The speed of evolution is measured in genetic changes of a species over time. A lot of genetic changes accumulated over time will produce a species that is very different from its ancestor species. If these genetic changes occur over a short period of time (thousands of years) the speed of evolution is fast.

Why would the genetic changes over time of a species change? Well, there are many possible reasons, e.g.:

• Changed generation time (increase of generation time means a decrease in possible genetic changes over time)
• Balancing selection (new mutations are lost from the gene pool)
• Strength of selection pressure (gene frequency is changed or genes are lost quicker when a population is under strong selection)
• Random effects on the gene pool, like bottlenecks or genetic drift.
• Changed mutation speed (e.g. because of correction mechanisms when copying DNA)
• Population size (more individuals means more possible mutations)