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Name: Christine
Status: student
Grade: 9-12
Location: OH
Country: USA
Date: N/A 


Question:
We learned that species adapt to their environment through mutations and inherited traits. If that's true, why, after hundreds of thousands of years of eating red meat and being in the sunlight, do scientists say both of those things are bad for a person? Wouldn't you think that the body would adapt to a diet of meat and high cholesterol? And wouldn't the body build up defenses against harmful UV light?



Replies:
Yes, and we have! First of all, evolution is not goal oriented. Mutations happen randomly, but then natural selection takes over and those with the favorable mutations survive long enough to reproduce.

Dark skin is dark because it contains melanin which is a natural form of "sunscreen". The initial guess as to why there are different skin colors is to protect against skin cancer. This was because it was noticed that the darkest skin occurs in areas with the most direct sunlight. But, skin cancer usually occurs after the age of reproduction. So evolutionarily speaking, that is not a viable answer. Now, the current hypothesis is that it protects against the loss of folate which is necessary during the development of the nervous system in an embryo. But skin that is too dark doesn't let in enough Vit D. So as people moved away from equatorial areas toward the poles, sun intensity decreased. Skin color is a balance between having enough Vit D and protecting folate levels.

As far as eating red meat, most heart disease occurs later in life past the age of reproduction, so it shouldn't be a selection pressure. The meat we eat today is raised to contain more fat because it tastes better, and we eat a lot more meat today than our ancestors were able to. Our teeth show that we have evolved to be omnivores, whether you choose to eat meat or not.

vanhoeck


Christine,

Organisms develop responses/defense to things that harm them through a process known as natural selection. There's natural variation in the gene pool, some of which is beneficial, and some of which is harmful. The helpful traits increase reproduction, and so the beneficial traits tend to accumulate over time. But that doesn't mean the harms are no longer harmful -- it just means organisms have found ways to manage or mitigate those harms. Moreover, some things that are harmful don't affect (or happen after) reproduction, so simple models of natural selection don't necessarily apply.

Let's start with a simple example of wounds (cuts, scrapes, etc.). The body has platelets to make scabs, and the immune system to protect against infection, and these have evolved over time. But if someone stabs you with a sword, it's still really bad. People who survive wounds live to reproduce -- people who don't survive wounds as well (for example, people with hemophilia), tend not to reproduce as much. Humans have evolved methods of dealing with wounds -- but getting stabbed is still bad.

Referring to your example of sunlight -- sunlight is actually both helpful and harmful. The body has build up defenses against sun to some degree -- peoples' skin darkens after prolonged exposure to sunlight, and there are genetic components to skin color too. People in tropical zones tend to have darker skin than people in polar zones. Sun can still harmful, but people with darker skin have some added defenses. However, sun is also important in making vitamin D -- if you don't get enough sunlight, you can have a deficiency of vitamin D (that's bad). In terms of natural selection, as long as the person makes it to reproductive age, then the selective pressure to change is gone (or nearly so).

Your example of diet is a little different. Human's diet has changed dramatically since industrialization (the last 100-200 years) adding much more meat. Before that, stable crops (wheat, potatoes, etc.) were the primary source of calories. In developing countries, meat is still a relatively minor component of diets compared to the US. Humans have had very little time to respond to this change on a global scale. However, there is a lot of evidence of pre-existing genetic variation that affects how peoples' bodies handle meat. For example, some peoples' bodies convert HDL ("good cholesterol") to LDL ("bad cholesterol") inefficiently, resulting in lower risk of heart problems. Other people convert LDL very efficiently -- and have problems because of it. This example is also interesting because heart problems tend to emerge *after* reproductive age is reached. So the simple models of natural selection don't work as well for long-term or late-life health issues.

Hope this helps,
Burr


How long do you suppose humans have had a diet high in fatty meat? Probably not for many generations. We have evolved to crave salt, fat, and sweets because those nutrients are rare in Nature. We haven't had a chance to adapt to abundant supplies.

Sunlight is a double-edged sword. Populations who have long lived in high-sun areas consistently develop dark skin as partial protection. But we don't want to be too well-protected, because UV is needed for us to generate Vitamin D from its precursors. So evolution has come up with a pretty good compromise based on those conflicting needs.

Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming



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