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

To answer the question with a complicating question, suppose the anecdotal preference for "tall" men and "short" women is really just an urge toward the norm? Suppose that women seek taller men in proportion to their self-perception as "shorter than average"--the closer a woman is to average (female) height, the less likely she is to seek a mate who is markedly taller?

In my unscientific experience, height matters less to people whose height is more or less normal.

Then, with some assumptions about the heights of offspring of couples whose heights are different, the tendency might be toward a stable average. As noted in other answers, nutrition is probably a big factor in recent gains in Europe, Asia, and Africa.

This would be hard to model mathematically and I am not convinced the result would shed much light on the question, especially (as noted above) in light of the polygenetic nature of the trait.

Answer 2

(This should have been a comment, but I don't have enough reputation yet. Sorry!)

You asked an interesting question, but I'm not sure about your reasoning.

Firstly, the "trend" you describe, that in the past century humans grew taller that ever before, is a phenomenon known to most European countries, one of the so called "secular trends": a uni-directional change in height (or growth rate) over a period of time. It was established that adult hight increased by 1 to 3 centimeters for each decade until 1980 (except for the times of World War 2, which had an opposite effect in some places). Such changes, however, happened much too quickly to be of genetic origin; and are much better explained by an increase in quality of life. In line with this explanation is the increase in height and growth tempo we see now in Third World countries, clearly the effect of improving life conditions.

Secondly, in my opinion, womens' preference for higher men can be another manifestation of their wanting simply the best male. I don't see any particular advantage for a woman to want a man higher that she is, but, as you pointed out, anecdotal evidence suggests otherwise.

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Based on:

• Cole, T.J. Secular trends in growth. Proc Nutr Soc 59, 317–324 (2000), PMID: 10946801.

See more on secular trends in:

• Cole, T.J. The secular trend in human physical growth: a biological view. Economics & Human Biology 1, 161–168 (2003), PMID: 15463971.
• Sargent, M. Biomedicine and the human condition: challenges, risks, and rewards. (2005).

Answer 3

@kate has what is probably the more correct answer for the observed pattern.

But as an experiment, I set up a basic simulation to approximate the conditions that you lay out:

1. Starting mean heights of 5'8" (172.72 cm) and 5'4" (162.56 cm) with standard deviations of 2.8" (7.112 cm). I used cm, because it's easier than dealing with inches.
2. Males will not mate with females that are taller than themselves.
3. Females will not mate with males more than 8" taller.
4. Males will not mate with females more then 8" shorter (follows from #3 above).

The problem that I quickly ran into was that, by truncating part of the normal distribution, the variance in height at each generation gradually decreased. After about 20 generations, the means weren't evolving because there was so little variation in height.

Human height is one of the most studied quantitative traits, going back over 100 years to some of the very first statisticians (Fisher, Galton). Height is a polygenic trait with very high heritability (h² = 0.8)¹. Genome-wide association studies have reported 54 genes involved in determination of human height².

Imagine that each of these 54 genes has just two alleles: a and b. a gives a +1 to height. b gives a -1 to height. So aa would be +2, ab or ba 0, and bb -2. The sum of all those alleles is correlated to height. So if all 54 were aa, then the height would be +108.

The problem comes in when people only mate with taller people. Over time, the proportion of b's will decrease, and the proportion of a's will increase, but only to a point. Once all the alleles are fixed at a, there won't be any room left. The genetic variation will be exhausted. Without the input of new alleles, height will cease to evolve.

1 Lettre, G. 2011. Recent progress in the study of the genetics of height. Hum Genet 129:465–472.

2 Visscher, PM. 2008. Sizing up human height variation. Nat Genet 40(5):489-90.