Defining a species is an important concept part of the discussion of evolution. Even today, there is still different methods to actually define and describe what makes a species. Generally, a species is defined as a group of individuals that can successfully reproduce, but there are differing concepts that define what that means.
1. Biological Species Concept- This concept says that a species is a species if the members of the population that can or could interbreed/ successfully reproduce in nature. In other terms, if two organisms cannot reproduce or produce fertile offspring, then they are geenetically isolated and are not considered a species. For example, a donkey and a horse can reproduce, but don't produce fertile offspring. Therefore, a mule is not a species because it is sterile. This is the most widely accepted species concept.2. Morphospecies/Phenetic species conept- An older concept that defines a species based on morphological or phenetic characteristics. It generally ignores geographic restrictions, so if two organism look similar, they are the same. For example, if two birds look similar but are seperated by a mountain range and therefore can't reproduce, they are still the same species.
3. Phylogenetic Species Concept- Relies on systematics and monophyly. Identifies a species by estimating how closely related populations are, and that the smallest phylogenetic group is a species. The tips of phylogenetic trees are the species, as seen above. In the example, C has a common ancestor with B, but not with A, so it is its own species.
Freeman, S. & Herron, J. (2007). Evolutionary Analysis 4th Ed. Speciation, pp 605 - 633.
Wolf, M., Chen, S., Song, J., Ankenbrand, M., & Müller, T. (2013). Compensatory Base Changes in ITS2 Secondary Structures Correlate with the Biological Species Concept Despite Intragenomic Variability in ITS2 Sequences – A Proof of Concept. PLoS ONE, 8(6), 1-5.