Which of these describe(s) variation within a species or population? (A population is defined as members of the same species living together in the same area and who are likely to breed with each other.)

fish use gills to obtain oxygen from water
evergreen trees have needle-like leaves
polar bears have differences in the number of hairs per square inch of their skin
hummingbirds have beaks able to collect nectar from flowers while woodpeckers have beaks able to collect insects from tree bark