Evolution And The Eye
Mar. 26th, 2007 09:55 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
mtvesselFeb 2007
Climbing Mount Improbable - Richard Dawkins - Penguin Books, 1996
* * * * *
It’s a pity that Richard Dawkins has allowed himself to become the poster child for atheism. I can understand how the persistent attacks of the creationists, with their airy and inaccurate generalisations about what evolutionary theory actually says, must have got under his skin, but there are more effective ways of combatting them than by claiming that God is a delusion. One approach is to follow Ted Chiang and move the argument onto their own ground by pointing out that a Creator who is responsible for the myriad horrors of nature as well as its myriad beauties cannot possibly be described as good, just or loving. Another is to explain the science more clearly, and fortunately that is what this book does.
The “Mount Improbable” of the title is a metaphor that cuts to the heart of the matter. For creationists, and for that matter many other people including respectable physicists such as Fred Hoyle, the many wonders of nature look too unlikely to have arisen by a “random” process such as natural selection. Hoyle’s memorable quotation was that the chances of a working enzyme arising by chance were equivalent to that of a tornado sweeping through a junkyard and spontaneously assembling a Boeing 747. Unlikely objects such as an enzyme or an eye or a spider’s web constitute the peaks of Mount Improbable (which is more a mountain range than a single mountain), and for such people there is simply no way that you could reach such peaks from an undifferentiated ground state. The crags are too high, the slopes (representing the probability of the functional changes needed for a putative proto-organ to acquire the characteristics of its final form) are too steep.
Dawkins does the sensible thing and agrees with this. Expecting a wing or an eye to spontaneously assemble from its component parts by chance is indeed about as likely as a tornado creating a Boeing 747. But evolution is not a chance process. Yes, it relies on heritable variations and in nature these are mostly produced by random events, but randomness is not necessary for evolution to work. A non-random process of mutation would be equally effective. All that natural selection requires is that variations in a population of replicators arise that affect the rates at which they replicate. After that, statistics does the rest. The replicators that replicate the fastest in a particular environment will take over the population. As Dawkins puts it, with natural selection you look round the other side of Mount Improbable and find the long, smooth, easily climbable slopes that lead to such “designoid” objects (his slightly inelegant term for objects that look designed but aren’t) as the wing, the eye and the spider’s web.
Because evolution is fundamentally an algorithmic process, the slopes of Mount Improbable should be readily modellable using computers, and Dawkins discusses several examples. The main problem with computer modelling is in determining objectively how the 3D shape of a designoid object affects reproductive success (it is telling that Dawkin’s own Blind Watchmaker program, which I saw him demonstrate - with some technical difficulties - in Blackwell’s Bookshop many years ago, uses human aesthetic judgement as its criterion for fitness rather than any objective measurement). Fortunately, there are some models that can be used. A spider’s web, for example, is essentially two-dimensional, and estimates of its “fitness” can easily be determined by comparing its effectiveness at catching randomly fired flies against the amount of silk needed to make it. Surprise surprise, when you run the model over several generations it rapidly evolves into the range of web shapes seen in nature.
The pinnacle of the book is the chapter on the eye, a touchstone for anti-evolutionists and an organ about which even Darwin had his doubts. In fact, eyes of various sorts have evolved independently at least forty times in evolutionary history (hence Dawkins’ witty chapter heading, The Forty-Fold Path to Enlightenment). Dawkins shows step by step how our camera-style eye could have come about and why each step would confer a reproductive advantage to its possessor. What’s remarkable is that evidence of the intermediate stages can still be found in species living today, showing that all the steps are plausible. Even more impressive is the modelling done by Nilsson and Pelger. Starting from a ground state of a flat sheet of light-sensitive cells, making some very conservative estimates of evolution rates (a mutation causes a variation of just 1% in some aspect of the physical structure) and using ray tracing to determine the sharpness of the image in a vertical slice through the centre of the proto-eye as the measure of “fitness”, it turns out that a working fish eye complete with graded lens can evolve in just 432,000 generations. If we take a generation time of one year (again, very pessimistic in my opinion), then the model shows that the essential elements of an eye can evolve in less than half a million years. If you’ll forgive the pun, this is a mere eyeblink in geological time (certainly too quick for fossils with intermediate stages to be likely). To put it in some sort of perspective, creatures that were physically big enough to have eyes first emerged about the time of the Cambrian explosion 550 million years ago, meaning that there is enough time in evolutionary history for the eye to have evolved from scratch over 1100 times.
