Natural Selection

In Darwin's Theory of natural selection, there are three conditions that must be met for natural selection to occur.  First, there must be variation of the trait within the population.  Second, the trait must be hertiable therefore passed down to the offspring.  Finally, there must be an increase in reproductive sucess connected to having this trait.  One example of natural selection is the prevelence of lactose tolerance in humans.  First, there is variation in the trait because some humans are able to drink a glass of milk without any digestive complications while others are not.  Second, those who are able to drink milk are able to pass down this trait to their offspring.  Finally, having this trait leads to a greater reproductive sucess because there is access to a new food source and other dietary benefits.  As a result, there is a 5% fitness incease among the population who is able to digest lactose.  This adaptation has occured several times in populations from east Africa to Europe to the Middle East and India.  The sucess of the adaptation can also be seen in the gene flow that passes this trait around the world.