So if we discovered the last Tasmanian Tiger out in the wild somewhere then it wouldn't count as a species any more because it's the last one and... aww, this is too nitpicky a semantic debate even for me.
The definition is obviously about potential: the last healthy Tasmanian Tiger could mate with another healthy Tasmanian Tiger and have baby Tasmanian Tigers.
A healthy mule can't mate with another healthy mule and have baby mules.
People who happen to be sterile or decided not to have kids can remain a part of the human species...
We have something like that:
http://www.npr.org/blogs/krulwich/2011/05/10/136029423/the-l...
"A species" is typically defined as something like "a group of individuals that can breed with each other".