Biological Species Concept:
A Species is a group of organisms that are physically and genetically so similar that they are able to interbreed and produce fertile offspring.
Members of a species share a common phylogeny (family tree).
Members of a species share a gene pool that is distinct from that of other species.
Different species rarely interbreed; if different species interbreed, offspring is usually infertile.
Infertile Offspring (Hybrids)
Female horse & male zebra → zorse
Female tiger & male lion → liger
Female horse & male donkey → mule