Species

There is no clear universally accepted definition of species. A commonly cited definition is that two organisms are in the same species if they can produce fertile offspring.

There are a number of problems with this definition:

1. It can never establish whether two males or two females are in the same species.
2. There are islands in the Galapagos where tortoises from one island can reproduce with tortoises from two other islands but the populations of those islands cannot reproduce with each other. In this case there appears to be no clear species barrier.
3. Some animals are genetically compatible but may not be anatomically compatible. The smallest and largest dogs might find sex to be impractical. Humans might be able to artifically fertilise an ovum and produce fertile offspring but the dogs could not do this without human assistance.
4. Neanderthals and Denisovans are often described as being different species to humans, but humans contain DNA from both groups, suggesting they produced fertile offspring.

As such species is a fuzzy definition.