Biological species.

  1. Similarity between different species.
    The eastern meadowlark and western meadowlark are similar in appearance, but are distinct biological species because their songs and other behaviors are different enough to prevent interbreeding in the wild.

  2. Diversity within a species.
    Though diverse in appearance, all humans belong to a single biological species (Homo sapiens), defined by our capacity to interbreed and produce fertile offspring.

Some species are able to interbreed, but if the hybrid offspring is sterile, the parental species are considered separate biological species.