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Tuesday, October 13, 2009

Half an eye IS better than none

Alright, Ginger, here you go. I'm going to do this off the top of my head because my references are at home (or on line, but if I start calling up this stuff I'll never take a break to get any work done).
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Evolution works in small steps. Lots of creatures are born with small genetic variations that did not exist in their parents (replicating our 3,000,000,000 base pairs of DNA is bound to make a mistake somewhere eventually). Sometimes that means you're born with Muscular Dystrophy; usually it means nothing at all: now and then it means you're a little bit better at something.
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One of the creationist arguments that evolution cannot be right is that this kind of small incrementation cannot possibly result in complex structures that don't work until all the parts are there. Having just some of the parts would be useless and not advantageous, so you couldn't get them all together by chance. An oft-taken example is the eye.
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But it simply isn't true that the modern human eye must be complete and perfectly functional for it to be advantageous. Certainly, when competing one human against another, being myopic is not as good as having clear vision. And being color-blind is not as good as being able to see the usual spectrum of colors. And having macular degeneration is disadvantageous too. But that's comparing partial vision to good vision, when you should be comparing partial vision to even less vision or none at all.
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Early in the course of animal life on earth, imagine that no creature can see. Then one creature happens to have a patch of cells that are somewhat sensitive to light. This may indeed be advantageous because it gives the creature an edge in knowing where to find shelter, or which direction to go to find food plants that grow in the light. Then one of this creature's descendents had a light-sensitive patch that was bigger, and so could start to detect the movement of something dark against the sky. This creature might avoid predators better than its peers and thus survive to reproduce more. Then in one of its descendants the light-sensitive patch developed in a sort of indentation, and this concentrated the light in one area, resulting in better resolution. And so on. The indentation deepens to a globe, an eye. The light-sensitive cells become sensitive to different wavelengths, giving color vision. The globe is capped with a lens of very stable, clear, flexible proteins (the crystallins, a family of enzymes used for completely different purposes, just happen to fulfil these critera, and now make our lenses) that helps focus light into sharp images. Every step of building an eye is more useful to some animals than the eye without that step. Examples of different eyes, down to simple light-sensitive patches, abound in the animal kingdom (and that's where I need my references to cite them for you).
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The very definition of what's a good eye varies with different animals. Cats' eyes are well developed for gathering all the light they can in the semi-dark, but that night-vision sacrifices daytime detail. Predators tend to have front-facing eyes with excellent acuity in a narrow zone in front where their prey is, with a concurrent sacrifice in peripheral vision. Prey animals, in contrast, have their eyes on the sides of their heads, making for excellent peripheral vision to spot that stalking lion but poor depth perception in front. Deep-living animals, if not completely blind, are often content with light-sensitive patches signalling 'up'.
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So in the course of evolution, changes that head toward something more useful than the previous version tend to let their animals live more successfully. What is better depends on what you need - whether you're prey or predator, living in bright light or shadows. Changes that don't help, or that hinder, either go unnoticed, or disappear from the gene pool, or are known to us as birth defects or genetic illnesses, or contribute to the diversity of the species. Just think of the color of your iris: living in most places, this color doesn't impact your physical fitness but it well may impact your sexual fitness by making you more desirable to the opposite sex (notably if your eyes are blue); in other places, like an equatorial desert where the sun is harsh, having a dark iris may protect your eye from too much light and thus have a real impact on your ability to function outdoors. Usually eye color is placed in the basket of normal variation without impact, but is it really...
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Anyway, the point is that the complex and magnificent thing that is an eye can indeed be developed through evolution, and there is plenty of evidence that it did so. Every part of the eye is explained, from its existence, position, color, eyelashes, tear ducts, even the awkward inside-out design (with the light-sensing retina behind other structures, and a central blind spot where the optic nerve enters - if one were to design an eye from scratch, surely this organisation would be avoided! but it's a leftover from earlier versions). A person denying evolution may then say, 'well, that's just a bad example, let me take another one.' But that one too will be argued and we'll just go on and on and on.
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I'll give you more if you ask for it, but it's time to prepare today's oncogenetics class. (cancer - now there's evolution gone wild!)
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11 comments:

GingerV said...

