[A]: The natural hybrids of the species are either totally absent or are very rare.
[R]: Because members of separate species do not interbreed usually

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that natural hybrids of species are either totally absent or very rare. This implies that there is a lack of hybridization occurring in nature between different species.

2. **Understanding the Reason**: The reason provided explains that members of separate species do not usually interbreed. This means that the lack of hybridization is due to the reproductive barriers that exist between different species.

3. **Defining Hybridization**: Hybridization occurs when two different species mate and produce offspring, known as hybrids. For example, when a horse (species A) and a donkey (species B) mate, they produce a mule, which is a hybrid.

4. **Examples of Hybrids**: The mule is a classic example of hybridization. However, it is important to note that mules are typically infertile, meaning they cannot reproduce. This is a common outcome of hybridization between different species.

5. **Reproductive Isolation**: The reason for the rarity of natural hybrids is largely due to reproductive isolation mechanisms. These mechanisms prevent different species from interbreeding and producing viable offspring. This can include behavioral differences, temporal isolation (mating at different times), and mechanical barriers (differences in reproductive structures).

6. **Conclusion**: Both the assertion and the reason are true. The assertion that natural hybrids are rare is correctly explained by the reason that members of separate species do not usually interbreed. This lack of interbreeding is what leads to the absence or rarity of natural hybrids.