Determining whether a given population of animals constitutes a distinct species can be difficult because no single accepted definition of the termexists. One approach, called the biological species concept, bases the definition on reproductive compatibility. According to this view,a species is a group of animals that can mate with one another to produce fertile offspring but cannot mate successfully with members of a different group. Yet this idea can be too restrictive. First, mating between groups labeled as different species (hybridization), as often occurs in the canine family, is quite common in nature. Second, sometimes the differences between two populations might not prevent them from interbreeding, even though they are dissimilar in traits unrelated to reproduction; some biologists question whether such disparate groups should be considered a single species.A third problem with the biological species concept is that investigators cannot always determine whether two groups that live in different places are capable of interbreeding. When the biological species concept is dificult to apply, some investigators use phenotype, an organism's observable characteristics, instead. Two groups that have evolved separately are likelyto display measurable differences in many of theirtraits, such as skull size or width of teeth. If the distribution of measurements from one group doesnot overlap with those of another, the two groupsmight reasonably be considered distinct species.