Biology -The species concept
The species concept
Although the domestic dogs shown below differ in appearance, they have
many similar characteristics. domestic dogs belong to the species_ Canis
Living things belong to the same species if they are structurally similar
(look similar) and can interbreed and produce viable  and fertile 
This definition works in most cases. However, it is far from perfect.
Difficulties arise if the species reproduces asexually where no
interbreeding occurs (which is the case with most bacteria) or the organism
fertilises itself (some plants do this).