1-3-1-7 Big Issues
1-3-1-7-1 The pace of evolution
Does evolution occur in rapid bursts or gradually? This question is difficult to answer because we can't replay the past with a stopwatch in hand. However, we can try to figure out what patterns we'd expect to observe in the fossil record if evolution did happen in bursts, or if evolution happened gradually. Then we can check these predictions against what we observe.
What should we observe in the fossil record if
evolution is slow and steady?
If evolution is slow and steady, we'd expect to see the entire transition, from ancestor to descendant, displayed as transitional forms over a long period of time in the fossil record.
In fact, we see many examples of transitional forms in the fossil record.
For example, to the right we show just a few steps in the evolution of whales
from land-dwelling mammals, highlighting the transition of the walking forelimb
to the flipper.
Transitional forms in whale evolution
What would we observe in the fossil record if
evolution happens in "quick" jumps (perhaps fewer than 100,000 years
for significant change)?
If evolution happens in "quick" jumps, we'd expect to see big changes happen quickly in the fossil record, with little transition between ancestor and descendant.
In the above example, we see the descendant preserved in a layer directly after the ancestor, showing a big change in a short time, with no transitional forms.
When evolution is rapid, transitional forms may not be preserved, even if fossils are laid down at regular intervals. We see many examples of this "quick" jumps pattern in the fossil record.
Does a jump in the fossil record necessarily mean
that evolution has happened in a "quick" jump?
We expect to see a jump in the fossil record if evolution has occurred as a "quick" jump, but a jump in the fossil record can also be explained by irregular fossil preservation.
This possibility can make it difficult to conclude that evolution has happened rapidly.
We observe examples of both slow, steady change and rapid, periodic change in the fossil record. Both happen. But scientists are trying to determine which pace is more typical of evolution and how each sort of evolutionary change happens.