Biological Species Concept

Asexual reproduction does not involve interbreeding, so the concept of reproductive isolation is no longer meaningful.

There is rarely a definitive moment marking the transition from one species to another.

Differences in size and shape of fossil bones cannot reveal whether there was reproductive isolation between the individuals from which the bones came.

Two non-interbreeding populations may be connected to each other by gene flow through another population, so there is no exact point where one species stops and the other begins.

Hybridization—the interbreeding of closely related species—sometimes occurs and produces fertile offspring, suggesting that the borders between the species are not clear-cut.

Scenario 1:
Mineralized remnants of large mollusk shells are found high above sea level across Europe. Not only does this finding indicate that these areas were once under water, but this also reveals that there were diverse varieties of mollusks in this area.

How will the biological species concept fall short of classifying these into distinct species?

Scenario 2:
Cichlid fish are very diverse and thrive in Africa and South America. It is not uncommon for several species to coexist in the same habitat. Occasionally, members of different cichlid species are able to breed and produce fertile offspring. These offspring not only are able to breed with one another, but they also are able to breed with members of their parents’ respective species.

How will the biological species concept fall short of classifying these into distinct species?

Scenario 3:
Two butterfly species inhabiting southern Canada are incapable of breeding with each other; however, each can breed with butterfly populations west of the Rocky Mountains as well as with some distinct butterfly populations in the Midwestern United States.

How will the biological species concept fall short of classifying these into distinct species?

Scenario 4:
A certain fungal species can only produce offspring via mitosis. As a result, progeny are genetically identical to the parents. Occasionally, mutations occur, causing the progeny to have a dramatically different appearance from their parents.

How will the biological species concept fall short of classifying these into distinct species?

Scenario 5:
The oldest known ancestor of modern horses appeared around 55 million years ago and was about the size of an average dog. Although there have been many branches to their evolutionary tree since then, horses today have features that resemble their oldest extinct ancestor and ancestors since.

How will the biological species concept fall short of classifying these into distinct species?