Height and natural selection in humans?

Question

I watched the documentary "Evolve" recently and in the segment on "size" Scott V. Edwards, Harvard evolutionary biologist mentioned the idea that humans might evolve to be 7' tall in 'hundreds of years'. (I think this may have been taken out of context... I have emailed him to find out, but do not expect a response from someone so busy)The reasoning goes that the trend in the past 100 years has been greater height, and women show a strong preference for men who are taller than they are. (Though a large share perhaps all of this difference has been due to diets higher in protein at an early age) I wonder, though, if this is only part of the story. The preference women have is not just for tall men, but for a man who is taller than she is. Likewise, men seem to prefer woman who are shorter than they are. There is even cultural pressure: the classic western image of a couple on wedding cake always shows a man who is about 4" (to scale) taller than the bride.Thus, women who are short have an advantage as they have a greater pool of men to choose from. (Colloquially, simply ask any 6' tall woman if she feels her height helps her find dates.) Let's say that men seek women who are shorter than they are, but no more than 8" shorter. Women seek men who are taller than they are but no more than 8" taller. Given that the current average height for men is 5'8" and for women it is 5'4" (and distributed normally SD 2.8") will we have selective pressure that leads to greater or lesser height? (This is, obviously, oversimplified, but it is a starting point.)

Answer 1

That's a great question!! Here are a couple of factors you may want to consider.

In addition to being influenced by a polygenic set of alleles that are the same for both sexes, height is also sexually dimorphic -- that is, if a man and a women have exactly the same set of the alleles mentioned above, he will ordinarily grown to be taller than she will (think hormones). So sexual selection or cultural preferences you cited will not necessarily cancel out and result in stablizing selection at all.

On the other hand, in the absence of sexual selection there is an optimal height for human beings based both on their environment (niche -- like job and, as you noted, food supply. If protein is scarce, shorter might be a better strategy) and based on the need for their various systems to function harmoniously (think breeds of dog that have hip problems). So the sexual selective pressure for the population to grow taller will perhaps be counterbalanced by the disadvantages of the parts no longer working together as well as they did at some earlier height.