The biological species concept classifies species as a group of actually or potentially interbreeding individuals. This means that any two individuals (well a male and female) within the group could breed and produce fertile offspring.     This is definition  is commonly used but there are some issues with it, and situations it can't be used in.      1) Not all species sexually reproduce      2) It can't be applied to the fossil record                The phylogenetic approach is now commonly used instead of the above, especially with increased knowledge of genetics. By comparing DNA sequences it can be found which individuals have a common ancestor (an organism the whole group evolved from). This (monophyletic) group can be classed as a species. This species classification system is better as it     1) can be applied  to asexual organisms          2) Can be used for extinct species if DNA is available       3) No need to test the population to see if they can interbreed as this can be very difficult   4) However it is subjective - the scientist has to decide how recent a common ancestor to use (as all individuals have a common ancestor at some point in evolution). Usually the most recent common ancestor of a population, that can be distinguished from other populations, is used.