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Name: Bik
Status: student
Grade: 9-12
Location: Outside U.S.
Country: USA
Date: Fall 2013


Question:
If natural selection allows human traits that are better suited to advance why do we still have mutations like diabetes? I know that mutations were a form of adaptation that was actually beneficial in some cases, like Sickle Cell Anemia. However, I searched around trying to find what Diabetes (type 1 because type 2 is usually due to human problems) was useful for. My first belief was that Diabetes could have helped individuals who did not have ample supple of food because (assuming the individual has hyperglycemia) they would be able to survive longer in search of food sources due to the naturally higher blood sugar. However, hypoglycemia is the exact opposite and would in fact make the individual starve faster. So what is your opinion on how Diabetes could be helpful?



Replies:
Not all genetic diseases have an evolutionary advantage - in fact, many if not most are distinct disadvantages. They arise not from a selection mechanism but rather random chance and poor luck. Population genetics studies can often help determine if a seemingly deleterious mutation (such as sickle cell anemia) might confer some previously unknown selective advantage; in most cases, though, this is not the case. This is why most genetic disease are so rare; they appeared in the population and persist through poor luck, not selection.

As for your specific question about diabetes, it is not actually tied directly to a simple genetic mutation. Both type 1 and type 2 diabetes are often linked to variations in a large number of genes rather than a single mutation, though they both also have significant environmental components. It is likely that the genetic factors of type 1 diabetes is due to poor luck rather than evolutionary selection - essentially, variations in how the immune system recognizes 'self' vs. 'non-self' means that it accidentally attacks the pancreas cells that produce insulin. I am unaware of any selective advantage type 1 diabetes-related genes might give an individual, though certainly the mechanism of disease action (the immune system) obviously has distinct selective advantages.

Evolution can be blamed for many things - including, for example, our sweet tooth and tendency to overeat, which in turn can predispose one towards develop type 2 diabetes in the future. But the underlying genetic variations and mutations that are related to developing specific genetic diseases are frequently the result of a random event rather than a selected one. This is even more likely in 'polygenic' diseases tied to many different genes. Since not all individuals with the variant/mutated genes are going to exhibit the disease (since it's dependent on many other genes and environmental factors), these variants can easily persist in the population without being selected against.

S. Unterman Ph.D.


While there are examples of possessing deleterious alleles and survival in response to a pathogen (for example, carrying one copy of the sickle cell gene being protective against malaria, as you mentioned), diabetes is not an example of this phenomena.

The first thing to remember is that there are 2 types of diabetes. Type 1 diabetes is more common in children, and would have been fatal in most instances. However, the genetic causes of diabetes are not as clear cut as when dealing with dominant and recessive genes. While genetics plays a definite role, other factors have also been proposed to contribute to development of the disease, including childhood infections and diet. Thus, there wouldn't be a clear drive to erase this trait from the population, because carriers and even some homozygotes would not be at a disadvantage when it comes to mating and producing offspring.

Type 2 diabetes tends to develop later in life, and while there is still a genetic predisposition, the contributing factors (obesity, sedentary lifestyle, etc) have increased over only recently in terms of the span of human history. While developing the disease would again likely be fatal, developing symptoms after the child-bearing years would have no effect on the stability of the risk in the human population.

There is also the low risk of developing diabetes despite having a genetic predisposition: roughly 1:25 to 1:100 for type I, although there are caveats to those numbers.

Thus, just because a trait exists in the human population, it isn't necessarily an example of molecular evolution. A trait would need to be maintained despite being deleterious during child-bearing years for a large portion of the population for that sort of scenario to be likely.

M. Mefford


Hi Bik,

Thanks for the question. I would not classify diabetes as a mutation. Yes, diabetes may be caused by a mutation. Specifically Type I or Juvenile Diabetes may be caused by a mutation. However, there are several other factors which can lead to diabetes. The only positive aspect I can think of about diabetes is that it can accelerate weight loss. The weight loss occurs since the body cannot use glucose and instead resorts to metabolism of fat stores (adipose tissue). A couple of months ago, there was a nice article in National Geographic about sugar. I would recommend that you look it up as it may provide some additional insight.

I hope this helps. Please let me know if you have more questions. Thanks Jeff


Maybe you need a reply from a hematologist, but the mistake here is the supposition that an adaptation is beneficial to survival when that is not always the case. An adaptation is an adaptation and the beneficial adaptations improve an organism's ability to survive and detrimental adaptations do not. Maybe the diabetic adaptation just slipped through the "survival of the fittest" process, riding the beneficial adaptations through to survival. My engineering experience tells me that systems that are not running in optimum condition (in this case well balanced blood sugar) may still run, but it is not optimum. The system is outside its design specifications and that is not a good, efficient system.

Sincere regards, Mike Stewart


I cannot think of anything advantageous to an individual from having diabetes (Type I or II). There is no good evidence that Type I diabetes is inheritable. Type I diabetes is probably not due to a mutation being passed on to offspring. One theory is that Type I diabetes is caused by a virus inflaming the pancreas which triggers an autoimmune response by the body to destroy the islet cells in the pancreas which make insulin. Type 2 diabetes gene inheritance (probably due to past mutation) is passed on to offspring obviously before the diabetes has caused death. So if Type 2 diabetics usually died before breeding age, then there would be much fewer Type 2 diabetics. Unfortunately, we are now seeing obese children in junior high developing Type 2 diabetes. While diabetics who are not under adequate treatment control do have hyperglycemia (high blood sugar), they are losing a lot of glucose and excess water in their urine (glycosuria) due to osmotic diuresis which can lead to dehydration and electrolyte imbalance. Type I diabetics have insufficient insulin, obese Type 2 diabetics usually have plenty of insulin, but have insulin resistance. Hope this helps.

Dr J. Cowsar San Antonio, Texas USA



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