Final answer:
Post-zygotic barriers occur after the A. formation of a zygote and include mechanisms that result in hybrid inviability or hybrid sterility. These barriers contribute to reproductive isolation and speciation by preventing interbreeding between different species after fertilization has occurred.
Step-by-step explanation:
Post-zygotic Barriers in Reproductive Isolation
Post-zygotic barriers are mechanisms that function after the formation of a zygote, which is the fertilized egg cell that marks the beginning of an organism's development. These barriers arise when individuals from different species mate and produce hybrid offspring. One common post-zygotic barrier is hybrid inviability, where the resulting embryo is not viable and does not survive to birth. This can happen due to genetic incompatibilities between the parents' DNA. Another significant post-zygotic barrier is hybrid sterility. In this situation, the offspring may grow to maturity but are sterile and cannot produce viable gametes, meaning they cannot reproduce. A well-known example of this is the mule, an infertile offspring produced by a female horse and a male donkey.
Prezygotic Barriers vs. Post-zygotic Barriers
Prezygotic barriers, on the other hand, prevent reproduction before a zygote can form by hindering fertilization. Barriers like different mating rituals, incompatible reproductive organs, and gametic incompatibility fall into this category. These differences are crucial for maintaining distinct species and prevent gene flow between them, reinforcing speciation.
Conclusion
In essence, post-zygotic barriers allow for the initial formation of a hybrid zygote, but prevent it from becoming a successful contributor to the gene pool of a population due to developmental issues or sterility.