Interpreting the evidence
You might think that describing the "what happened when" of macroevolutionary
change would be easy to do. But it isn't so easy when you're dealing
with events long past, and a history that we didn't observe and that
can't be repeated. For example, imagine a paleontologist examining fossils
from the two adjacent rock layers pictured below. In the earlier rock layer,
there are only a couple of early echinoderms (the clade containing starfish
and sea urchins). In the later rock layer there are many echinoderm species
with quite distinct body forms. These rock layers present snapshots of life
through time, and our job is to fill in what happened between the snapshots.
Any one of the following scenarios is consistent with the evidence in the
series of snapshots.
1. The lineage experienced a sudden burst of speciation and morphological change:
2. The lineage experienced slow and steady speciation and morphological change:
3. The lineage did not experience much speciation or morphological change during this time period. Most of the
echinoderms from the later rock layer were living at the time that the earlier rock layer was laid down but they
didn't get preserved. Perhaps, the "new" echinoderm species were living in a different geographic
area when the earlier rock layer was laid down, migrated into the area, and then happened to get preserved in the later rock layer:
Figuring out the real story behind the fossil pattern is the sort of problem
that keeps paleontologists up all night!