A Book Review by: Paul Harris, OD
It seems that one of the central questions debated in many different circles over the past 30 years or so has been to what degree this or that is determined by our genes OR the cumulative effect of our life experiences and what we gain from them. In our own field and in particular the condition of myopia this debate rages heavily. About 15 years ago Barry Millis, OD from Pennsylvania gave a paper based on the writings of Bateson and he was one of the first that I encountered who talked about nature and nurture being codependent on each other in a way that was so compelling that I felt like a breath of fresh air had entered the room. Millis had freed me to not engage into the dialogue of nature VS. nurture as that was an old game not worth paying one’s ante to get a seat at. Ridley has taken the approach of Bateson and Millis and seated it in science and the results of the genome project. This work should be a must read for all members of the heath care field and the sciences.
The book is divided into 10 chapters with a prologue and an epilogue. In chapter one he sets the stage giving us a view of what has been learned about genes in humans and other species. “The human genome contains about 3 billion ‘letters’ of code. The difference between two individual human beings amounts, on average, to 0.1 percent, so there are 3 million different letters between me and my neighbor.” Interestingly, “The difference between a human being and a chimpanzee is about 15 times as great, or 1.5 percent, … which is about 10 times as many letters as there are in the whole bible.” (page 26)
These 3 billion letters of genetic material make up about 30,000 genes. If the number of genes in a chimpanzee is similar then a 1.5 percent difference means that there are only about 450 of these genes that are different between us humans and chimpanzees! Stated another way, it means that 29,550 genes are identical between us. However, it is possible that the 1.5 percent difference is scattered across the entire genetic code in just such a way that each and every one of the genes is different, very slightly, but different nonetheless.
In a section where Ridley is exploring many of the changes in our species including the massive explosion in brain size he brings up the notion that there may not have had to have been a massive change in the genetic code, rather that a small change in a gene that shuts growth down gets suppressed for longer. This is very similar to the butterfly effect from Chaos theory, better called; sensitive dependence on initial conditions. One could get very large changes with huge ramifications from a small change in the genetic code.
An analogy used by Ridley is quite strong in helping understand his view of genes. He discusses doing an analysis of the words used by Dickens in his novel David Copperfield and J.D. Salinger’s The Catcher in the Rye. He states that there is, “Probably at least 90 percent lexical concordance between the two books. Yet they are different books. The difference lies not in the use of a different set of words but in the same set f words used in a different pattern and order. Likewise, the source of the different between a chimpanzee and a human being lies not in the different genes but in the same set of 30,000 genes used in a different order or pattern.” (page 31)
I tended to think of genes and just being there and doing their thing from the beginning and throughout life just the same. Need a protein? Transcribe the DNA forming RNA etc. through the whole sequence. Obviously it had to be much more complicated and it is. “The function of many genes is therefore to help switch other genes on or off. And the susceptibility of a gene to being switched on or off depends on the sensitivity of its promoters.” This idea of switching on and off a gene depending on various conditions, some of which occur within the body and some of which are triggered by the environment are the key to understanding how nature and nurture interact.
“The beauty of the system is that the same gene can be reused in different places and at different times simply by putting a different set of promoters beside it.” The brain researcher Karl Pribram stated that an analogous type of thing was occurring in brain processing. A particular part of the brain looked like it was doing job “A” when analyzed in a certain way. For example think of the seminal work of Hubel and Wiesel on receptor fields. Pribram stated that the same section of the brain is used for different purposes when that type of computing is needed.
Ridley continues, “To make grand changes in the body plan of animals, there is no need to invent new genes, just as there is no need to invent new words to write an original novel. All you need to do is switch the same ones on and off in different patterns. Suddenly, here is a mechanism for creating large and small evolutionary changes from small genetic differences.”
Once the human genome project completed its work some scientists were concerned about the apparent small number of genes. Their concern was that there weren’t enough genes to act as blueprint of how to make a human body. “Bodies are not made, they grow. The genome is not a blueprint for constructing a body; it is a recipe for baking the body.” Nerves aren’t told specifically where they are supposed to go and exactly how they are to connect to their neighbors. Rather, there is a recipe that they follow to bake a brain.
In a section on instincts Ridley talks about the work of two pioneers in the fields of language and brain development who, until reading Ridley, I had been unable to relate to. It had been said by some in our profession that we are born with all the marbles. My background was strongly Piagetian and this led me to reject the notion of being born with too much in there. Now, I didn’t go so far as B.F. Skinner who might have argued a tabla rasa I had read some Noam Chomsky and he argued that the child was born with all the “marbles” for language. Ridley states, “Chomsky argued that the universal features of human language, invariant throughout the world, plus the logical impossibility of a child deducing the rules of a language as quickly as it does merely from the scanty examples available to it, must imply that there was something innate about language. Much later Steven Pinker dissected the human ‘language instinct,’ showed it had all the hallmarks of a Swiss army knife blade – structure designed for function – and added the notion that what the mind was equipped with was not innate data but innate ways of processing data.” So Ridley helped me understand what Chomsky was saying in a way that have helped me come to the point where I can agree with Chomsky. (page 63-64)
The All Powerful Gene – NOT!
