UC Berkeley
Image Caption
Expansion and stasis: The isolated population expands into its past range. Larger population size and a stable environment make evolutionary change less likely. The formerly isolated branch of the mollusk lineage may out-compete their ancestral population, causing it to go extinct.
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This image is part of a series:
Punctuated equilibrium (1 of 8) Stasis
Stasis: A population of mollusks is experiencing stasis, living, dying, and getting fossilized every few hundred thousand years. Little observable evolution seems to be occurring judging from these fossils.
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…
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.
Punctuated equilibrium (5 of 8) Reintroduction
Reintroduction: Sea levels rise, reuniting the isolated mollusks with their sister lineage.
Punctuated equilibrium (7 of 8) Preservation
Preservation: Larger population size and a larger range move us back to step 1: stasis with occasional fossil preservation.