This struck me as impossible and I believe that Rennie fell into the trap of what I call integral fallacy- the fallacy of treating a bunch of discrete events as a giant blob of one continuous phenomenon (akin to taking the integral of a discrete function). It means, essentially, that you're ignoring the specific, discrete events that result in a phenomenon, and then simply use the phenomenon as if it's a big continuous thing.
The passage of DNA from parent to offspring requires two and only two things- the offspring has to reach sexual maturity, and the offspring has to successfully mate. A trait is advantageous if and only if it increases the likelihood of one of these two things with respect to the competition. In other words, for a species to evolve, many individuals with inferior traits must die (and be taken out of the gene pool) before reaching sexual maturity or procreating. Remember, evolution is survival of the fittest, and survival implies that some survive and others die.
Specifically, the introduction of milk as a food source would result in a change in the ability for adults to digest milk if those who could digest milk as adults were more likely to survive until sexual maturity, and once there, more likely to copulate. But, remember, it must also work in the opposite direction - if you can't digest lactose as an adult, you must be less likely to make it to sexual maturity and less likely to mate, again, with respect to your competition. From a simple thought experiment perspective, it seems that the ability or inability to digest milk as an adult neither gives an advantage nor disadvantage for or against surviving to sexual maturity, nor does it give an advantage for copulation and therefore, reproduction. It seems, therefore, that we have both adult-non-lactose-digesters and adult-lactose-digesters freely passing on their lactose digestion trait to their offspring with the trait having no effect whatsoever on the gene pool.
Think about it. We walk erect and have highly developed brains because, and only because (take a minute to digest these two things- because and only because), those who walked hunched and had small brains died before copulating and those who walked erect and had bigger brains had a higher chance of surviving to sexual maturity and reproducing. Otherwise, we'd have a whole range of humans, walkers and non-walkers.
How does this relate to milk? Well, how on Earth could having the gene that allows adults to produce lactase, the enzyme that breaks down lactose, increase your likelihood of reaching adulthood or enhance your odds of copulating? It does neither, and therefore falls into the integration fallacy. There is no integral "evolutionary pressure" without the discrete evolutionary pressure, and there is no discrete evolutionary advantage to an adult (post-sexual maturity) having the ability to digest milk. Why? Because they're already sexually mature adults when the trait kicks in.
Now, it could be argued that having milk as a protein source enhances the vitality and therefore, sexual attractiveness of adults, but that's a stretch. And besides, Rennie implies that the mere presence of a plentiful protein source is all the evolutionary pressure necessary for humans to evolve. Absent any impact on the likelihood of gene passage (and therefore trait passage), it is, to put it simply, inconsequential to human evolution. As a point of clarification, I am not claiming that humans have not evolved the ability to digest lactose, only that it's introduction as a food source did not cause it.
To take this argument to a more generic and interesting level, think about these two events- sexual maturity and copulation - and how things have changed from the dawn of civilization to now. Today, in modern society, at least in the West, virtually no child fails to make it to sexual maturity (a tragic badge of shame that we still wear is that we have failed to spread this good fortune to the rest of the world). Further, virtually no human being has a highly unlikely chance to reproduce. Of course, we have social forces and religious factors, etc., but from a strictly genetic perspective, with reproductive clinics, online dating sites, more and more protective and egalitarian social norms, etc., etc., there is no gene that favors or disfavors reproduction. Of course this is a bit of an exaggeration, but between the dawn of civilization and today, the odds have tilted dramatically toward universal gene passage to offspring (again, in the West).
Therefore, I submit that human evolution is dead or severely retarded because we have evolved socially to the point where we all, communally, believe we should do whatever we can to make sure a) that everyone survives and b) that everyone has the opportunity to reproduce. Our social evolution, therefore, has ended (or severely retarded) our genetic evolution.
The irony is that the traits that evolved to make us social beings created our desire to live communally and care for each other communally. This has fostered our shared value of doing everything we can to make sure that very few children die and that any adult who wants to reproduce can. We even pay for these things communally with shared health care costs and shared education and daycare costs. The net effect is that our compassion has effectively arrested our evolution.
2 comments:
Kapauldo, Isn't there a clear link between the adulthood characteristic of ability to digest milk and the adolescent charateristic of ability to digest milk? One can easily imagine a pre-cow society in which 100% of babies can digest milk and 10% of children can digest milk. Since we're still pre-cow, neither the tolerant nor intolerant children have any advantage in making it to adulthood and therefore we expect the mix of adults to be 90/10 intolerant/tolerant. As the society switches to a milkcentric diet, the costs in human life would not be alarmingly high -certainly not enough to abandon the switch to milk or even be able to infer milk as the cause- but, nevertheless, would begin the process of selection by which the children who can not digest milk face lower probabilities of reaching sexual maturity compared to their lactose tolerant counterparts. With this selective pressure favoring milk digesting children, who are destined to become milk tolerant adults, then given enough generations, we would expect to see a shift in the ratio of the tolerant to the intolerant. Furthermore, as the ratio increases, the initial convenience of the cow which was high enough to appeal to a low ratio society is amplified and the pressure becomes increasingly strong! The question remains why would a lactose intolerant band adopt cow milk as a major food source.
yes, *if* and *only if* the selective pressure is on pre-sexual maturity ability to digest milk. but, that's precisely my point. he claims that there was evolutionary pressure on post-sexual maturity ability to digest milk. additionally, your argument presumes that the age range of lactose digestability spans from birth to old age, and that it's a continuous range, and that there was (you speculate) selective pressure to expand that range. however, it is also possible that the pre-post sexual maturity ability to digest milk is a binary trait, on or off, corresponding with our need to nurse.
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