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

@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.

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.

---

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

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 4

The tendency for mean height to be pushed upward is natural when you consider two facts
(that so far have been left out here):

• With the higher costs to females in general for reproducing, they are the "choosier" gender, so when it comes to mate selection, females' preferences matter more. Therefore, a tendency for males to prefer shorter females (if that is even true) would not have an equal and opposite effect.

• There is not a 1:1 ratio between males and females who pass on their genes. The standard deviation on number of children is higher for males, and a higher proportion of males than females produce 0 offspring, thus "truncating part of the normal distribution", every generation, more for males than for females. Again, even if there were just as much preference for short females by males as there is preference for tall males by females, there would still be a trend upward in mean height. This fact has more to do with the speed of the change rather than whether or not the change will happen, though.

Two facts that have been discussed already but not widely acknowledged here should be explicitly mentioned again:

• Male and female height each influence height in both male and female offspring. This matters because even if there were preference not for the male to be too much taller than the female, this would not stop the trend upward in height because females would be constantly be trending upward in height. Thus, females would not serve as any kind of "anchor" for height.

• Height changes are due to more than just genes (whose effects could "top out" in the absence of mutation, as kmm mentioned), but in any case mutation is not negligible for human height (as Marta Cz-C mentioned), so a lack of possible genotypes for the extreme height we are talking about is not an issue. Actually, this is obvious from the fact that there already are (fertile) people taller than the threshold mentioned in this question.
  (The only real reasons for an upper limit on human height, which have not be brought up here, are nicely presented in this video: How big can a person get?, and those lead to a maximum height somewhere above the 213 cm (7 feet) we are talking about.)

Some things that have been mentioned here, though true or possibly true, are not really relevant to the question:

• A limit on how much taller the father is than the mother for the typical child (imposed by female or male preference) does not change the answer to the main question (direction of mean height change, if any), only the speed ("in 'hundreds of years'"). (This follows from the "anchor" comment above.)

• The fact that there are many other factors than height for mate selection, and even that many of these factors are more important than height, also does not change the answer to the main question. (That is unless the other factors which correlate with lower height put downward pressure on it. I don't think anyone would argue this, but I will provide an example because I think it is interesting: If shorter males were, on average, more intelligent than taller males, then even if females preferred taller males (intelligence being equal), if they also preferred more intelligent males then human height could go down (while mean human intelligence goes up).)

Of course, generalizing all of this to humans in the present and in the future (switching language from "males" and "females" to "men" and "women") is predicated on some assumptions:

• that women, particularly the ones who have children, do prefer and get taller men. This could fail to be the case in the face of competition from women who also prefer taller men but do not have children. This could also change due to reversal of the preference for height or due to increased preference for other factors that correlate with lower height combined with a reduction in the lustre of male height for females. As one example, the ever-decreasing importance of physical competition between males and the ever-increasing level of safety from unarmed attack in the modern world means that height (and other measures of strength) matters less and less. As another example, the reduction in face-to-face workplace settings, which are no doubt at least partially responsible for taller men being paid more than shorter men, should lead to wealth (which is a factor on its own) being less of a correlate with height and therefore reduce selection for height.

• that the preferences of women will continue to matter more than the preferences of men, for mate selection. If women and men were completely rational actors, this one could already have changed due to recent decreases in reproduction costs for females (mostly due to medical changes, such as lower death rate associated with pregnancy) and increases in reproduction costs for males (mostly due to legal changes, possibly to the point where the costs for males are higher than for females). The way women and men think and feel with regard to mate selection is very strongly biased toward the normal conditions in our evolutionary history, though (higher female investment), and are hard to act against.