Rates of evolution vary

It has been observed that there is an inverse relation between the rate of evolution and the time interval over which it was measured: the observed cases of rapid evolution have tended to be for shorter intervals than the cases of slower evolution.

Here is an explanation:

If the direction of evolution fluctuates, the rate of evolution measured over a short interval is inevitably higher than the rate measured over a longer time interval because the short-term changes cancel out. For example, it has been shown that the Galapagos finches’ beaks evolved to be larger in times of food shortage and smaller in times of abundance (the finch Geospiza fortis is shown opposite). Over time, these changes in size cancel each other out.