Here's why:

* Environmental Factors: The rate of evolution is heavily influenced by environmental changes. Rapid changes, like climate shifts, disease outbreaks, or the introduction of new predators, can drive rapid evolutionary responses. Stable environments tend to see slower evolutionary change.

* Genetic Factors: The rate of evolution is also influenced by the genetic makeup of a population. Populations with high genetic diversity can adapt more quickly to changes, while those with limited diversity may struggle to adapt.

* Randomness: Evolution is not a perfectly predictable process. Genetic drift (random changes in gene frequencies) can have a significant impact, particularly in smaller populations. This can accelerate or slow down the pace of evolution in unexpected ways.

Examples:

* Rapid Evolution: The evolution of antibiotic resistance in bacteria is a prime example of rapid evolution driven by environmental pressure (the presence of antibiotics).

* Slow Evolution: The evolution of large, complex organisms like humans often occurs over extremely long periods, with slow, incremental changes accumulating over millions of years.

Key Point: While evolution is a constant process, its pace is not constant. It's influenced by a complex interplay of factors, resulting in periods of rapid change interspersed with periods of relative stasis.