well thank you for a good read with my early coffee. It is interesting to me that you choose eyes for your example, as I am virtually blind in my left eye. "crossed eye' or 'lazy eye' at birth not corrected in the 50s until I was 3, then had to be recorrected several time (5 & 7) by then the eye just wasn't used and eventually is considered 'blind'. My mother alwaays said it was the doctor that misused the forcepts but when one of my children was born the same I assumed genetic. I also had a child with a cleft palet. Now they are saying this is from bad diet during pregnacy but I do not fully believe that either... I am one of 7 children (besides personality defects handed down) there was only my lazy eye. with the next generation of 20grandchildren there were a cleft palate, a curved spine/hip misalignment, and a leg that was 1 inch too short at birth. So now not just genetic misfires caused by natural selection built in, but possible enviornmental cause/effect. (the first 4 childdren all girls lived just off the Utah desert during the 1950 underground nuclear testing)
Sorry this is way too much information but a true interest of mine.
hugs from Brasil
(Camillo and I will spend FIVE days in Paris at Christmas - I am thrilled)

NanU said...

There are plenty of anecdotes suggesting that lazy eye (I have one, well-corrected and it sees well but just as if the light were dimmer on that side) and clefting runs in families, but it can actually be quite hard to separate genetics from environmental effects there. Because families tend to share an environment, and even dietary/behavioral tics that the neighbors next door don't have, environment can look like genetics. Plus there's the possibility that having a condition requires both a genetic predisposition and an environmental factor together... Prenatal folic acid is very important for proper body-plan development, but there are genetic influences in how well a woman takes advantage of the folic acid in her diet!

Five days in Paris - that will be fun! Will you be staying in the city the whole trip?

swiss said...

the eye thing, is there even any point in trying to explain to such people when one can just get enthused with the structures themselves. i'd recommend the eye by simon ings as a lightweight browse through the differing stuctures of the eye. it's rather lovely.

cancer genetics sounds cool. i studied genetics once upon a time but alas was too thick in the lab to put it to any use. i do need to write some stuff about the genome tho so i'll be scouring your blog for things!

Barry said...

I'm very impressed Nancy. Right off the top of your head? Obviously there is a lot of impressive stuff in that head!

NanU said...

Well, Barry, I do admit that it was only a few months ago that I read one of Richard Dawkins' books (which I didn't like as much as I hoped to - he goes too far in ridiculing his opponents, making sure that people on the fence won't be converted to the science side), and in it he goes through the eye thing at length. So while I didn't look up any sources, the subject has been near the top of the pile.

Reya Mellicker said...

I am not a fundamentalist in any way, nor can I really understand why anyone would/could take the Bible literally. It has been translated umpteen times, so who knows what it originally said? And who knows what those guys who wrote it intended to pass on?

Love this post. Thanks, Nancy.

action figure toy said...

if I think about evolution, did you guys ever wonder, you know...In the bible, Adam and eve are our ancestors.

Then, how we could be like this?? Born into another different races. It means we are evolutions too...

NanU said...

Evolution isn't something that happened in the past, then stopped. It's going on all the time. It's going on in people today, right this minute.
(I'll address that in Friday's post, or maybe tonight if I have time.)
Humans are all one species, regardless of superficial differences in build, height, hair, coloring, etc! I think the idea of Adam and Eve needs to be taken very figuratively. At some point we are all related. But that point is very far back in time, and there's not any way to finger a particular pair of individuals as the 'progenitors' of our species.
Adam and Eve don't need to be biologically "true" to be the starting point for the Christian creation story. They can be socially "true" as the first people to be self-aware, concious of right and wrong, and capable of making rational choices about their behavior. That's the point of the Bible story anyway.

EastwoodDC said...

Nicely done SG!

There is a good video (and more) on this topic at this PBS site: http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html

GingerV said...

we leave Brasil on the 219 and return on 30 - fly into Paris - then to Rome to see Camillo's sister then back to Paris on the 23rd - leave on 29
we have a complete of museums, churches and restaruants we want to see. I bought long underwear - 7 years in the tropics I will be cold.....

NanU said...

Ah, Rome! I adore Rome. I would love to go back and stay all year!