Humans have a need to classify and simply information. One way we do this in biology is by naming and classifying all the organisms that have been discovered. A scientific name is given to all organisms so that they can be distinguished from one another. This name consists of its genus and species. For example, the scientific name for Humans its Homo sapiens, Homo being the genus we belong to and sapiens being our species. Species is the smallest unit of biological taxonomy. It contains organisms that are of one kind or appearance while a genus may contain one or more species and a family many contain many genus and so forth up the taxonomic chain (species, genus, family order, class, phylum, kingdom).
Distinguishing species is very important task to several areas of biology. It is of course important to taxonomists since its their job to classify organisms, but it also important to the study of evolutionary biologists, geneticists, systematicists, and other biologists who do their research with model organisms. In order to do their research, they may need to know which organisms are part of a particular species.
You would think that, if all you have to do is determine if two organisms are of the same kind, defining a species and placing organisms in that category would be easy. But it is not as easy as you might suppose. While some organisms are easy to distinguish as separate species, for example no one would assume that a cat and a mouse are the same species. Other organisms might be more difficult to distinguish as two species such as distinguishing between like two insects that are similar in look. Biologists use many different traits including morphology (look), behavior, biochemistry and genetic makeup to distinguish and categorize organisms into species. Using all these different types of traits means that we could possibly define a species in several different ways.
One definition of species that biologists use is called the biological species concept. It was established in 1942 by an evolutionary biologist named Ernst Mayr. It states that a species is defined as a population or groups of populations whose members have the potential to interbreed with one another in nature and to produce viable fertile offspring but who cannot produce viable offspring with members of another species. This means that a biological species is genetically isolated or distinct from other biological species.
For example, a lion is its own distinct species. It can reproduce with other lions but cannot reproduce with tigers in nature. Now wait a minute you might say, I have heard where lions and tigers have interbred and produced offspring so how are they still separate species? The key is that they have to be able to reproduce in nature and that they have viable and fertile offspring. In the case of lions and tigers, they don't live in the same environment so they would never interbreed in nature. Secondly, while their offspring (when they reproduce accidentally in captivity) is viable (meaning born alive and able to live) it is not fertile therefore you could never cross two ligers or tigons (as lion/tiger crosses are called) to each other.
There are some problems with this definition of species. One is that it doesn't encompass all the organisms that one might come into contact with. It makes no allowances for asexual species such as plants that sprout from cuttings, or for fossils. We cannot know whether two fossil species were able to reproduce with one another since they have long be extinct. In the case of fossils or asexual species, other definitions of species must be used for research purposes. While the biological species concept is not perfect, it is a good workable definition of species for most biologists and researchers.