Issue #4 - Survival of the Thesis

How is a species defined?
Asked by: Adam Ellison

Being able to tell the difference between a poisonous plant and an edible fruit is an ability that humans have developed over centuries of evolution. Over time this natural observation and experimentation developed into the concept of what a species is. Building on this, biologists have documented and studied many life forms in the hope that they can gain a stronger understanding of each and every organism, how and why it exists, and its place within nature. The increase in detailed research has resulted in questions being asked about the exact definition of ‘species’, fuelled especially by Darwin and Wallace’s theories of evolution, which puts animal and plant life forms in a continuous state of evolutionary change.

Modern biological classification has its roots in the work of Carl Linnaeus, who studied animal and plant life closely, grouping species according to shared physical characteristics. The development of the hierarchy of biological classification, in which species is the smallest unit, has allowed biologists to categorize species in more precise detail. This system consists of: Life, Domain, Kingdom, Phylum, Class, Order, Family, Genus and Species. Classification has been refined over the years, shaped by Darwin and Wallace’s theories, and the increasing study and understanding of genes and DNA. However the more precisely scientist tried to define what a species is, the more questions and debates arise.

Ernst Mayr tried to solve this problem by creating and promoting the Biological Species Concept (BSC). The BSC system defines species as “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups". Despite the popularity of the BSC system, it does not go far enough to define all life forms. Some organisms do not fit within the BSC, such as asexual organisms who do not interbreed, hybrid offspring of two distant relatives such as captive tigers and lions, and types of bacteria that are known to evolve horizontally across phylum. These flaws, as well as the system’s inability to define the point where a species splits and becomes a new independent species, has cast doubt over the BSC and its ability to solve the species problem.

Multiple theories continue to exist with the debate raging on, over what the universal definition is and whether it can exist. A clear and consistent definition of species may never be agreed upon, with the concept remaining loose and open for scientists to choose a theory that best fits their research area.

This might be an unsatisfactory answer for many and I would be interested to hear your thoughts on the subject. Do you have an all-encompassing concept that will save the day? Do you disagree with any of this article? Please leave a comment below.

Other Species Theories
typological species, morphological species, biological/isolation species, biological/reproductive species, recognition species, mate-recognition species, evolutionary/Darwinian species, phylogentic, ecological species, genetic species, phenetic species, microspecies, cohension species and evolutionarily significant unit.