What does fossil evidence suggest about the timing of pelvic spine loss in ancient lake populations?

• A. 1 million years ago
• B. 10 million years ago
• C. 100 million years ago
• D. 50 thousand years ago

Answer: (B)

Adaptive change in response to moving into lakes drives pelvic spine loss. Fossil records indicate that this reduction happened in ancient stickleback lineages around ten million years ago, during the Miocene, when many lakes existed and freshwater populations repeatedly formed. This timing shows that the trait is a deep, parallel response to freshwater environments, not a recent development.

The other timeframes don’t fit with what the fossil record and what we know about stickleback evolution show: a million years ago or fifty thousand years ago would be much too recent to account for the widespread, repeated loss seen across ancient lake populations, and hundred million years ago would predate the origin and diversification of these lineages in their freshwater habitats.