"Species" is a rather arbitrary concept. Sometimes we divide species by whether they can produce fertile offspring, but many times we divide them by much fuzzie

"Species" is a rather arbitrary concept. Sometimes we divide species by whether they can produce fertile offspring, but many times we divide them by much fuzzier criteria such as where they live, how they behave, or what they look like. If there was an offshoot of humanity that found themselves facing a plague and only those with trisomy 21 survived it, we might well have regarded that population as a different species.