Hence
When the Central American Isthmus closed the seaway about 2.8 million years ago, it joined land populations, prompting migrations and extinctions, while marine populations on either side became isolated. Some marine populations diverged or went extinct. The closure also changed ocean currents, salinity, and temperatures.
The land bridge formed gradually: seaways became shallower and narrower, so exchange between the oceans and the spread of animals changed over time rather than all at once. The final cutoff of surface-water exchange is dated to about 2.76 million years ago, based on changes in marine plankton and differences in salinity.
After the isthmus closed, it became a barrier to coastal marine species. Related populations in the Caribbean Sea and the eastern Pacific evolved separately; in some pairs of fish, this divergence can be traced through differences in their DNA. Ocean currents also shifted: water from the Caribbean began flowing north, helping change temperatures and salinity in the region.