We have all seen the headlines, “The gene for X has been discovered!” Ridley states just what most of us feel when we see this. He states, “… That this does much mischief, not the least because of the reputation genes have garnered for being invincible bulldozers of all that stands in their path.”
He spends time on “heritability”, which is the degree to which a behavior or a characteristic is passed on by the genes. In this discussion he helps see how the answer one gets very much depends on how the initial question is asked. As well this relates to how the manner in which a research protocol is set up will affect its outcome. “Heritability depends entirely on context. The heritability of personality may be high in a group of middle-class Americans who have experienced equivalent, even identical, patterns of nurture. But through a few orphans from Sudan or the offspring of headhunters from New Guinea into the sample and heritability of personality would probably drop rather fast; now environment would matter. Hold the environment constant and it’s the genes that vary.”
Ridley also helps to clear up a misconception I had about the number of genes involved in a trait and it heritability. “Heritability is usually highest for those features of human nature caused by many genes rather than by the action of single genes. And the more genes are involved, the more the heritability is actually caused by the side effects of genes rather than the direct effect.” So the idea of a single gene determining this or that is highly unlikely.
So how does nature and nurture interact? “Nature can only act via nurture. It can act only by nudging people to seek out the environmental influences that will satisfy their appetites. The environment acts as a multiplier of small genetic differences, pushing athletic children toward the sports that reward them and pushing bright children toward the books that reward them.” “Genes become limiting only when they are malfunctional.” (Page 93)
Chapter four is entitled, “The Madness of Causes”, which delves into the currently held mental models that search for and expect to find simple causative relationships between this and that. Ridley quotes William James, “The word ‘cause’ is an altar to an unknown god.” Ridley states, “As long as we are unable clinically to group illnesses on the basis of cause, and to separate dissimilar causes our views about etiology will necessarily remain unclear and contradictory.” Within our own field so many of the names of the probes and the “diagnoses” assume known causes. We have tests named positive relative accommodation, that purport to probe only that and to lead to a diagnosis of an accommodative problem, which is then linked specifically to our patient’s symptoms. Once we find the cause we feel satisfied somehow that their salvation is near at hand.
"Development accommodates to the environment: it is capable of coping with different circumstances and still achieving a result that works. If different developments can result from the same set of genes, then different genes might also be capable of achieving the same outcome. Or to put it in technical terms, development is well ‘buffered’ against minor genetic changes.” (Page 129) This leads to much more complication that one might have thought intrinsic in the system and helps us recognize that there will always be need for good clinicians in health care. One can hope that the general medical profession does not get so seduced by looking for simple causes that is forgets what being a doctor means.
There is a section on critical periods that seems a bit naïve to me, but it comes from the assumption of already accepting the notion of critical periods. “Without visual experience in the first month of life, the brain cannot interpret what the eye sees.” (Page 164) In citing the work of Hubel and Wiesel where they sewed shut the lids of monkeys and looked at the changes in the visual cortex he states, “The effect is irreversible. It is as if the neurons from the two eyes compete for space in layer 4C and those that are active win the battle.” On just the next page though in talking about some work with mice and changing critical periods he states, “The mice with extra BDNF (brain-derived neurotrophic factor) went through the critical period faster. Their brains set two weeks after eye opening instead of three. This was the first demonstration that a critical period could be adjusted artificially.” I would contend, and I believe that this has been the position of Stephen Cool, Ph.D. that what we do clinically in vision therapy does a similar thing but rather than speeding up a critical period we reawaken a new period of plasticity.
In the year 2000, “A Japanese scientist, Takao Hensch discovered that a mouse lacking a gene called GAD65 failed to sort its eye inputs in response to visual stimuli. But these same mice did sort their inputs if injected with the drug diazepam. … In the mice lacking GAD65, the scientists could bring on plasticity with diazepam at any time, even during adulthood. But only once. … If you shut the eye of a mouse, BDNF production in its visual cortex drops within half an hour. ” (Page 166-7)
Ridley takes the reader through many more aspects of the interactions of nature and nurture. This is one of those books that will need to be revisted several times to cull from it all that is relevant to our field.
Available from Amazon (NOTE: The hard back and paper back versions of this book have different titles.)