human evolution in action -- sexual dimorphism

I recently read this report: Short women more successful with men. A study indicates that below-average-height women are more likely to be married and have children than taller women. It’s also been well established that above-average-height men are more likely to be married and have children than shorter men.

Given these two facts, isn’t this a great answer to the question: “are humans evolving”? In the long run, women will become shorter and men taller (that’s sexual dimorphism). There are obviously limits to the height difference (and perhaps we’re already at them). But this shows that the human species is still subject to the same basic forces of natural selection as every other species.

What do you think?

My speculation, FWIW. I think there are three facts to consider here:

  1. Below average height women find it easier to get a partner taller than themsleves.

  2. Above average height men find it easier to get a partner shorter than themselves.

  3. Average height has been getting higher over the years. (No cite but probabaly easy to demonstrate.)

So, my conclusion [dons white coat and serious expression] : Women are not be getting shorter since the average height for women keeps getting higher. Facts 1) & 2) will always be true as long as women prefer taller men and men prefer shorter women (than themselves).

The story is a statement of the obvious and adds no new insights. [removes white coat]

Just a guess.

I believe in natural selection, but in the case of height I don’t think that the “sexual dimorphism” of which you speak will ever take place.

Yes, there is a cultural selection of tall men over shorter ones, and of smaller women over taller ones.

But genetically speaking, that won’t be passed down to their children. IIRC, each individual gets a set of height genes from each parent. A tall parent can pass on short ones, medium ones, or tall ones. The same situation occurs for a short parent. So if the tall man and the short woman get together, it’s not a guarantee that their daughters will all be small and their sons will all be tall.

And if that’s not the case, then it’s not being passed on to offspring - so I really don’t think that humans are evolving in height terms.

Sorry, I know it’s probably not what you wanted to hear.

But the current sexual selection tends to increaser the difference in height of women and men. This is a separate issue than the average height over the entire species.

Just to clarify: humans already exhibit sexual dimorphism (you’re probably refering to the increase in sexual dimorphism).

You’re correct that there is no guarantee. However, the article comments on the fact that height is a sex-linked characteristic. This means the height of women and the height of men may evolve independently. In fact, a mutation that increased the correlation between father/son heights and mother/daughter heights would be beneficial. (Beneficial in the sense of greater reproductive potential.) Likewise a mutation that decreased the correlation between father/daughter heights and mother/son heights would also be benficial. Thus I see a long term potential for greater height differences between women and men.

You seem to be making the assumption from the above, that, shorter than average women are marrying taller than average men. That is NOT what the facts state but would have to be true for your “dimorphism” to be true.
This also doesnt really speak to true averages in the population in general just to statistical data of 2 select groups that by definition do NOT represent the average.

It may be sex linked, but that does not mean it is sex linked genetically, and none of this means that there will be an increase in sexual dimorphism. For the increase to take place, the genes controlling height must be either Y chromosome linked (only expressed in males) or X linked recesive*. Since neither of these seem to be the case, we will not see increase in height differences over time. From the article:

So we don’t really know what genes correlate to height, but the above tells me it is probably not linked to either the X or Y chromosomes. The above quote is odd, because it does not follow any type of inheritance that we really understand. However, the fact that characteristics seem to pass mother -> son rather than mother -> daughter and father -> daughter rather than father -> son is further evidence that there will not be an increase in height difference. Instead, the correlation seems to indicate that there would be a decrease in height difference. We haven’t really seen that either, so the answer to why women are shorter than men overall is probably due to hormonal development rather than genetics (a man having the same height genes as a woman will be a little taller than the woman because of testoterone, estrogen, etc.), and should stay roughly the same from generation to generation.

Aditionally, think about sexual selection for a moment. The height difference has probably reached an equilibrium point and will not increase or decrease in the future. There is a limit at which height difference becomes a disadvantage. I’m 5’9" and prefer the women I date to be 5’7" or 5’8". I do not prefer them to be under 5’.

*there are other methods of X linked expression that could cause differences in the sexes, but I don’t want to get into “partially recesive” genes, additive genes, heterogenous expression, and other more complicated methods right now. I’m trying to keep it simple

Please keep in mind that natural selection and sexual selection are not synonymous terms. Height differences between males and females do not impose any detriment or advantage in survivablility, thus are likely not subject to natural selection.

