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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
descendent, displayed as transitional forms over a long period of time in the
fossil record.
In the above example, the preservation of many transitional
forms, through layers representing a length of time, gives a
complete record of slow and steady evolution.
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.
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| 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 descendent.
In
the above example, we see the descendent 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.
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