I should add that Dawkins explains all this much better than I have done, and to cap it off he discusses other sorts of eyes than our own, including, inter alia, the eye of the scallop which works on the same optic principles as a reflecting mirror telescope, the three different ways in which insect compound eyes solve the problem of combining images from their ommatidia (the tube-like structures that form the facets) into a brighter whole, and the quite extraordinary eye of the crustacean Ampelisca, which has the same camera-like structure as our own but where the retina consists of ommatidia, implying that it has evolved from a compound eye.
This latter illustrates the other important rule of the algorithmic process of evolution, which is that you can never go down the slopes of Mount Improbable, only up. A mutation that causes an organ to perform less well will never be selected for, even if it makes possible an alternative path to a higher peak. This is why insects are stuck with the basically disastrous design of the compound eye (which would need to be at least a metre across to produce as good an image as a camera-like eye) and why Ampelisca couldn’t evolve a retina like our own. Which, by the way, is also the product of a dodgy short-term evolutionary design decision - our rods and cones are orientated the wrong way round, with the cell bodies and neuronal connections on the front surface and the light sensitive parts buried inside, which means that they are less sensitive than they could be and results in the infamous “blind spot” where the optic nerve has to pass through the retina to get to the brain, a design defect which requires elaborate neural processing to correct. This would seem an ideal opportunity to comment on the failure of creationists to explain these duff design features and to suggest that if there is a Designer out there, then He is deeply incompetent, but surprisingly Dawkins fails to do so. This book was written ten years ago and perhaps he was less militant then.
Sadly, after this masterly exegesis Dawkins has exhausted the Mount Improbable metaphor, and the remainder of the book is a ragbag of chapters which, though extremely interesting in themselves, have little to do with what went before and may well confuse the general reader. Nonetheless, for its clear explanations of complex processes, and the frequent “wow, I never knew that” moments as Dawkins hits you with another amazing example of an evolutionary solution to an engineering problem, this is a book that should be read by everyone. Especially creationists.
Climbing Mount Improbable - Richard Dawkins - Penguin Books, 1996
* * * * *
It’s a pity that Richard Dawkins has allowed himself to become the poster child for atheism. I can understand how the persistent attacks of the creationists, with their airy and inaccurate generalisations about what evolutionary theory actually says, must have got under his skin, but there are more effective ways of combatting them than by claiming that God is a delusion. One approach is to follow Ted Chiang and move the argument onto their own ground by pointing out that a Creator who is responsible for the myriad horrors of nature as well as its myriad beauties cannot possibly be described as good, just or loving. Another is to explain the science more clearly, and fortunately that is what this book does.
The “Mount Improbable” of the title is a metaphor that cuts to the heart of the matter. For creationists, and for that matter many other people including respectable physicists such as Fred Hoyle, the many wonders of nature look too unlikely to have arisen by a “random” process such as natural selection. Hoyle’s memorable quotation was that the chances of a working enzyme arising by chance were equivalent to that of a tornado sweeping through a junkyard and spontaneously assembling a Boeing 747. Unlikely objects such as an enzyme or an eye or a spider’s web constitute the peaks of Mount Improbable (which is more a mountain range than a single mountain), and for such people there is simply no way that you could reach such peaks from an undifferentiated ground state. The crags are too high, the slopes (representing the probability of the functional changes needed for a putative proto-organ to acquire the characteristics of its final form) are too steep.
Dawkins does the sensible thing and agrees with this. Expecting a wing or an eye to spontaneously assemble from its component parts by chance is indeed about as likely as a tornado creating a Boeing 747. But evolution is not a chance process. Yes, it relies on heritable variations and in nature these are mostly produced by random events, but randomness is not necessary for evolution to work. A non-random process of mutation would be equally effective. All that natural selection requires is that variations in a population of replicators arise that affect the rates at which they replicate. After that, statistics does the rest. The replicators that replicate the fastest in a particular environment will take over the population. As Dawkins puts it, with natural selection you look round the other side of Mount Improbable and find the long, smooth, easily climbable slopes that lead to such “designoid” objects (his slightly inelegant term for objects that look designed but aren’t) as the wing, the eye and the spider’s web.
