GMAT Reading Comprehension (RC) | Determining whether a given population of animals constitutes a distin

(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.