Final answer:
It is true that species are defined by the ability to reproduce and form fertile offspring, a concept first recognized by John Ray. This biological species concept helps distinguish between different species based on reproductive isolation. However, the concept is not always easily applied, especially in the case of asexual organisms or those that can produce hybrids.
Step-by-step explanation:
According to John Ray and the biological species concept, it is true that groups of plants and animals could be distinguished by their ability to reproduce with each other, leading to the term species.
Specifically, a species is defined as a group of individual organisms that can interbreed and produce fertile, viable offspring. This reproductive capacity distinguishes one species from another, preventing individuals of different species from producing fertile offspring in natural conditions. Reproductive isolation, whether geographic, behavioral, or genetic, contributes to the formation of new species.
Despite this clear definition, applying it in the real world can be challenging. This definition primarily applies to organisms that reproduce sexually and does not easily cater to asexual organisms or ones where hybrid offspring are possible. The ability to interbreed is fundamental to defining a biological species.