Sunday, November 01, 2009

I was hoping for X-ray vision...

We often forget the process of evolution did not stop when Darwin first outlined how it works.  So while it's really hard to discern, our species is still selecting the best package of genes for survival.

Here are what we might be choosing.
Several traits did indeed appear to be undergoing selection.  From the amount of this selection, we can predict the percentage change in the trait that we expect to see after ten more generation of reproduction (roughly 300 years from now).
Total cholesterol: going down.  Projected to drop 3.6% in ten generations
Weight:  going up a tad, projected to increase 1.4% in ten generations
Height:  we’re getting shorter projecting a drop of 1.3% (2.1 cm) in ten generations.
Systolic blood pressure:  Going down, as predicted. Projected to drop 1.9% in ten generations.
Age at menopause:  Going up; projected to rise 1.6% (0.8 years) in ten generations.
Age at first reproduction: Going down. Projected to drop 1.7% (from 26.18 to 25.74 years).
So women, at least, are getting shorter,  stouter, and reproducing earlier and over a longer period of time.   This is evolutionary change.  Based on this study, we can tentatively say, with more assurance than I used to, that yes, our species is still evolving.
But there are two important caveats to this study, both of which were recognized by the authors.
1.  The “inheritance” of the trait includes not just genetic inheritance, but cultural inheritance.  Humans pass not only their genes to their offspring, but aspects of culture that may mimic a genetic inheritance.  For example, parents who eat a lot may induce their kids to eat a lot, and some of the correlation of weight between parents and their kids may be due not to shared genes, but to shared food.  Parents who for cultural reasons have their kids early may induce their own kids to produce grandchildren early.  As the authors say, “We are not able to differentiate the effects of genes and culture with these data.”
This is a bit of a problem, because the evolutionary projections are based on assuming that all of inheritance is genetic.   It’s hard to get around, since distinguishing genetic from cultural inheritance involves difficult work using data from adopted children or twins raised together versus apart.  Nevertheless, we can probably assume that some selection is acting on these traits, discounted by the degree to which parent/offspring resemblance reflects cultural similarity.
2.  The predictions may be hard to verify, because they assume that the environments of our species — that is, the environments that are relevant to each trait — will remain constant over the next few generations. Cholesterol is predicted to drop from 224 to 216 mg/100 ml blood over the next ten generations, but this assumes that diet is constant.  If people stop eating fatty foods, the drop may be even steeper because of this dietary change.  Conversely, if people flock even more frequently to fast-food joints,  the predicted drop may be negated by an increased intake of burgers and fries.
These problems mean that one has to be a bit careful about not only predicting the degree of selection, but also testing those predictions in the future. Yesterday Stearns was on NPR’s Science Friday, discussing how the authors deal with the conflation of culture/environment and genes, and why the prediction of increasingly plump women may not be what it seems. [More]

Discussions of this nature help me keep in mind how transitory our lives are, and how we invest so much importance on stuff that will have minute impact on human history as a whole.

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