The biological species concept is one of the most widely accepted definitions of a species. It defines a species as a group of organisms that can interbreed and produce fertile offspring, and are reproductively isolated from other such groups. While this concept works well for many sexually reproducing organisms, it has important limitations and cannot be universally applied.

Key Limitations of the Biological Species Concept

One of the major limitations of the biological species concept is that it does not apply to organisms that reproduce asexually. This includes species that reproduce through parthenogenesis—a form of reproduction where offspring are produced without fertilization by a male gamete. Parthenogenetic species can be found among certain fish, lizards, insects, and other invertebrates. In these organisms, the concept of interbreeding and reproductive isolation loses its meaning, as there is no requirement for mating between individuals.

Why Parthenogenetic Species Are Excluded

• No sexual reproduction: Parthenogenetic species do not require a mate to reproduce. Each individual can produce offspring on its own, making the concept of interbreeding irrelevant.

• No reproductive isolation: Since reproduction does not involve mating, there is no mechanism for reproductive isolation, which is a cornerstone of the biological species concept.

• Clonal reproduction: Offspring are often clones of the mother, or may have shuffled genetic material from the mother only, further complicating the application of the concept.

Other Cases Where the Concept Fails

• Asexual organisms: Bacteria, many plants, and some fungi reproduce asexually, so the biological species concept cannot be used.

• Fossil species: The reproductive compatibility of extinct organisms cannot be tested, making this concept inapplicable to fossils.

• Self-fertilizing organisms: Some species reproduce by self-fertilization, further challenging the concept.

Evaluating the Options

Let’s consider the provided options:

1. Parthenogenetic species – The biological species concept cannot be applied, as explained above.

2. Sympatric species – These are species living in the same geographic area; the concept can be applied if they reproduce sexually and are reproductively isolated.

3. Species producing viable hybrids – The concept can be applied, though it may be complicated by hybridization; however, if hybrids are fertile, the boundary between species becomes blurred, but the concept is still relevant.

4. Species of aquatic ecosystem – The ecosystem type does not affect the applicability; if the species reproduce sexually, the concept can be applied.

Conclusion

The biological species concept cannot be applied to parthenogenetic species. This is because the concept is based on the ability to interbreed and produce fertile offspring, which is not relevant for organisms that reproduce without mating.

Correct answer:
(1) Parthenogenetic species