Once on an island, there were lots and lot of caterpillars. The island was called "The Kingdom of Caterpillars" due to the numerous amount of caterpillars. There were blue and yellow of them. Likewise, no other animals had ever disturbed their natural habitat.
One day, some birds flew to the island to search for food. They found a huge amount of caterpillars and since then, they decided to live there.
The birds have gradually invaded the island. They started to eat and hunt caterpillars to survive. The birds have also started to produce their offsprings.
Due to the fact that there were two different colors of caterpillars - yellow and blue - the birds had a bias on yellow caterpillars. They thought yellow caterpillars were prettier, so they only hunt and ate blue caterpillars.
This caused the population of blue caterpillars to decrease and being replaced by yellow caterpillars. At the same time, the population of the birds were also increasing.
No longer later, the blue caterpillars are extinct, leaving only yellow caterpillars and birds alive. Since the birds still didn't want to eat the yellow caterpillars, some birds died and some emigrated.