Reproductional isolation is a way to prevent closely related species to mate each other. This is due to reproductive barriers which can be either prezygotic (before fertilization) and postzygotic (after fertilization) barrier. Behavioral isolation is an example of postzygotic barrier. It is based on behavior, including mating sounds and rituals. Thanks to behavior isolation, the different species (in this example gentoo penguins and emperor penguins) will have different mating rituals so they could attract only individuals of their own species, respectively.
Weather refers to short term atmospheric conditions while climate is the weather of a specific region averaged over a long period of time. Climate change refers to long-term changes.
Index fossils are used by scientists to help them figure out the dating of discovered fossils or the correlation. They are also very known to appear in a (very) certain/specific age range.
