Sunday, July 8, 2007

Lactose Tolerance

I want to interact with another science article from a recent New York Times. Titled "Spread Globally, Evolved Locally," this piece (by Nicholas Wade) discusses some truly exciting research from the field of human genomics. Researchers have identified parts of the genome where differences exist among human populations and discovered that such differences include adaptations for lactose tolerance, malaria resistance, and salt retention (to name a few).

I want to focus on the lactose tolerance. But first, a couple of general comments. While the article is liberal with the use of the word 'evolution,' let's start by acknowledging that it's talking only about 'microevolution,' the changes that occur over time within a species. We might further emphasize that we're discussing a species that originated only tens of thousands of years ago. The article itself recognizes that
The ancestral human population is thought to have originated in northeast Africa about 50,000 years ago.
All of this is consistent, of course, not only with a theistic understanding but also a specifically biblical one. I could, therefore, quibble with the use of the term 'evolution' (with all that it is meant to connote), when 'microevolution' or (better yet) 'adaptation' would have more accurately described the research findings.

My real issue with the interpretation of these research results, though, is two-fold: the unsupported inference that natural selection is the driving force and the assumption that the genetic differences involve random mutations. These two ideas form, of course, the standard neo-Darwinian dogma, but the evidence of these studies--if considered objectively--would seem to suggest design or purpose rather than the randomness and chance associated with naturalism.

According to the article, lactose tolerance arose in (at least) four different human populations in four different parts of the genome. So far, so good. Identifying the genome differences and accurately describing the adaptation with which those differences are associated--that's great science. But it is really naive to expect us to believe that those genomic differences are the result of random mutations. For one thing, we now have an idea of how truly rare are instances of beneficial mutations compared to fatal or harmful (those that lead to natural abortion, death, or reproductive unfitness of the individual), or neutral mutations. We can also demonstrate that humans (like all other higher animals) are not good candidates for mutational advance on the basis of demographic parameters--few offspring, long inter-generational times, low population numbers (again, to name but a few). Moreover, other geneticists would be quick to suggest that the differences herein identified likely represent not so much a new gene, but a differential activation (by a sort of 'controller gene'), a turning off (or on), an effecting of a potential that was already (always?) in the genome.

On this (more objective) view, this latest research offers evidence against the random mutation idea and for the notion that the human genome exhibits even greater design--call it 'inherent potential' if your worldview can't accept the idea of design (and then make a note to upgrade your worldview before too long)--than was previously recognized.

But this research would seem to undermine not only the idea that the mutations are random but also the other half of the evolutionary paradigm--that natural selection is the driving force. On the dogmatic view of the author (and, apparently, the researchers themselves), natural selection is not only a very powerful force but the only mechanism that need be invoked (that, after all, is still the textbook explanation). In the case of the origin of lactose tolerance, does this really make good sense?

Picture a population of northern European cattle-herders 5,000 years ago (when this adaptation is seen to have arisen). One herder develops a gene mutation that enables him to drink the milk from his herd without a rash or other negative side effects. On the naturalistic evolution view, this adaptation is so significant that his ability to survive and reproduce so exceeds that of others of his generation and clan that his progeny soon outnumber theirs by 10 to 1. (On the traditional view, a mutation arises in one individual and is dependent on reproduction to spread through a population generation by generation. On the more recent view--that the capacity for this genetic change was inherent in the genome, and 'switched on'--the right conditions could cause many individuals in the same generation to have this same adaptation.) We are meant to believe that those who still could not drink milk (without developing a rash on the inside of their elbows) died before having children, while this one adaptation endowed the milk drinker with overwhelming survivorship and productivity. In other words, environmental factors somehow dictated that the ability to process milk efficiently was the make-or-break characteristic for this population of humans (though they had been surviving and reproducing up until that point).

But we must not stop at believing that this was the case for northern Europeans. The research shows that three other human populations experienced similar environmental conditions in which a gene change (albeit a different gene in each case) that conferred tolerance for lactose was likewise the make-or-break characteristic. According to the article...
That lactose tolerance has evolved independently four times is an instance of convergent evolution. Natural selection has used the different mutations available in European and East African populations to make each develop lactose tolerance.
Authors (and researchers) who cavalierly mention convergent evolution seem to be unaware that the discovery of more and more "instances" of it argues against the naturalistic paradigm. If there is no purpose or design associated with these changes within a population, then it should be rare indeed that environmental conditions "select" for a similar adaptation in different populations.

Here's the bottom line... These exciting research discoveries--when viewed objectively--support a theistic or design understanding of the documented adaptive changes within recent human populations and undermine the 'natural selection acting upon random mutation' paradigm. That the author and researchers are unable to recognize this is a testimony to their lack of objectivity and of the extent to which they have chosen to work within the artificial constraints of naturalism. While some marvelous research is taking place, their naturalistic misinterpretation of the results stifles the scientific advance toward truth to which such research might otherwise lead.

1 comment:

Ian R said...

Dear Mr Gerhardt,

Thank you for this interesting blog entry. As an (evolutionary) biologist and someone with a particular interest in the relationship between science and society, I take a great interest in the arguments of young earth creationists and those who oppose naturalist evolution.

I have a number of problems with what you have written, but feel compelled to interject regarding one key point, if you do not mind.

I refer you to your words:

"We are meant to believe that those who still could not drink milk (without developing a rash on the inside of their elbows) died before having children, while this one adaptation endowed the milk drinker with overwhelming survivorship and productivity. In other words, environmental factors somehow dictated that the ability to process milk efficiently was the make-or-break characteristic for this population of humans (though they had been surviving and reproducing up until that point)."

I am afraid you have made a fundamental error, albeit a very common one. Natural selection does not primarily act at the level of the population. The authors of this paper would almost certainly not claim that lactose tolerance would spread throughout the population for this reason. Rather (and you make the point yourself very well elsewhere):

“On the naturalistic evolution view, this adaptation is so significant that his ability to survive and reproduce so exceeds that of others of his generation and clan that his progeny soon outnumber theirs by 10 to 1.”

I am not sure how you come up with your subsequent argument (earlier quote), given this earlier explanation that it is differences between reproductive successes of individuals within populations. Perhaps you have not considered how lactose tolerance could increase reproductive success of individuals within a population relative to others? If an individual’s fitness were improved even slightly by his or her ability to exploit such a food resource, then any gene conferring such an advantage should spread. It wouldn’t have to make the difference between life and death in all cases. It would merely need to confer a slight advantage on average. We don’t need to consider all the ways in which the availability of milk could be even slightly beneficial. It’s not a great stretch of the imagination.

A herding population with wholesale lactose-tolerance may well have outcompete a herding population with wholesale lactose-intolerance. But this is not what would lead to the first population being tolerant in the first place. Slight differences between reproductive successes of individuals within populations are always what will matter the most.

Thanks

Ian