Depression as a Tool of Adaption?

I know this is not law school related, but I felt like playing amateur scientist for this evening.

I am interested in the concept of evolution.  Whatever trait we have originated because of a need in the past to help the species survive.

Now, people love to go on about "women be different than men!" jive, and only think about evolution as fast runners outstripping a pursuing lion, but there are all sorts of other traits that serve other purposes.

Before I explain the title of my piece, let me back up and give a back story.

A friend of mine told me that she was being treated for Vitamin D deficiency.  I was curious about this condition, so I looked it up on the internet.  I read that people of different races needed different amounts of Vitamin D, and that African-Americans needed much more than people of European descent, and that people in more northerly climates get most of what they need from eating sea food.

As you know, people in Norway and Iceland are pale and blond, so I began to wonder how those people effectively adapted to that environment.  After all, it's not like the pale people from ancient Africa decided to go north one day.  Something happened where they all eventually became that way. 

Even with genetic mutation, how can one explain such uniformity in personal characteristics?  Dark skin and dark eyes tend to be a dominant trait.  If a person from Iceland has a child with a person with dark skin, that child is going to tend towards darker skin.  It's not like you can simply breed it out of the population on that scale like lab rats.

...Or CAN you?

Now, we know that illnesses from lack of sun exposure do show up in the form of Ricketts, but, on the whole, illnesses of the body tend to be slow acting.  If your family has a tendency towards heart disease, it's not going to be genetically bred out on its own because the illness is going to strike long after that person has popped out a multitude of children.  As such, people in northerly climates were probably already eating a sizable amount of sea food and probably could have gotten by with some level of nutrition.  They may not have lived a long, healthy life, but still would have survived long enough to have children.

So, then, my thoughts turned to this:

What part of the United States has the highest suicide rate in the entire country?

Seattle.

Seattle also has the most cloudy and rainy days of about any city in the nation.

Although everyone acknowledged the link between the two issues, I started looking at it differently.  I then asked myself, "what if depression is a tool of adaptation to the surroundings?"

When one thinks about it, depression is not only a genetic issue where it tends to run in families.  It can be triggered by the environment---particularly when a person is not adapting well to their surroundings.  They lost a job.  They lost a family member.  They were rejected by the community for being different.  They have run out of food.  They are in the middle of a war.  In response to their conditions, they either become lethargic and do nothing, or become restless and are spurred into running away from the situation.

Obviously, the people who did not commit suicide were better-tolerated to that environment, but it is a deeper issue than simply saying that people who were intolerant to the weather simply killed themselves.  It can also be a passive act.  It causes social isolation and lack of sexual drive, which obviously means fewer children.
  But, the illness can also cause a group of angry malcontents who simply become tired of the situation and leave.  If people are ill because of nutrition, it directly affects their mental abilities.  The body, sensing that it is not getting what it needs, creates a depressed state in the mind to seek immediate change in environment.  If that person simply stays in their current environment, they are less likely to socially participate and genetically contribute to the blood line.  As opposed to physical illness from stress, such as diabetes and heart disease that generally will not kill people until well after they had children, depression is much more immediate in effect.  Considering the fact that depression usually rears its head during the teenage years (the time when a person could start having children), it may simply be a necessary part of a person's makeup to ensure that they are well-motivated to try and find suitable surroundings in which to raise their children. 

In essence, the depression from malnutrition is signalling the body that this is an inappropriate environment to have children.

Let's say, hypothetically, that in the case of a country like Iceland, a large group of people once settled there that once looked as dark as people in the Mediterranean.  Illness from lack of sun and an inability to get everything that they need from nutrition would begin to scourge the population.  Over the centuries, the people whose bodies could not adjust to the level of sunlight in the place engaged in self-destructive behaviors, had fewer children, or simply moved somewhere else, while the people who could better tolerate the environment stayed, had more children, and continued to thrive from the available resources.