Please help asap The graph below shows changes in the number of families of organisms that existed during a time period in Earth's history. Which of the following describes what would most likely be happening between points Y and Z on the graph?

Adaptive radiation is occurring following a mass-extinction event.
A mass extinction is occurring as a result of changing environmental conditions.
Genetic variation is increasing as a result of the fusion of hybrid zones.
The total population of organisms is increasing as a result of genetic drift.