It is a chronic sleep disorder which leads to excessive daylight drowsiness.
It may be due to a genetic factor and unusual signal being released in the brain as a result of absence of the brain chemical called hypocretin/orexin, which controls sleep. It is often seen as the effect of the immune system erroneously working against the production of hypocretin in thr brain.
It results in unexpected attacks of sleep, abrupt loss of muscle tone and hallucinations.
Natural selection is a mechanism of evolution. Organisms that are more adapted to their environment and are more likely to survive and pass on the genes that aided their success. This process causes species to change and diverge over time.
The equator receives the most solar radiation in a year. The difference in the amount of solar energy the land receives causes the atmosphere to move the way it does.
