What prevents horses and donkeys from hybridizing to form a new species?
a. Limited hybrid fertility
b. Limited hybrid viability
c. Hybrid breakdown
d. Gametic isolation

Verified step by step guidance

1. Understand the concept of hybridization: Hybridization occurs when two different species mate and produce offspring. However, for a hybrid to lead to the formation of a new species, it must be fertile and capable of reproducing successfully over generations.
2. Review the reproductive barriers: Reproductive barriers are mechanisms that prevent species from interbreeding or producing viable, fertile offspring. These barriers can be prezygotic (before fertilization) or postzygotic (after fertilization).
3. Analyze the specific case of horses and donkeys: When horses and donkeys mate, they produce a hybrid offspring called a mule. However, mules are typically sterile, meaning they cannot reproduce to form a new species.
4. Identify the correct reproductive barrier: The sterility of mules is an example of a postzygotic barrier known as 'limited hybrid fertility,' which prevents the hybrid from contributing to the gene pool of a new species.
5. Match the correct answer to the question: Based on the explanation, the correct answer is 'a. limited hybrid fertility,' as it directly addresses the inability of the hybrid (mule) to reproduce and form a new species.

Key Concepts

Hybrid Fertility

Hybrid fertility refers to the ability of hybrid offspring to reproduce. In the case of horses and donkeys, they can produce a mule, which is typically sterile. This sterility prevents the hybrid from contributing to the gene pool of either parent species, thus limiting the potential for the formation of a new species.

Hybrid Viability

Hybrid viability is the ability of hybrid offspring to survive to reproductive age. While mules (the offspring of horses and donkeys) are usually healthy and viable, their inability to reproduce means that they do not contribute to the continuation of a new species. This concept highlights the importance of survival and reproductive success in speciation.

Gametic Isolation

Gametic isolation is a prezygotic barrier that occurs when the gametes (sperm and egg) of two species are incompatible, preventing fertilization. In horses and donkeys, even if mating occurs, the sperm may not successfully fertilize the egg due to differences in reproductive mechanisms, further preventing the formation of viable hybrids.