Final answer:
John Ray defined species based on their reproductive ability to interbreed and produce fertile offspring, a foundational concept of the biological species definition still used today.
Step-by-step explanation:
John Ray defined species by their reproductive strategies, a concept we still use today. The formal biological definition of a species includes groups of actually or potentially interbreeding organisms that can produce fertile offspring together. This definition has practical challenges, particularly when studying asexually reproducing organisms, or when dealing with the fossil record where breeding behaviors cannot be observed. In these situations, species can be determined by shared morphological or genetic characteristics, as proposed in the phylogenetic species concept. Additionally, Ernst Mayr extended the understanding of the biological species concept to incorporate a focus on reproductive isolation between populations.