Genetic Mutations and Evolutionary Change
A single genetic mutation is the first step on the evolutionary path; a subtle change in the genotype that may affect phenotype. The cell replication process is fraught with errors, and mutations are an inevitable by-product of cell division (when the genetic material is duplicated).
Most of the time these mutations are harmless; they do not have a deleterious effect on the organism. Sometimes the mutations go the wrong way; they are bad for the organism and can prove to be fatal.
Occasionally a mutation may confer a benefit, an advantage that permits an organism to adapt to a change in its environment. For example, a genetic mutation that prevents an animal from succumbing to a particular disease - i.e. resistance to a viral pathogen. The advantageous mutation is then "selected for" and passed down to subsequent generations.
When these genetic changes generate differences within the organisms of a population, but do not bring about a new species, it's known as microevolution. It accounts for some of the variations seen in some species. And it happens faster for animals in warmer climates.
Examples of microevolution are;
House sparrows - house sparrows in the north of America have larger bodies than those in the south.
Pathogens - microevolution by bacteria and viruses that confers resistance to therapeutics.