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Punctuated equilibrium (6 of 8) Expansion and stasis 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. << Back to search results

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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 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.

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.

Punctuated equilibrium (8 of 8) This process would produce the following pattern in the fossil record This process would produce the following pattern in the fossil record: Evolution appears to happen in sharp jumps associated with speciation events.