From: Understanding Evolution at evolution.Berkeley.edu

The basic ways in which micro evolutionary change happens is: Mutation, Gene Flow, Genetic Drift and Natural Selection. An example would be observing an increase in the frequency of brown colored genes and a decrease in the frequency of blue colored genes in a beetle population.

1. Mutation: Some blue genes randomly mutated to brown genes. Since any particular mutation is rare, this process alone cannot account for a big change in allele frequency over one generation.
2. Gene Flow: Some beetles with brown genes immigrated from another population. Resulting in an increase in the frequency of brown genes in the beetles.
3. Genetic Drift: When the beetles reproduced, just by random luck more brown genes than blue genes ended up in the offspring than in the parent population.
4. Natural Selection: Beetles with brown genes escaped predation more frequently than beetles with blue genes. So that more brown genes got into the next generation.

Mutations are critical to the development of diverse life forms, a phenomenon known as speciation.
Causes of Speciation:
1. Geographic Isolation: Scientists think that geographic isolation is essential for the process of speciation to begin. It dosen't need to be a physical barrier like a river that separates groups. It might be an unfavorable habitat between the 2 populations that keeps them from mating with one another. This isolation causes a reduction in gene flow. Even in the absence of a geographical barrier, reduced gene flow across a species range can encourage speciation.
2. Allopatric speciation: Speciation that depends on an external barrier to gene flow. Allopatry may start the process off, but the evolution of genetically based barriers to gene flow is necessary for speciation to be complete. Speciation requires that the 2 incipient(beginning to develop) species be unable to produce viable offspring or!!! that they avoid mating with members of the other group. Once gene flow is stopped or reduced, larger genetic differences between the species can accumulate.