Punctuated equilibrium (2 of 8) Isolation
Isolation: A drop in sea level forms a lake and isolates a small number of mollusks from the rest of the population.
Punctuated equilibrium (3 of 8) Strong selection and rapid change
Strong selection and rapid change: The small, isolated population experiences strong selection and rapid change because of the novel environment and small population size: The environment in the newly formed lake exerts new selection pressures on the isolated mollusks. Also, their small population size means that genetic drift influences their evolution. The isolated population undergoes rapid evolutionary change. This is based on the model of peripatric speciation.
Punctuated equilibrium (4 of 8) No preservation
No preservation: No fossils representing transitional forms are preserved because of their relatively small population size, the rapid pace of change, and their isolated location.