How does evolutionary theory explain appearance of complicated organs, like eyes?

Question

Let's say, for an animal it is essential to have a long tale, the longer the better. So, the tale is going to get longer in the following generations until it reaches optimal length or environment changes, so long tale is not required anymore. No problem.
But what about eye? It's a very complicated thing. I cannot accept that animal without eyes can have it in the following generation complete and ready. So it will take multiple generations to get from blind predecessors to guys with functioning vision, with ability to distinguish light and darkness at least.
So, generations 'in between' won't have a vision and won't have any evolutionary benefits, instead these unlucky will own a completely useless and redundant system that doesn't work.
So it looks like the mother nature is able to foresee multiple plies in future?

Answer 1

> "How does evolutionary theory explain appearance of complicated organs, like eyes?"

By starting with simple organs, like eyespots.

If you go to Google or the search box at the top of this page and type "evolution eye", you will find many descriptions. I linked the top two hits under Sources below ... including PBS's "Evolution of the Eye" page.

But the basic idea is that eyes evolved from better and better use of a certain abundant class of proteins called "photopigments" ... proteins that react to light. Plants of course use them for photosynthesis, but even plants can react and *move* (yes, move) in response to light (opening and closing flowers during the day and night, changing the angle of leaves to track the sun during the day, etc.)

In animals these collected into what we call "eyespots", which are nothing more than regions of concentrated photopigments on the skin surface. E.g., jellies use light to know which way is the ocean surface in order to keep itself oriented ... and to know and when it's daylight up top to know when to start its daily migration up and down to the most abundant food sources.

From there, the complex eye evolved from a *long* series of simple modifications, each one providing a *slight* advantage in *slightly* better eyesight ... those individuals born with this modification just survived better and reproduced more offspring with the same modification. The basic process of natural selection ... for millions of generations, millions of years.

The reason we know (the evidence) that these modifications and intermediate stages are useful, is that we find examples of these intermediate stages still in use in eyes in organisms today.

These modifications include the following (and please note that this is not a list of sequential "steps" ... one step doesn't develop to completion and then "stop" before the next step begins, many developments are happening *simultaneously*:

- Higher concentration of eyespots on one side of the body (e.g. the top) lets the organism know which way is up.
- The number (or density) of photopigment-containing cells in the eyespot increases, producing better and better acuity ... this process is constantly occurring at the same time as ALL of the following modifications.
- Even a slight indentation of the eyespot causes an improvement in knowing the direction of light.
- The more cuplike this indentation, the better the directionality.
- The smaller the opening of the cup, the more the light is restricted to a specific direction.
- Muscular control over the size of the opening improves vision in different lighting conditions (day/night),
- Some clear mucus over the opening allows the fluid inside the opening to be cleaner than the outside water.
- If the mucus is denser, but still clear, more light can get in.
- If the mucus develops into one or a series of membranes you have the beginnings of a lens.
- By varying the pressure of the fluid or mucus inside the membranes at the opening, the organism can begin to focus light.
- Again, remember that the concentration of the photopigments in the eyspot is constantly getting higher while all of these mutations are occurring ... the eyespot has become what we would now call a retina.
... So now we have a primitive retina, pupil, and lens.
- And while all of this is happening, nerve cells dedicated to detecting objects, movement, up-down, etc. are developing in the retina.
- Mutations in the photopigments cause tuning to different wavelengths of light ... multiple photopigments allow detection of color categories.
- Other helper features are slowly appearing ... muscles to move the eye independent of the organism, to keep the eye clean, to protect the eye, the lens develops into a UV filter to protect the eye from damaging rays, optic nerves for better transmission of information to the brain (which is itself undergoing its own development at the same time as, and in many ways driven by, the eye), more and more complex pathways from eye to optic nerve to visual centers, etc., etc.
- After millions of years of this incremental development, simple improvement upon simple improvement, you have the accumulation of incredible complexity.

That same type of explanation can be used for any example of "irreducible complexity", any complex organ.

(E.g., echolocation has a similar development ... but my answer is already too long ... it's hard to explain 100 million years of small improvements in a small space like this. Again, just type "evolution echolocatio" into a Yahoo or Google search box.)

And this is why the "irreducible complexity" argument is such a weak argument against evolution. People who raise this argument start with a completed organ, and then point out that it can be completely disabled by removal of a single part. Nobody denies this ... but it does not affect evolution theory one bit because evolution does NOT work that way. An organ is not a collection of separate "parts" that evolved separately as completely useless structures until one day being assembled into something hugely useful. The evolution of each "part" is completly inseparable from all the other "parts" with which it evolved.

Answer 2

I think this is called the law of 'irreducible complexity', the idea that all the parts of the eye had to develop simultaneously. This is an often-used argument against evolution.

But there are more and less primitive eyes! In fact several different kinds of eyes have evolved at the same time, in parallel, like the compound eyes of insects, and a few others besides the kind we have.

There is another example of this theory that I like even better. In order for bats to use echolocation, they had to develop voices that could produce just the right frequency, ears that could hear that frequency (which we humans can't hear), and a brain that could sort out the echoes to form mental maps of the environment. Yet a bat can locate a mosquito in mid-air and plot an intersecting course such that the mosquito lands in his mouth. A bat can eat his weight in mosquitos in 24 hrs.

Answer 3

Doggone it, I hate it when secretsauce gives a great answer. The only thing I have to add to his is that eyes evolved more than once, independently. Cephalopod eyes work quite well, but weren't built quite the same way as ours.

Answer 4

Richard Dawkins answers this question quite well in the following u-tube video.