With the severe changes to natural environments, many species are coming into contact with other species from which they were once separated, and sometimes members of the different species mate and produce hybrid offspring. One example of this is the pizzly bear, a cross between a polar bear and a grizzly bear. One concern from such hybridizations is that the hybrids tend to not survive long enough to reach reproductive age, many not even completing development. What isolating mechanism is being described here?

Postzygotic isolating mechanism

Explanation:

Reproductive isolation can occur due to prezygotic or postzygotic isolating mechanism. Prezygotic mechanism prevents fertilization of gametes and can occur due to different mating behavior, different mating time period etc. Postzygotic mechanism occurs after the formation of zygote i.e. the organisms are able to mate and fertilization also occurs. However the resultant zygote may not survive. It might not survive immediately or might die before reaching the reproductive maturity. Such a situation would still come under reproductive isolation because the resultant offspring is not viable and not able to pass on the genes to the next generation.

oxygen +1 most likely