A species is often defined as the largest group of organisms where two hybrids are capable of reproducing fertile offspring, typically using sexual reproduction. While in many cases this definition is adequate, the difficulty of defining species is known as the species problem.his definition can be extended to say that a species is a group of organisms that could potentially interbreed—fish could still be classed as the same species even if they live in different lakes, as long as they could still interbreed were they ever to come into contact with each other.