There are certain organisms that you hear about a lot in evolutionary biology. In some cases, like Drosophila flies or E. coli bacteria, that’s because the organisms are easy to use in experimental studies. Other organisms, like Hawaiian silversword plants or Galapagos finches, come up frequently because they’re fantastic examples of evolution happening out in the “real world”. And then there are those rare cases where an organism is both a fantastic example of evolution in the field, and a convenient organism to work with in more controlled circumstances. The three-spined stickleback (Gasterosteus aculeatus) is one of those doubly-convenient organisms.
There are populations of three-spined sticklebacks in the ocean and in many freshwater streams and ponds. The oceanic populations have been around for a long time, but the freshwater populations are all relatively recent in evolutionary terms - they’re found in bodies of freshwater that were formed after the ice sheets retreated about 12,000 years ago. These populations appear to have evolved independently of each other, but they share a number of similar traits.
One of the more notable of the traits concerns the bony “armor” along the sides of the fish. The marine populations typically have a line of over 30 bony plates along their sides. The freshwater populations typically have only 6-9 of these plates. Why this is the case is a classic evolutionary biology question: do the freshwater populations lose the armor because there is a real advantage to losing the plates, or do they just lose them because there’s no real disadvantage to losing them.
Casey Luskin cites a news story about a recent scientific paper to support his view that the loss of the armor is just the result of the freshwater populations not facing the selective pressure seen in the oceans: