Hyach

The Hyach are a spacefaring humanoid species who were members of the League of Non-Aligned Worlds and later the Interstellar Alliance.

Biology
Hyach are humanoids with reddish skin, bulbous heads and no hair. They are closely related to the extinct Hyach-do, which evolved on the same planet as them. It is estimated that the Hyach and the Hyach-do diverged from a common ancestor about a million years ago. The two species remained genetically compatible and interbred regularly throughout their history.

After the extinction of the Hyach-do, the Hyach eventually discovered that their population was decreasing steadily. It turns out that interbreeding with the Hyach-do increased the Hyach's fertility. Without the influx of Hyach-do genes, their fertility rates dropped with each generation. As of the 23rd century, the Hyach species is declining and in danger of dying out eventually unless they find a way to reverse the process.

Culture and society
Hyach society is a gerontocracy, ruled by a Grand Council of Elders.

History
Hyach civilization dates back to the 5th millennium BC. At first, they competed with the Hyach-do for resources and territory, but eventually managed to coexist with them. Mixed marriages between Hyach and Hyach-do were considered a scandal, but still happened frequently. Unbeknownst to them, interbreeding with the Hyach-do contributed greatly to the Hyach's fertility. This situation continued until the 11th century AD, during which religious laws were created establishing inter-species unions as immoral and later illegal. At some point, the practice was punishable by death. Finally, when even that was seen as insufficient, the Hyach launched a systematic extermination campaign against the Hyach-do. The ensuing genocide took centuries. By the 15th century AD, the Hyach-do had been rendered completely extinct. Centuries later, the Hyach realized that their own population was declining steadily due to the lack of a genetic factor which affects their fertility.