Humans and stickleback fish are similar in at least one wayboth populations started out as a single group that went on to colonize new habitats. And in both cases, the colonizers became isolated from the main group.
The stickleback is a fish that lives in both fresh and salt water. As the last ice age ended, glaciers pulled back and formed new fresh water lakes. Sticklebacks colonized these new lakes and lost touch with their fellow sticklebacks.
This set up an excellent natural laboratory to study how populations change over time when they are isolated from each other. Some of these fish have had up to 10,000 generations to adapt to their new environment.
Humans followed a similar pattern. They started out in Africa and then colonized the rest of the world. Each colonizing group became isolated from one another and so ended up adapting to their new environment.
Both humans and sticklebacks sometimes landed in a place where lighter skin was favored. And they both used the kit ligand gene (kitlg) to become lighter.
The Kit Ligand Gene
Sea-living sticklebacks are usually dark colored but some of the lake dwellers have become much lighter. The researchers compared the DNA of the dark and light fish to try to figure out how the fresh water fish became lighter.
Stickleback fish are ideal for this kind of study because scientists know a lot about their DNA. Scientists need well characterized DNA to easily find genes involved in new traits. When the researchers looked at the DNA, they found one gene, the kit ligand gene (kitlg), that appeared to be responsible for the different colors.
The kitlg is also involved in human coloring. When scientists have compared Africans to Europeans and East Asians, they have found that part of the reason for the difference in skin color is due to the kitlg.
The kitlg is involved in coloration because of its involvement in melanocytes. Melanocytes are specialized cells where pigment is made. The kitlg is needed for some of these melanocytes to survive.
Animals (including people) have different versions of this gene. Some versions lead to lots of melanocytes and dark skin. Other versions lead to fewer melanocytes and lighter skin.
Lighter skinned sticklebacks and people tend to have the version of this gene that leads to fewer melanocytes and lighter skin. However, these lighter versions are also found in darker skinned populations.