Chapter 3
Phylogenetic fuses and evolutionary
‘explosions’: conflicting evidence and
critical tests
Richard A.Fortey, Jennifer Jackson and Jan Strugnell
ABSTRACT
Evolutionary radiations are often considered to have been times of rapid
origination of major clades as represented by their sudden appearance in the fossil
record. The Cambrian evolutionary ‘explosion’ is considered as an example in this
chapter. Whether or not this ‘explosive’ phase was preceded by a prolonged period
(phylogenetic ‘fuse’) of more cryptic evolution poorly represented by fossils can, in
principle, be tested by molecular estimates of divergence times. However, these
methods themselves have several limitations, not all of which have been
acknowledged in previous work. We critically examine some of these limitations
concerning phylogenetic uncertainty, the unreliability of calibration points, neglect
of confidence intervals, and problems with accurately relating sequence changes to
rate variation. Many of these problems are cumulative with time and hence add to
the difficulties in accurately determining deep divergences, as within the
Precambrian. The techniques considered preferable are summarized.IntroductionHow faithfully does the fossil record reflect the major events in evolution? Where fossils
are abundant the answer might seem to be self-evident, but even in this case there may be
factors at work that introduce the possibility of spurious patterns. For example, Smith et al.
(2001) have shown that the volume of rock exposed for a given age influences the
taxonomic richness recovered from the same time slice. Hence fluctuating diversity
curves may reflect little more than sedimentation/outcrop extent, which is in turn related
to former relative sea level. The greatest mass extinctions are real phenomena, but minor
extinction events and ‘radiations’ should be viewed with more circumspection. If the
fossil record is putatively poor there is all the more reason to view negative evidence with
a sceptical eye before claiming major patterns. We know that marine copepods are among
the most abundant elements in the marine realm, yet their fossil occurrences are
negligible. Nobody would conclude from this that such a great crustacean group only
achieved its current importance post-Pleistocene. Yet assumptions about the
completeness of the fossil record are often implicit in hypotheses concerning major
evolutionary events. We examine some of these assumptions here, and point to ways in