I would imagine any lines are shadows or accumulated dirt. I don't see the species of wood making any difference.

Species will likely matter to some extent, but the grade of the hardwood you choose will dictate color variations to a greater extent (i.e. those black lines, etc). The lower the grade the more variation/character the floor will have. The higher the grade, the clearer and less varied the wood will be. The higher grades also come with a higher price tag. Using just a clear poly on the floors vs a colored stain should give you a nice light natural look.

Red or white oak are economically friendly options that are harder than pine. Maple and birch offer durability and clearer/less varied boards, but the price is higher than the oak options. All 4 offer you the light color you prefer.

Hope this helps!

Maple will have more durability. Not Birch. Birch is softer than red oak.