Re: Why humans differ widely in size?

Great question Gaby!
Humans are among the most versatile animals in the world. We have adapted to nearly every climate or situation the world has to offer. Part of our ability to do this is the amount of individual variation we have in our species. This variation, which can take the form of size, is based on many factors both environmental (climate, nutrition and disease) and genetic (heredity).

Let's start with the environment: A person living in a healthy environment is more likely to grow taller because their body can spend more energy on growing rather than fighting off disease. A person might not grow to their maximum possible height if they don't get enough to eat or don't get enough of a particular nutrient, like calcium or protein. Environment can also affect the body size and shape of a whole population of people over generations. For instance, people who live in hot climates are more likely to have longer limbs (Allen's rule) because they dissipate heat more quickly. Likewise, people living in very cold climates tend to have more massive, round bodies (Bergmann's rule) to hold in heat longer. There's many other environmental conditions (altitude, amount and quality of food and water, dense vegetation etc.) that favor either tall or short body types.

Then there's the genetic side of things. Obviously, you're more likely to be tall if your parents are tall. But it should be mentioned that height is not controlled by one pair of genes, but by many. It is a polygenic trait, so, many sets of genes contribute to height. For (a very simplified) example: different sets of genes control the length of the spine and the length of the femur (which is why people tend to be closer in size when they are siting than when they are standing). So each person gets a mixture of different instructions for each part of the body. Depending on the mixture of genes coding for length and height one gets, they could be tall, short, or more likely, somewhere in the middle.

In your question you also bring up sexual selection. Historically, this has also been an important factor in determining the height (as well as many other traits) of the offspring. Sexual selection is still alive and well in modern times- just open up the personal ads and you will see ad after ad of people describing what traits they desire in a mate. I don't believe sexual selection is as important in determining height (and other traits) as it once was, because given the higher population and comparative ease of survival in an industrial society, there just isn't as much competition to squeeze certain traits out of the gene-pool.

For more info on this topic, check out these other Madsci question responses:

www.madsci.org/posts/archives/may2001/991259808.An.r.html
www.madsci.org/posts/archives/jan99/916624846.Ev.r.html
www.madsci.org/posts/archives/jun2000/960243185.Ge.r.html
www.madsci.org/posts/archives/apr2001/986167663.Ge.r.html

Hope I've been helpful!