Because evolution is fundamentally an algorithmic process, the slopes of Mount Improbable should be readily modellable using computers, and Dawkins discusses several examples. The main problem with computer modelling is in determining objectively how the 3D shape of a designoid object affects reproductive success (it is telling that Dawkin’s own Blind Watchmaker program, which I saw him demonstrate - with some technical difficulties - in Blackwell’s Bookshop many years ago, uses human aesthetic judgement as its criterion for fitness rather than any objective measurement). Fortunately, there are some models that can be used. A spider’s web, for example, is essentially two-dimensional, and estimates of its “fitness” can easily be determined by comparing its effectiveness at catching randomly fired flies against the amount of silk needed to make it. Surprise surprise, when you run the model over several generations it rapidly evolves into the range of web shapes seen in nature.
The pinnacle of the book is the chapter on the eye, a touchstone for anti-evolutionists and an organ about which even Darwin had his doubts. In fact, eyes of various sorts have evolved independently at least forty times in evolutionary history (hence Dawkins’ witty chapter heading, The Forty-Fold Path to Enlightenment). Dawkins shows step by step how our camera-style eye could have come about and why each step would confer a reproductive advantage to its possessor. What’s remarkable is that evidence of the intermediate stages can still be found in species living today, showing that all the steps are plausible. Even more impressive is the modelling done by Nilsson and Pelger. Starting from a ground state of a flat sheet of light-sensitive cells, making some very conservative estimates of evolution rates (a mutation causes a variation of just 1% in some aspect of the physical structure) and using ray tracing to determine the sharpness of the image in a vertical slice through the centre of the proto-eye as the measure of “fitness”, it turns out that a working fish eye complete with graded lens can evolve in just 432,000 generations. If we take a generation time of one year (again, very pessimistic in my opinion), then the model shows that the essential elements of an eye can evolve in less than half a million years. If you’ll forgive the pun, this is a mere eyeblink in geological time (certainly too quick for fossils with intermediate stages to be likely). To put it in some sort of perspective, creatures that were physically big enough to have eyes first emerged about the time of the Cambrian explosion 550 million years ago, meaning that there is enough time in evolutionary history for the eye to have evolved from scratch over 1100 times.
I should add that Dawkins explains all this much better than I have done, and to cap it off he discusses other sorts of eyes than our own, including, inter alia, the eye of the scallop which works on the same optic principles as a reflecting mirror telescope, the three different ways in which insect compound eyes solve the problem of combining images from their ommatidia (the tube-like structures that form the facets) into a brighter whole, and the quite extraordinary eye of the crustacean Ampelisca, which has the same camera-like structure as our own but where the retina consists of ommatidia, implying that it has evolved from a compound eye.
This latter illustrates the other important rule of the algorithmic process of evolution, which is that you can never go down the slopes of Mount Improbable, only up. A mutation that causes an organ to perform less well will never be selected for, even if it makes possible an alternative path to a higher peak. This is why insects are stuck with the basically disastrous design of the compound eye (which would need to be at least a metre across to produce as good an image as a camera-like eye) and why Ampelisca couldn’t evolve a retina like our own. Which, by the way, is also the product of a dodgy short-term evolutionary design decision - our rods and cones are orientated the wrong way round, with the cell bodies and neuronal connections on the front surface and the light sensitive parts buried inside, which means that they are less sensitive than they could be and results in the infamous “blind spot” where the optic nerve has to pass through the retina to get to the brain, a design defect which requires elaborate neural processing to correct. This would seem an ideal opportunity to comment on the failure of creationists to explain these duff design features and to suggest that if there is a Designer out there, then He is deeply incompetent, but surprisingly Dawkins fails to do so. This book was written ten years ago and perhaps he was less militant then.
Sadly, after this masterly exegesis Dawkins has exhausted the Mount Improbable metaphor, and the remainder of the book is a ragbag of chapters which, though extremely interesting in themselves, have little to do with what went before and may well confuse the general reader. Nonetheless, for its clear explanations of complex processes, and the frequent “wow, I never knew that” moments as Dawkins hits you with another amazing example of an evolutionary solution to an engineering problem, this is a book that should be read by everyone. Especially creationists.