Also note that the trend, per the OP’s article, seems to indicate that a) short women are preferred by men, and b) tall men are preferred by women. This does not necessarily correlate to preferential pairings between shorter-than-average women and taller-than-average men. More likely, this translates to “for a given male, a shorter female is preferred to a taller one”, and “for a given female, a taller male is preferred over a shorter one”.

This suggests is a possible mechanism for increasing the height difference. Some hormones are, of course, sex-linked. A mutation that causes sex-linked hormones to increase the height difference of individuals would be beneficial.

I agree this is a definitely possibility, as I mentioned in the OP. Does anyone know of any studies comparing height differences of the sexes on a historical basis? Or even comparing height differences between contempory post- and pre-industrial cultures?

I don’t believe I’m making this assumption. My reasoning is:[ol][]Shorter women have more children, thus being short is an adaptive trait for women.[]Taller men have more children, thus being tall is an adaptive trait for men.Therefore, any gene which tends to produce shorter women and taller men will be advantageous.[/ol]Genes which make both sexes taller or make both sexes shorter are not selected for by the population pressures 1. and 2. And, as mentioned above, genes affecting sex-linked hormones are reasonable candidates.

Oops, you’re right about that. I’ve been sloppy in making the distinction between the two. Human sexual dimorphism is almost certainly due to sexual selection and not due to natural selection. (I can think of scenarios where it would be natural selection, and so don’t want to rule it out entirely.)

Oops, you’re right about that. I’ve been sloppy in making the distinction between the two. Human sexual dimorphism is almost certainly due to sexual selection and not due to natural selection. (I can think of scenarios where it would be natural selection, and so don’t want to rule it out entirely.)

There is definitly nothing to suggest such a gene exists particularly not 1 single gene that would accomplish both traits.

Anyway you are trying to assign way more information from this study than you actually obtain. These statistics are nothing but a minor curiosity. No offense. :slight_smile:

Hmmm…I seem to recall that there are in fact “height multipylier” genes that are found only on the Y chromosome. Time for a bit of internet searching.

Catch you later

Incidently, one of the “related articles”, on the sidebar to the article in the OP is this one, wherein it is thought

And from another related article:

Obviously, there is far more at work here than simply sexual selection of height extremes.

After a short time searching the web, I found a cite that is several years old that refers to the proposed existence of genes on the Y choromsome that cause the gene effect for height (which are found on other chromosome pairs in both males and females) to be multiplied in males. Although I cannot remember the citation, I have read in the past 18 months or so that the existence of these height multipliers on the Y choromsome have been verified during the human genome project.

Refer to this URL to read the article!
In addition, testosterone is a definite influence on skeletal growth…since normal females have no Y chromosomes, they don’t get the effects of their height genes multiplied, and since they also have lower testosterone levels than their male counterparts, skeletal growth will be less during puberty.

Just to confuse matters, testosterone not only causes skeletal growth, but also causes the growth centers of the long bones to calcify, thereby causing the growth spurt and then the slowing down of growth.

There are a number of other sexual dimorphism traits that are due to genes on the Y chromosome…the physical differences are also due to testosterone too, of course.

Hope this helps.

Dimorphically yours…and appreciating the other forms…


Way to go, Darwin’s Finch!

Thanks for the cites, Darwin’s Finch and sunstone.

How about from the original article:

It is true that “more likely to have children” does not necessarily imply “have more children”. For typical distributions of the number of children per woman, it is probably safe to assume that they are equivalent. (So, you are right to point out my lapse in logic, but I believe my conclusion is still reasonably valid.)

Actually, my reasoning doesn’t depend on whether or not such a gene exists, but whether it could exist. For if it could exist, then when it occurs (via mutation) it will spread in the poulation.

To argue against myself:

From the first link in D. Finch’s post:

This is an indication that women are at the lower bound of adaptive heights. If men are near the upper bounds of adaptive heights (anyone know?), then the sexual height difference can’t increase much further. Only if women’s mean height decreases (and/or men’s mean height increases) without reducing the lower (or increasing the upper) limit can the height difference increase.