For two populations of organisms to be considered separate biological species, they must be
a. Reproductively isolated from each other
b. Unable to produce living offspring
c. Physically very different from each other
d. A and C are correct
e. A, B, and C are correct

Verified step by step guidance

1. Step 1: Begin by understanding the concept of a biological species. A biological species is defined as a group of organisms that can interbreed and produce viable, fertile offspring under natural conditions.
2. Step 2: Analyze the options provided in the problem. Option (a) mentions reproductive isolation, which is a key criterion for defining separate biological species. Reproductive isolation prevents gene flow between populations, maintaining their distinctiveness.
3. Step 3: Evaluate option (b), which states that the populations must be unable to produce living offspring. This is incorrect because biological species can produce living offspring; the key is whether the offspring are viable and fertile.
4. Step 4: Consider option (c), which suggests that physical differences are necessary for species separation. While physical differences may exist, they are not a requirement for defining separate biological species. Reproductive isolation is the critical factor.
5. Step 5: Combine the correct elements from the options. Option (a) is correct, and option (c) is not essential for defining biological species. Therefore, the correct answer is (a) only.

Key Concepts

Biological Species Concept

The Biological Species Concept defines a species as a group of organisms that can interbreed and produce viable, fertile offspring under natural conditions. This concept emphasizes reproductive isolation, meaning that members of different species do not mate or produce offspring that can survive and reproduce.

Reproductive Isolation

Reproductive isolation refers to mechanisms that prevent different species from interbreeding. This can occur through prezygotic barriers, such as temporal or behavioral isolation, or postzygotic barriers, like hybrid inviability or sterility. Effective reproductive isolation is crucial for maintaining distinct species.

Speciation

Speciation is the evolutionary process by which populations evolve to become distinct species. This can occur through various mechanisms, including allopatric speciation, where populations are geographically separated, or sympatric speciation, where new species arise within the same geographic area due to factors like behavioral changes or polyploidy.