So now we have a clearer picture of how a population of a given initial makeup evolves and why, but what sets that initial condition?  How does a population get to the point where we start the clock and begin to trace how a population evolves?

1. 1)Mutation: we’ve already discussed how a mutation can occur in an individual.  In a population of N individuals this will set an initial condition of a fraction of 1/N with the trait caused by the mutation. 
   

2. 2)Migration: Imagine there are initially two separate populations, A and B, each with N individuals.  Populations A and B evolve completely independently from each other.  Imagine population A is completely made up of individuals  sharing a given trait (this trait is fixed) while population B is completely made up of individuals without the trait (if it ever existed in population B it was lost). If population B migrates into the same territory with population A and they merge to create a new population, it will produce an initial condition of a population of size 2N with an initial fraction of 1/2 sharing the trait.  The new population will evolve from this point with the dynamics described in the previous sites.  Scientists can use observations of the fractions of a merged population with a trait native to one of the original populations to measure when migration occurred.
   

3. 3)Catastrophic Event: Sometimes an event in a population’s environment can drastically change the makeup of the population.  For example, say a population is made up of both individuals with a natural ability to swim (e.g. with webbed feet) and individuals without that ability (not webbed feet).  If there is a particularly rainy season one year causing flooding it will kill a larger portion of the individuals with non-webbed feet than individuals with webbed feet.  The resulting population will begin the usual process of genetic drift again after the floods with an initial population that has a high fraction of individuals with webbed feet.