Barbarian, regarding the numerous transitional forms of eyes within different phyla:
These facts convinced people who weren't evolutionists....It comes down to evidence.
Good evening Barbarian, I'll ask nicely that you refrain from referring to hypothesis as "facts"
Those structures are demonstrably present in living creatures today. That's about as solid as facts can be.
and opinions as "evidence".
It's reality. If you doubt the finding, you can check numerous repeated examinations of the data, or you can go and repeat the process yourself. It always comes up that way.
While that was a nice explanation, none of it is observable, testable, or repeatable,
I just showed you how it's directly observed. It's been repeatedly confirmed. If you want to repeat the investigation, you'll find the same thing.
and as such is quasi-science.
I'm sure you realize that isn't the case now.
True, those are the opinions and ideas of educated people but that would fall into the category of philosophy.
As I said, it comes down to evidence. Darwin's prediction of numerous intermediate eyes has been repeatedly confirmed.
Barbarian observes:
A bit of dark pigment allows a cell to become more sensitive to light.
What allows a cell to be sense light is the 11-cis-retinal protein.
In some highly-evolved organisms. But as you just saw, all that's really necessary is a darker pigment like meletonin, which was, as the research found, derived from a different function. So it's no surprise that some more complex photopigments are derived from meletonin. That's how evolution works.
Barbarian observes:
A bit of dark pigment allows a cell to become more sensitive to light.
No, it's light. You see, visible light, impacting a dark object, is absorbed by the object and transformed to heat, which is easily detected by an organism. So the simplest light-sensitive cell is just a cell with extra pigment. And that is what we see in nature.
I know you wouldn't post something you know to be untrue, so I will just say that is not accurate.
Comes down to evidence. And the evidence shows that's how it happened. The simplest known photopigment is just meletonin, sensitive because it's darker, and incidentally, because it still works as an antioxident, and therefore, can convert a photon to other energy.
What allows a cell to be sense heat is the transient receptor potential cation channel, subfamily V protein, specifically TRPV3. Sensing light is not a simple matter of extra pigment.
I just showed you that it is. Bacteria and protists like Euglena detect light and heat and usefully respond to it, without any of that. As I said, you can do the experiment yourself, and you will find that your skin can detect light on a sunny summer day. And you will find that a snug-fitting t-shirt with a black spot on the back will make an even better eye.
In order for your argument of molecular evolution from detecting heat to light to work you should provide scaffolding that wouldn't interfere with the cell to demonstrate how TRPv3 could, through gradual successive steps, evolve into 11-cis-retinal.
You've gone a bit off the path here. The evidence is that the simplest photosensitive pigment is merely a dark substance that allows light to be transformed to thermal energy.
However, any intermediate gene sequences between TRPv3 and 11-cis-retinal would be invisible to natural selection
All gene sequences are invisible to natural selection. Only phenotypes are visible to natural selection. However, as you see, a gene that caused a dark pigment to collect in a cell would make it more sensitive to light, and thereby more useful to the organism.
so what possible mechanism could there be to take it from point A to B?
Such an advantage would be subject to natural selection. And of course, the new structure would then be open to favorable mutations that could make it more sensitive.
Is there evidence for this? Yes, there is. In frogs, for example, their skin cells can detect light, due to a primitive photopigment, melanopsin. And not surprisingly, humans still retain some of that primitive arrangement:
Melanopsin was originally discovered by Ignacio Provencio and his colleagues in 1998, in the specialized light sensitive cells of frog skin.[5] In 1999, Russell G. Foster showed that entrainment of mice to a light-dark cycle was maintained in the absence of rods and cones. Such an observation led him to the conclusion that neither rods nor cones, located in the outer retina, are necessary for circadian entrainment and that a third class of photoreceptor exists in the mammalian eye.[6] In 2000, Provencio determined that melanopsin was expressed only in the inner retina of mammals, including humans, and that it mediated nonvisual photoreceptive tasks.[7]
The first recordings of light responses from melanopsin-containing ganglion cells were obtained by David Berson and colleagues at Brown University.[8] They also showed that these responses persisted when pharmacological agents blocked synaptic communication in the retina, and when single melanopsin-containing ganglion cells were physically isolated from other retinal cells.[8] These findings showed that melanopsin-containing ganglion cells are intrinsically photosensitive,[9] and they were thus named intrinsically photosensitive Retinal Ganglion Cells (ipRGCs).[10] They constitute a third class of photoreceptor cells in the mammalian retina, beside the already known rod and cone photoreceptors.
