What is a species?
There are two main 'qualities' of a species:
They are similar to one another but different from members of other species. They have similar genes and resemble each other. They have similar immunological and development features, and occupy the same ecological niche.
They are capable of breeding to produce living, fertile offspring. This means that when a species reproduces sexually, any of the genes of individuals can, in theory, be combined with any other. i.e. they belong to the same gene pool.
The binomial system
The binomial system came into practice because without a worldwide convention for naming species scientists could not be sure that new species were actually new species. The binomial system gives organisms two names:
A generic name. This denotes the genus to which the organism belongs. It is a bit like a 'surname'.
A specific name. This denotes the species to which an organism belongs. It is a bit like a 'first name'.
The names are all Latin or Greek, and are written in italics, if printed, or underlined, if handwritten. These names are subject to change, as our scientific knowledge and understanding increases.
Principles of classification
Classification of species allows better communication between scientists, and less confusion. There are two kinds of classification:
Artificial classification. This divides organisms according to differences that are useful at that time. They are described as analogous characteristics where they have the same function, but different evolutionary origins (e.g. a butterfly and a bird will both have wings used for flight, but they originated very differently).