Species, speciation... and definitions

Species is such a difficult word to define because of the drastic variety of creatures on the planet. Since no universal definition exists, scholars have defined the term in four concepts. The biological concepts divide species by reproductive isolation. It states that a species is a group of individuals that can mate with each other and not with other species. This definition is only true for individuals with reproductive data, which excludes every asexual organism and extinct organisms whose reproductive data was not collected. The morphological concept groups individuals in a species by their body shape and other structural features. However, morphological characteristics can provide subjective division. For example, some people may view one physical characteristic as a dividing trait between species, but others may see the characteristic as unimportant. The ecological concept classifies species based on their interaction with the environment, i.e. food, adaptation, and survival instincts. However, this classification is not reliable because every individual in a species is different, meaning they will respond differently to scenarios. Finally, the phylogenic concept divides species by their place on the tree of life. A species can be found as the smallest branch of individuals on the tree with a shared ancestor. However, this concept produces an excessive number of species that cannot be easily distinguished by the naked eye, many more species than any other concept previously discussed. 

Since nature is constantly adapting and mutating into more and more species, I think creating a universal definition for species is nearly impossible. We still have not identified every species on the earth, and as time continues, the number of species is going to continue increasing. For research purposes, I think the closest universal definition of species would be the phylogenic concept because that is one of the only definitions that does not allow for the subjective division of organisms, even though this results in a large number of species.