Imagine two dogs: one with short hair and one with long hair. Now imagine these two dogs leaving Noah’s Ark, reproducing, and spreading out across the world. Some of the dogs travel North and enter into colder regions. The short hair dogs suffer from the elements and are not able to survive. They die out, but the long haired dogs survive because they have the necessary protection from the cold. They are able to reproduce and pass on their genes, creating more long haired dogs who can withstand the harsh cold environment. However, these dogs will only be able to produce more long-haired dogs because the short-hair DNA information is now lost. On the reversal, if the long haired dogs stayed near the equator, it would be too hot for them to survive. They would die out while the short haired dogs would thrive and reproduce. This illustration shows why we see high densities of long haired and short haired dogs in cold and warm climates, respectively.
The medium length hair dogs below have the DNA for long hair and short hair, so they can produce a variety of hair lengths. If two long haired dogs reproduce, they will isolate the genes and only produce long haired dogs. This explains why we would see a certain trait carried on in animals or people in specific areas. This can change when alternate DNA is introduced, such as a long haired dog breeding with a short haired dog.
Apply this concept to other traits we see. People with darker skin live in sunnier regions where their skin is not affected as much by the heat and they have lower risks of skin cancer. Light skinned people would not do well in these hotter regions because they are at risk of skin cancer. On the other hand, people with dark skin who live in colder, less sunnier climates would not receive as much Vitamin D in their skin, thus they would be at risk for rickets and other vitamin D deficiency disorders.
When a specific trait like light-colored skin is lost and only dark-skinned people remain, the light-colored trait will not return until a dark-skinned person has a child with a light-skinned person. The child’s skin shade would be determined on the dominant and recessive genes. If that child had the light-colored skin, he/she would be able to pass on that trait to another generation. However, if the environment was not favorable to them (if they got cancer, for example), then their chances of survival or successful reproduction would be lower. Neither the dark-skinned nor light-skinned people “evolved” to their surroundings, but rather, their already existing favorable traits allowed them to survive and thrive in that particular environment.