Why are Bulgarians so dark?

Bulgarians are the darkest people in Europe. Why are they so dark, to their Balkan counterparts? Such as Greeks. Everyday Bulgarian... show more Bulgarians are the darkest people in Europe.
Why are they so dark, to their Balkan counterparts? Such as Greeks.

Everyday Bulgarian people:
http://www.theepochtimes.com/n2/images/stories/large/2010/10/07/PHOTO1-WEB.jpg
http://www.neatorama.com/wp-content/uploads/2011/02/rivard.jpg
http://www.avatarhosting.net/pics/50/izbori.jpg
http://www.dnevnik.bg/shimg/zx450y250_1037163.jpg
http://gallowayministries.org/wp-content/uploads/2012/12/DSCF9469.jpg
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