Final answer:
Differences in plant species over geologic time are explained by evolution and tectonic plate movements. The breakup of supercontinents like Pangaea and Gondwana led to geographic isolation and unique species distributions, while the equilibrium between speciation and extinction over time influenced overall biodiversity.
Step-by-step explanation:
Earth scientists explain differences in a plant species over geologic time through the process of evolution. This process, alongside the movement of tectonic plates, contributes to the diversity and distribution of plant species. For instance, before the supercontinent Pangaea broke up around 200 million years ago, broad plant groups were distributed worldwide. After the breakup, these groups appeared uniquely in different regions, evolving separately because of geographic isolation caused by the tectonic shifts.
The concept of biogeography highlights that the geographic distribution of organisms is deeply intertwined with evolutionary history and that the movement of continents over geologic time has impacted where species occur. The breaking apart of Gondwana led to flora like the Proteaceae family being spread across now distant continents such as Australia, southern Africa, and South America. The distinct plant assemblages we find today in different parts of the world, including endemism in areas like Australia, reflect these deep historical processes.
Furthermore, the dynamic equilibrium between speciation and extinction rates over geological time leads to the biodiversity changes observed in the fossil record. Continuous macroevolutionary processes, supplemented by environmental changes such as earthquakes and continental shifts, have shaped the adaptation and survival of plant species, resulting in the diversity we see in different geological periods.