Ernst Mayr, during the 20th century coined a species as a group of actually or potentially interbreeding species which are reproductively isolated from other such group. Within Mayr's definition, a species can be used to describe a group of individuals connected by gene flow that is isolated, in terms of genes, from other individuals in that group. There exists gene flow between organisms within a species, but not outside of that group. These descriptions are widely regarded as the most apt but there is some difficulty still in terms of a pure definition of the concept. Where does one species end and another begin? Does a specific boundary exist between one species and another? Due to the lack of concrete knowledge known about the evolutionary history of most organisms these questions are often quite difficult to answer.…show more content… Reproductive isolation or the biological species concept versus the relationships in branches on evolutionary trees highlight the hypotheses on the origin and development of species. Speciation is a gradual, slowly developing process, so it is common to find populations that are only partly reproductively isolated. This semi isolation means that species from different lineages can engage in gene flow, to a certain extent, sometimes to the point where their lineages merge again, but this is rare. The major point is that there is no clear cut, precise way to define species, as some take certain characteristics to mean other things, for example, in terms of analogy, most people are seen as either children or adults, however different people could classify a child as an adult or an adult as a child due to factors such as physical characteristics or emotional maturity. It's not an exact