Great Question! The Talmud itself deals with this. In reality, the modern concept of adaptation to the environment is rooted in the Talmud written 1800 years before Darwin was born!
The Sages teach us that in order to help different people survive in their respective climates and conditions, God gave them physical characteristic traits adapted specifically for their environment. For example, the Talmud states that God gave people who live in swampy regions large feet to help them contend with the soft footing. And to those who live in eastern desert climates God gave special eyelids to help them contend with the sand which kicks up.
In the same line of reason, we can clearly postulate that God adjusted the melanin in the epidermis of people who live in different longitudes to contend with the varied exposure to the angles of the sun's rays. Or, simply put, God did it.
This is clearly the way God created the whole world. Many times in Tanach we see that God created everything in kindness, with the innate ability to survive and contend with its environment. Not only does God give all life its capability to survive, but he created the inanimate world with many different manifestations of nature to protect and enhance life.
For example, King David in Psalms says that God gives "snow like wool." What kind of comparison is this? Snow is cold and wool is used to warm and protect? Yet, careful observation gives us the answer: Snow acts like an insulation to protect the ground and its vegetation from the extreme cold of winter. The more to the poles we traverse the more snow there is, further insulating the ground underneath. Many animals even have the innate understanding to burrow themselves in the snow during extreme temperatures.
In his inspired wisdom, King David perceived this concept of ground insulation marvelously. Differences among people, too, can be seen as helpful adaptations from a benevolent Creator.