Although evolution can sometimes be a rapid process, the changes that endure and become permanent tend to take a long time, researchers at Oregon State University said.
In contrast, they said, rapid evolutionary changes in local populations often don't last for long or spread through a species.
"Rapid evolution is clearly a reality over fairly short time periods, sometimes just a few generations," OSU zoologist Josef Uyeda said.
"But those rapid changes do not always persist and may be confined to small populations. For reasons that are not completely clear, the data show the long-term dynamics of evolution to be quite slow."
For a major change to persist and for changes to accumulate, it took about 1 million years, researchers said.
This occurred in a "remarkably consistent pattern" across a wide range of species, they said.
"We believe that for changes to persist, the underlying force that caused them has to also persist and be widespread," Uyeda said.
"Evolutionary adaptations are caused by some force of natural selection such as environmental change, predation or anthropogenic disturbance, and these forces have to continue and become widespread for the change to persist and accumulate," he said.
"That's slower and more rare than one might think."