Scientists have proposed more than two dozen ways to define species. One commonly used definition, called the biological species concept, holds that populations belong to the same species if they can contribute genes to a common gene pool. Reproductive barriers block gene flow between different species. Gene flow between populations tends to hold the gene pool of a species together. Three of the most prominent definitions of species are the biological species concept, the phylogenetic species concept, and the morphological species concept. For each characteristic, identify the a