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 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 difficult to apply, some investigators use phenotype, an 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 not overlap with those of another, the two groups might reasonably be considered distinct species.

With which of the following statements regarding the classification of individual species would the author most likely agree?

Phenotype comparison may help to classify species when application of the biological species concept proves inconclusive.

Because no standard definition exists for what constitutes a species, the classification of animal populations is inevitably an arbitrary process.

The criteria used by biologists to classify species have not been based on adequate research.

The existence of hybrids in wild animal species is the chief factor casting doubt on the usefulness of research into reproductive compatibility as a way of classifying species.

Phenotype overlap should be used as the basic criterion for standardizing species classification.


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