Biology Reference
In-Depth Information
6
.
Patterns over evolutionary time,
present mass extinctions
This chapter is not intended to give a detailed discussion of all we know
about eras of stasis and radiation in evolutionary history, but concentrates
on a few recent studies that provide evidence for saturation (equilibrium)
and nonsaturation (nonequilibrium) in evolutionary history. It also gives
a brief account of human impact on diversity.
The fossil record and interpretations
There have been three marine evolutionary floras and faunas, i.e., the
Cambrian, Paleozoic and Modern, each with its own degree of diversity,
and each subsequent one with higher diversity than the previous one
(Jablonski and Sepkoski
1996
). Benton (
1995
,
1998
), re-analysing fossil
evidence, has shown that there has been an exponential increase in the
number of families of continental and marine organisms in geological
time to the Recent. Courtillot and Gaudemer (
1996
) analysed the same
data, and arrived at the somewhat different conclusion that equilibria
were reached several times but re-established at higher levels after each
mass extinction; but they still found an increase over geological time.
Jablonski (
1999
, see also Jackson and Johnston
2001
) has shown that fossil
data are solid and that the general trend of increasing diversity has
not been changed by more recent data collected between 1982 and
1992 (Figure
6.1
): there was a sharp rise in diversity in the Cambrian,
a Paleozoic plateau interrupted by several mass extinctions, and a sharp
rise since the Triassic, also interrupted by several extinction events. Thus,
the fossil findings lend strong support to the view that saturation, overall,
has not been reached.
However, the apparently overwhelming evidence for a substantial and
even exponential increase in diversity of taxa over much of geologic time
has been doubted. Rosenzweig (
1995
, references therein) believes that
the increases as suggested by the fossil record over much of the time span
are, to a large degree, not real. An important error is introduced by the
