 999, 998 of them must die before ever producing offspring.
It is that constant competition for survival that translates tiny advantages into massive differences. If a fish expresses a hearing mutation that means that only 49, 999, 900 of her offspring die rather than 49, 999, 998, then within just a few thousand years every fish in the world will be carry that mutation.
This doesn’t require that the vast majority of other fish die due to lack of suitable hearing. All that it requires is that they die at a rate that 0.001% higher than that of the mutants. What they die of doesn’t matter. If lacking the mutation means they get just a little bit less food and die just a few days sooner during a famine, or it means that move just a thousandth of a second sooner when that shark leaps out at them, it doesn’t matter. Allt hat matter sis that they die just a little tiny fraction more frequently. Not a majority of them, not even a significant minority of them. Just one in ten thousand more than the fish that carry the mutation.
As Inner Stickler says, they don’t have to run faster than a shark. they just need to run slightly faster than there siblings.
Nope. The gene is found within the mutant. That is all that is required. It will be inherited by 50% of the children regardless of the mother’s genetics.
I’m not sure why you believe that the genetic makeup of the other parent matters in any way at all. If I have the genes for black hair, and my wife has none of those genes at all, then >50% of my children will have actual black hair and 100% of them will inherit the genes for black hair. The fatc that my wife doesn’t carry those genes doesn’t have any effect on whether my children inherit them.
At worst, my wife’s genetics may affect whether the gene’s are expressed, just as my genes inhibited the expression of her “blonde” genes. But that doesn’t mean that all my children don;t carry those blonde genes. They do. All it means is that it will take a little bit longer for them to be selected for. If blonde genes are beneficial, it might take 5, 000 generation for them to dominate the gene pool rather than 1, 000.
Nope, all that is required is that he has a *single *child so the mutation is not lost.
At some point his descendants must produce marginally more children than non-mutants, but not a gaggle more. Just one milllion and one children to every one million produced by non-mutants will guarantee that the mutation will dominate given sufficient time
It doesn’t matter who they mate with,
There is no reaosn for cuh an assumption, but even if it were recessive…
Yes, that’s the point. The children die at the same rate. They don’t die any faster. So even if the mutation doesn’t give him any advantage at all he will produce 2 kids and 4 grandkids and 8 great grandkids.
Them in a “natural” setting the great grandchildren will *inevitably *marry their cousins. And then the recessive gene gets expressed. So at least one of his 16 great-great grandkids will express the mutation and that kid will have an advantage. That means that the superman, long since dead, will produce 33 great, great great grandkids when every other man alive only produced 32.
Even if that is the *sole *advantage that the mutation ever gives, in just 4 more generations the superman will produce 69 million descendants and every other man will have produced just 67 million.
But of course that isn’t the only advantage. Every generation will see more children expressing the gene just through random shuffling. And the increased survival odds of those children will lead to even more expression as a greater proportion of potential partners carry the gene.
Even if the gene only confers a one in a hundred survival advantage to those expressing the gene, not just carrying it, then within 10 generations fully 10% of the population will carry the gene. Within 16 generations 90% of the population will *express *the gene.
A tiny probability with a sustained advantage produces huge changes over time.
It’s not distant. For most of human history most people were marrying their first and second cousins. Remember for HGs your entire marriage pool was usually less than 100 people. That represented everyone in the entire world that you could legally have sex with. It wasn’t distant inbreeding. It was first and second cousin marriage, and everybody you would ever meet was your third cousin at the very least.