To understand the two key principles of evolution, being microevolution and macroevolution, you must have an understanding of how different types of selection work.

• Natural selection leads to changes in the gene pool of a population, which causes an observable change in the phenotype of a population.

• Stabilising selection is when the environment is not changing, so selective pressures will act against deleterious alleles which will alter the optimal phenotype.

• Directional selection leads to changes in phenotype over time, and is caused by changes in the environment which will lead to selective pressures favouring new or more extreme traits.

• Disruptive selection operates in favour of extremes, such as cyclones or droughts, causing a change in the phenotype of an individual.

Microevolution refers to any change in the gene pool of a population. The change is small, hence the term "micro", and only various allele frequencies in the population are changing. If the gene pool is changing, then it is said to be undergoing evolution.

Macroevolution is the term used to explain major evolutionary changes which occur above the species level. Macroevolution is a rare occurrence, and it usually occurs due to extreme geological events, for example, the Cambrian explosion. In macroevolution, there is a clear and observable change in a species from one period to another.