| Line | Determining whether a given population of animals |
| constitutes a distinct species can be difficult |
| because no single accepted definition of the term |
| exists. One approach, called the biological species |
| (5) | 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 |
| (10) | 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 |
| (15) | 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 |
| (20) | that investigators cannot always determine whether |
| two groups that live in different places are capable |
| of interbreeding. |
| When the biological species concept is difficult |
| to apply, some investigators use phenotype, an |
| (25) | organism’s observable characteristics, instead. |
| Two groups that have evolved separately are likely |
| to display measurable differences in many of their |
| traits, such as skull size or width of teeth. If the |
| distribution of measurements from one group does |
| (30) | not overlap with those of another, the two groups |
| might reasonably be considered distinct species. |