http://en.wikipedia.org/wiki/Melanopsin
Metazoan opsin evolution reveals a simple route to animal vision
PNAS October 29, 2012
All known visual pigments in Neuralia (Cnidaria, Ctenophora, and Bilateria) are composed of an opsin (a seven-transmembrane G protein-coupled receptor), and a light-sensitive chromophore, generally retinal...The first opsin originated from the duplication of the common ancestor of the melatonin and opsin genes... and an inference of its amino acid sequence suggests that this protein might not have been light-sensitive.
Just for clarification, they began with a light sensitive cell, inferred it might not have been light-sensitive a long time ago, but it is now.
So the evidence shows. Remember, science works on evidence.
They weren't kidding when they said "Our results entail a simple scenario of opsin evolution." I should think they would be embarrassed to publish that.
Remember, most of their readers are familiar with biochemistry, so they had no worries about that.
Also, that article said nothing whatever about the origin or possible molecular evolution of 11-cis-retinal, the actual protein that is sensitive to light.
As you see, there are much simpler molecules that are sensitive to light. You've looked at a jet aircraft and decided that it's impossible, because primitive men could not build jet engines.
Evolution of vertebrate retinal photoreception
Philos Trans R Soc Lond B Biol Sci. 2009 October 12; 364(1531): 2911–2924
...A phylogenetic tree of animal opsins, based on the recent study by Suga et al. (2008), is illustrated in figure 2a, with the two main families involved in photoreception denoted as r-opsins and c-opsins. Between these two groupings is shown a less well understood cluster of opsins that includes the photoisomerases of protostomes and RGR (retinal G protein-coupled receptor) of the vertebrate RPE (retinal pigment epithelium), together with the peropsins of both protostomes and vertebrates, as well as the neuropsins and Go-coupled opsins.
The r-opsins comprise the rhabdomeric opsins of protostomes together with the melanopsins of chordates, and couple to a Gq cascade. The c-opsins are always found in ciliated photoreceptor cells, and include the teleost multiple tissue (tmt) opsins and encephalopsins, together with the ciliary opsins of chordate photoreceptors, the latter of which generally couple to a Gt cascade. It has recently been discovered that the ‘cnidopsins’ of jellyfish (cnidarians) clade with the c-opsins (see §2c below). Since cnidarians diverged from bilateral animals long before the protostome/deuterostome split (see fig. 2 of Larhammar et al. 2009), it can be concluded that the separate classes of c-opsins and r-opsins were already present in primitive metazoa prior to the divergence of bilateria and cnidaria.
Barbarian observes:
There are organisms that detect and react to light without any specialized cells whatever.
I'm sure you wouldn't post anything you thought was untrue, but that is inaccurate.
One of my degrees is in bacteriology. Many bacteria are able to detect and usefully respond to light. They are single cells, so they cannot have specialized cells to detect light. Many protists are also able to do this without any specialized cells at all.
Cells need a special protein to detect heat or light.
As you now understand, your skin has the ability to detect light, without any specialized proteins for vision.
As I said:
Remember it was a light sensitive CELL that still defies explanation.
The cells of your skin are sensitive to light. And we understand why. The cells of frog skin are very sensitive to light, and we know why that is, also.
Evolution of Melanopsin Photoreceptors: Discovery and Characterization of a New Melanopsin in Nonmammals
Philos Trans R Soc Lond B Biol Sci. 2009 October 12; 364(1531): 2911–2924.
Photoreceptor cells don't work like that.
See the above. Turns out that they do.
Here is how a photoreceptor cell works...
One highly evolved one works that way. But, as you see, evolution first produced much simpler forms, which then evolved to more efficient ones.
Only when they all exist together do they make a light sensitivity possible.
As you see, this isn't the case. Much simpler systems exist for light detection. In bacteria, no specialized cells exist. And they don't have the elaborate form of photodetection you learned about.
This system is has no explanation by molecular or chemical evolution.
I hope you'll do some reading and learn about some of this. It's a very interesting process.
