Limits and Stasis
David Campbell (bivalve@isis.unc.edu)
Mon, 17 Nov 1997 15:58:36 -0400The basic principle behind living fossils is reflected in the popular adage
of "If it ain't broke, don't fix it." Natural selection does not
necessarily cause change. It can be stabilizing, directional, disruptive,
or nearly neutral. In the case of stabiizing selection, any mutations that
cause substantial change away from the norm are detrimental. This results
in strong maintenance of the current state of that trait. For example, if
both very large and very small individuals are at a disadvantage, the
average size of individuals in the population will tend to remain constant,
with little variation. If larger individuals do better, then the trend
will be increase in size; if both larger and smaller do better than medium,
then two distinct populations will tend to develop; if size doesn't matter,
then it will vary widely. Also, some organisms may mutate more slowly
than others. Bacteria reproduce asexually and are rather resistant to
mutation, so it's not surprising that they seem to change more slowly than
larger animals. Additionally, by the end of the Precambrian they had at
least 3 billion years of evolution as bacteria behind them, while large,
multicellular animals were quite a novelty. However, comparing fossil and
modern bacteria is somewhat problematic because they are so small and
because biochemistry is the main way to diagnose modern forms.
As alluded to, the change in mollusks is similar to that of the
more publicized vertebrates. Some living genera extend back to the
Paleozoic-these are anatomically primitive and have specialized for niches
not much else likes (such as soupy mud). Among the Eocene species here in
the southeastern U.S., only two have a chance of being alive (modern
species in the genus are poorly understood). Even in the lower Pliocene,
about 90% of the species are no longer living; elsewhere in the world, this
number is lower.