‘Thinking positively’:

Thinking any and all observed phenotypes are observable phenotypes because of some evolutionary reason. Phenotype X must have an evolutionary advantage! Even if Phenotype X appears detrimental/neutral, if Phenotype X wasnt advantageous, it wouldnt exist, right? Survival of the fittest! Evolution! YAY!!!

No, not always.

Evolution is a dirty, messy process. Sometimes shit just happens. There is no reason. No higher purpose, from deities or evolution. It just happens. Chance.

Viruses are happy to provide us with examples of this. First there was HPV. Now herpes.

The Herpes Simplex Virus-1 Transactivator Infected Cell Protein-4 Drives VEGF-A Dependent Neovascularization

HSV-1 activation in the eyes is one of the leading causes of blindness in the US. HSV-1 likes to bounce through the nerves connecting your mouth, nose, and eyes. If it activates in your eyes a few times, shit hits the fan.

Obviously, our corneas are clear. Thats how we can see. But when HSV-1 has been going to town in your eyes, you start getting blood and lymph vessels growing in places that should be clear. Surgery and corneal transplants can clean up the mess a bit, but after the damage is done, it can come back, and having a lot of blood and lymph vessels around increase cornea transplant rejection (an event that is normally pretty rare– you dont even have to genetically match people for cornea transplants).

What is the ‘reason’ for HSV-1 causing blindness like this? It must give the virus some kind of evilutionary/fitness advantage, right? Maybe replicating in the eye causes lots of shedding in tears, and then you wipe your eyes and pick someone elses nose and the virus increases transmission? Or maybe you go blind and you cant work anymore and you can only make money setting up a ‘KISSES $1′ booth in your front yard, thus increasing the viruses transmission?

Or maybe, shit just happens.

Altering blood and lymph vascularture requires activation of a protein, vascular endothelial growth factor-A. Any time your body needs to do that, VEGF-A is called to action. Naturally, VEGF-A is also responsible for the altered blood/lymph vessels resulting from HSV-1 activation.

Turns out, the promoter for VEGF-A kinda looks like the promoter driving the production of HSV-1 early genes. So the HSV-1 transcription factor ICP4 is grabbing onto the VEGF-A promoter, thinking ‘DIRP! I IZ MAKIN MORE VIROOS! DIIIIIIIRP! I IZ HALPING!!’… when it is actually inducing the expression of VEGF-A.

It doesnt ‘want’ to do this. There is no evolutionary ‘reason’ for ICP4 to do this. But when youve got blind molecules operating by sequence ‘feel’ like this, ICP4 cant tell the difference between its promoter and the VEGF-A promoter, leading to a completely unintended side-effect of no benefit to the virus or the host, not negative enough or at the right time to have any effect on transmission of the virus.

HSV-1 leading to blindness is a mess caused by a messy process. An accident of evolution, not an ‘end goal’ of evolution.

Comments

  1. #1 John Marley
    March 2, 2012

    ‘Thinking positively’:

    Thinking any and all observed phenotypes are observable phenotypes because of some evolutionary reason. Phenotype X must have an evolutionary advantage! Even if Phenotype X appears detrimental/neutral, if Phenotype X wasnt advantageous, it wouldnt exist, right? Survival of the fittest! Evolution! YAY!!!

    No, not always.

    QFT.

    This is one of the oversimplifications that I repeatedly try to convince my xian relatives is not actually the Theory of Evolution. No matter what Pastor says.

  2. #2 Garx
    March 2, 2012

    I can testify that HSV1 is a nasty virus. I was blinded by it. Fortunately, though, only in one eye.

  3. #3 CherryBombSim
    March 3, 2012

    “It doesnt ‘want’ to do this. There is no evolutionary ‘reason’ for ICP4 to do this.”

    Well, there is a reason, it’s just not adaptation.To be honest, I had a pretty naive adaptationist understanding of evolution, even after a pretty rigorous academic background in geology and work experience dealing with evolution on a daily basis to do stratigraphy. It’s only after all of this, and largely due to blogs like this, that I have come to a more sophisticated understanding of evolution.

    John Marley, it would be nice if everyone had a deep understanding of evolutionary theory, but not necessary, I think. Compare to General Relativity: Most people do not have the background needed nor the desire to get the background needed to understand it rigorously. They recognize that there are smart, driven people out there who DO understand it, and that is good enough. Evolutionary theory is not an easy subject, if you want to understand it deeply enough to actually interpret observations. It requires more math than most people have, and also grinding through many, many examples like the HSV-1.

  4. #4 TylerD
    March 4, 2012

    I’ve been fascinated with the possibility of neutral phenotypes for a while now, though I’ll admit I find Kimura’s diffusion equations easier to understand than the actual biology behind it.

  5. #5 mo (one of Abbies's elk)
    March 5, 2012

    You know, it may be that the activation of the VEGF-A promoter has a positive effect on the fitness of HSV-1 in its physiological targets, but not when it infects lens cells.

  6. #6 Rui
    March 5, 2012

    Abbie: I really appreciate the colloquialisms here and there. It added that nice touch of comic relief when talking about a slightly depressing subject.

    DIRP

  7. #7 EvilYeti
    March 7, 2012

    Uh, this is a great example of evolution.

    Individuals with ineffective immune systems get eyeball herpes and go blind. If we lived in a less egalitarian society these people would be weeded out of the gene pool eventually as its kind of hard to do the hunting/gathering thing when you can’t see.

  8. #8 abithi
    March 9, 2012

    The Information that you have given is awesome…

  9. #9 Randall Lee Reetz
    March 13, 2012

    OK, yes, this is obvious to anyone willing to think about evolution from a perspective outside of biology. The weird and sticky notion that evolution selects for optimization is nothing but a reflection of which ideas meet with the most resonance in human minds. Optimization provides local (time/space) advantage, but the same optimization (the beaks of Darwin’s finches) also reduces the generality and range of options from which adaptation (to change) is drawn. The optimizers win in the here and now, but always loose in the there and then that defines what matters most to an evolving system. Optimization leads to highly successful dead ends. Evolution, instead depends on the freaks at the fringes, the novel weirdos who need only just survive the trek over the ridge to valleys far more fecund. What seems never to be asked, the question that matters the most, Why does evolution happen at all? Answering this question brings all of these little confusions into the light of understanding. What is the ultimate fitness metric? Independent of domain? Anyone care to take a stab at answering this foundational question?

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