Evolution (and randomness)

[BlueScreen]

I went to see an eye doctor today, and being a physicist I got thinking about what a complex system the eye is. How does something like that evolve?

It's all good having small changes in cells and stuff. But what kind of mutation creates a lens system accidentally? And how much more random processes does it take to breed a creature with a perfectly functioning lens system with aperture control and receptors?

There is no reason for a creature with a half constructed lens system to have an advantage over any others of it's species. And I can't see it evolving in one step.

These parts of evolution have always puzzled me. I don't believe in intelligent design or anything, I'm looking for science answers. I've never taken biology. What are the generally held theories on evolution these days, and how do they describe the processes driving it.

 

[nozflubber]

I don't think complexity starts with new mutations - it HAS to begin small and subtle for reasons you stated. Basically, somewhere along the line a biological mutation was made in cells that made those particular cells react/sensitive to different forms of electro magnetic radiotion. Think of something starting off with small/simple structure.... like primitve spider eyes or something, that was the first photo-sensitive cells that gave advantages. the rest could just be developmental history

 

[JocktheMotie]

[YOUTUBE="http://www.youtube.com/watch?v=Yj_lNQerUJ4"]Evolutionary steps of the Eye[/YOUTUBE]

Here you go.

 

[BlueScreen]

Cool . Thanks to both of you.

 

[Feops]

Can't wait to get home and view that clip. I'd love to hear how something so complex came about.

At a guess.... basic photosensitivity seems simple enough. An attaction to light (energy) would be a basic survival trait. Plants do that just so they don't grow stupid and they don't have eyes.

From there, perhaps multiple distinct photosensitive patches to discern direction? From there, I could see a socket developing to better estimate depth and focus energies on increasingly specialized photosensitive patches for color and shade. There would be huge evolutionary pressure as the blind would be destroyed quickly.

Little stumped at this point on how the ball could form. Maybe a film, then a lens, then increasingly specialized lenses?

 

[cascadeco]

In the 'Blind Watchmaker', Dawkins devotes a handful of pages to this very subject - the eye.

Basic premise is that an organism with even the most basic 'vision' - photosensitivity - could have some sort of survival advantage over one with none at all.

Using humans as an example -- someone with vision impairment, who requires glasses but still sees fuzzy shapes and can navigate around objects without glasses, is going to obviously be in much better shape than someone whose vision is so impaired that they can only see vague shadows/lights. And someone who can only see vague shadows and hardly anything at all is going to be a bit better off than someone who can't distinguish anything at all.

And...just remember it's not like evolution is something that has a set goal in mind. It's nothing more than those individuals who have better survival mechanisms, and thus produce more offspring (in theory), who will then shift the overall gene pool of the population to include their traits. So then slowly the entire population shifts in that direction over time.

 

[sculpting]

Many species of bacteria actually posses a photosenstive clump of cells at one end that allow them to sense light. Also I think I'd look towards really simple sea based animals and see if they had the same features.

 

[ygolo]

Here is a series of lectures that is rather illuminating.

RichardDawkins.net

 

[Qre:us]

The evolution of the eye is quite fascinating as the eye is such a complex organ. Convoluting this matter is also how the eye does what it does....which we have yet to get a clear picture about. Esp. in terms of visual perception where the retinal image is not a direct translation by the brain (as what the image is on the retina, we do not 'see'), but, rather there is *something* going on with visual processing that allows us to see what we see. E.g., why are images upside down on our retina? What is the evolutionary advantage of this method versus direct translation of a right-side image on retina, to, processing in brain?

It's not simply about figuring out how the eye came about, but, to comprehensively answer this question, we must first find out HOW the eye/brain works, and work backwards.

* aside - I think it was Stratton and another dude, Kohler, who made these upside down glasses (glasses inverted images/world), and had peeps wearing them. With practice, we can get used to navigating in an upside down world. this raises quite a few questions regarding visual perception and what it means for the interpretation of images by the eye to the brain.

 

[Blackmail!]

like primitve spider eyes or something, that was the first photo-sensitive cells that gave advantages.

Actually, some spiders developped very efficient eyes through evolution, with stereoscopic, binocular and even color vision. Think for instance to the jumping spiders (Salticidae). Even if their range is more limited than ours, their accuracy is quite similar. Thus, you can tell when these spiders are staring at you.

Even if these animals are very tiny, they have a huge brain compared to their body mass, and it is mostly dedicated to image processing.

 

[Blackmail!]

Actually, there is a fact that proves our eyes weren't "designed" at all, because they are far from being perfect. Their design contains many oddities or absurd details we inherited through evolution, but they weren't removed because they weren't impairing enough.

For instance, there is a layer of cells in front of the retina. Octopi eyes don't have this defect, they are better constructed than mammal ones.

I don't think complexity starts with new mutations - it HAS to begin small and subtle for reasons you stated. Basically, somewhere along the line a biological mutation was made in cells that made those particular cells react/sensitive to different forms of electro magnetic radiotion.

The way we react to light is indeed quite simple. When some proteins called photopsins intercept even a very small amount of energy like a photon, they turn into a more energetic isomer. This process is called photoisomerization. A few moments later, when the extra-energy dissipates, they return to their primary form.

Rods cells are in fact made of several disks stacked up together, each covered with these proteins (here rhodopsins). The more light, the deeper it gets into the cell, and thus the more disks are eventually affected. Hence, the rod cell transmits a signal that is conveyed to the brain as a single pixel whose grayness vary according to the amount of light.

Cones cells perceive colours. They are also made of several disks stacked up together, but of increasing diameters, hence the "cone" shape. The light here is diffracted like in a prism, since the goal is colour recognition. Again, the more disks touched by light, the more intense the primary colour will be (whether the cell is configured to recognize blue, red or green wavelengths: you have three variants ; and the cone angle and geometry will be slightly different each time, because the required diffraction won't be the same).

And after that, the brain processes all the pixels together: first, a black and white image (rod cells) that gives great acuity and precision, and just after that it "paints" it with the informations received by the blue, the red and the green layers (cone cells, which are less numerous and less efficient than rod cells)...


So you see, when you consider each phase separately, the mechanisms used in our eyes use simple chemical and optical properties, nothing really fanciful!

 

[Oleander]

I think we are wrong to put so much emphasis on random mutation though. Even Darwin recognised sexual selection, which amounts to selective breeding. Interference from religion pushers obstructs the very serious problems with simple 'Darwinism' by making the intelligent defend that before they dare to question it. It's always possible that the Cosmos is evolving as an entirety and therefore influences specific evolutionary trends.

Certainly Darwin was a man of his time who believed that the sum is no greater than its parts and all composites could be explained from the bottom up by their simplest components' actions. We now think in terms of ecological systems and evolutionary theory should take more account of Lamarckian influences operating through feedback as whole environments develop as a unity, not as haphazard individualitie happening to come together.

I don't believe there is one single Theory of Evolution Many influences go into evolution. Some of them almost support the concept of a Designer (though I'm inclined to a Cosmos that evolves and deigns itself as it goes along - the way it has evolved determines the way it will evolve). Many genes lie dormant for millions of years before a mutation allos them activation. If Creationists were not so stupid, they would use this as a powerful argumetnt to say that millions of years ago, God anticipated future conditions and provided a gene for them.

Not that I believe that - the gene happened to be there and might never have been activated, Somewhere in our junk DNA there might be genes to allow us to drink seawater and eat sand in a radioactive world. We'll never know unless that happens and the few whose mutation switches them on survive.