Question

At what point are two populations descending from the same ancestral stock considered separate species? A. When they can no longer produce viable, fertile offspring B. When they look significantly different from each other C. When they can interbreed successfully and produce offspring D. When their habitats are separated by a significantly large distance so that they cannot meet

Short answer

A. When they can no longer produce viable, fertile offspring

Step-by-step solution

Understand the Concept of Species

A species is defined as a group of organisms that can interbreed and produce viable, fertile offspring. Viable means the offspring can survive, and fertile means they can also reproduce.

Evaluate Option A

Option A states that two populations are considered separate species when they can no longer produce viable, fertile offspring. This aligns with the biological definition of species, as reproductive isolation is a key factor.

Evaluate Option B

Option B states that two populations are different species when they look significantly different. This is not a definitive factor because physical appearance alone does not determine reproductive capability.

Evaluate Option C

Option C states that two populations are separate species when they can interbreed successfully and produce offspring. This contradicts the definition of species, as it suggests they are the same species if they can interbreed.

Evaluate Option D

Option D states that two populations are separate species when their habitats are significantly distant, preventing them from meeting. While geographic isolation can lead to the formation of separate species, it is not the defining criterion.

Conclusion

After evaluating all the options, it is clear that Option A is the correct answer as it aligns with the biological definition of species.

Key concepts

Reproductive Isolation

Reproductive isolation is a critical factor in speciation. It occurs when two populations of the same species can no longer interbreed and produce viable, fertile offspring. This isolation can be due to various barriers:

• Prezygotic barriers: Factors that prevent mating or fertilization, such as temporal isolation (different mating seasons) or behavioral isolation (different mating rituals).
• Postzygotic barriers: Factors that occur after fertilization, such as hybrid sterility (offspring cannot reproduce) or hybrid inviability (offspring do not survive).

Reproductive isolation ensures that genetic differences accumulate over time, leading to the emergence of new species.

Species Definition

The biological definition of a species revolves around the ability to interbreed and produce viable, fertile offspring. This means that if two populations can mate and their offspring can survive and reproduce, they are considered the same species. Conversely, if they cannot produce viable, fertile offspring, they are recognized as separate species.

• Viable offspring: Offspring that can survive to maturity.
• Fertile offspring: Offspring that can reproduce successfully.

This definition highlights the importance of reproductive capabilities over physical appearance or geographic distribution.

Geographic Isolation

Geographic isolation occurs when physical barriers divide populations. These barriers can include mountains, rivers, or vast distances. While geographic isolation alone does not define species, it plays a significant role in speciation. Isolated populations experience different environmental pressures and genetic drift, leading to divergent evolution. Over time, these differences can result in reproductive isolation, thereby creating new species.
Examples of geographic isolation include:

• Mountain ranges separating terrestrial animal populations.
• Rivers splitting populations of aquatic organisms.

Such isolation is often the first step in the speciation process, particularly in the case of allopatric speciation, which occurs when a species is split into two or more geographically separated groups that evolve independently.

Viable Offspring

Viable offspring are essential for the continuation of a species. Viability refers to the ability of the offspring to survive and grow to maturity. For two populations to be considered the same species, their offspring must not only survive but also be fit enough to reproduce.
The following are crucial for offspring viability:

• Genetic compatibility: Ensuring that genes from both parents can work harmoniously.
• Environmental suitability: The offspring must thrive in their habitat without facing excessive threats.

If hybrids (offspring from different populations) are not viable, it solidifies reproductive isolation and confirms that the populations are distinct species.