Trends in Ecology & Evolution
Evolutionary explosions and the phylogenetic fuse
Section snippets
New evidence from molecular data
Thanks to the increased availability of DNA sequence information, molecular tests of distant divergence events are becoming common9, 10, 11, 12, 13, 14, 15, 16(Appendix A). Divergence dates from molecular data are generally expected to be earlier than those from palaeontological data because they simply estimate the point at which genetic intermixing ceases while fossils record when a species has developed diagnostic characters, as well as attained fossilization. However, the extent of this
The Cambrian evolutionary explosion
The base of the Cambrian marks not only the appearance in the historical record of animal phyla, but also of many of their component major classes. There is no question that this time marked the nearly simultaneous appearance of hard shells (and hence commonly-preserved fossils) in many animal groups, but this may be decoupled from the appearance of the phyla and classes themselves. Among the arthropods, for example, there are Cambrian records of crustaceans, and stem group arachnoids, as well
The origin of modern bird and placental mammal orders
The history of birds and placental mammals presents many interesting similarities. Modern forms are abundantly represented in the favourable depositional environments of the late Palaeocene/early Eocene, but Mesozoic bird and mammal fossils are relatively rare and mostly represent `archaic' forms (e.g. multituberculates and enantiornithines) not closely related to their modern counterparts5, 7, 33. However, recent fossil discoveries have started to reveal tantalizing evidence of a Cretaceous
The phylogenetic fuse
Evolution has proceeded by innovation on the one hand and radiation on the other; and the two are not necessarily coupled. The fastest radiations from the recent past—the insect and bird endemics which evolved on Hawaii over the past five million years or so would be an example—have produced striking opportunistic adaptations but not, apparently, fundamentally new designs. Conversely, there are examples of major Baupläne such as represented by phyla today— chaetognaths or echiuroids are two
Acknowledgements
We thank many colleagues for stimulating discussions and access to unpublished data, especially David Wagner, David Penny and Tom Kemp. Chris Simon, Michael Benton, David Archibald and an anonymous reviewer provided helpful suggestions that improved the manuscript. Matt Wills generously provided a diagram. Financial support was provided in part by the UK NERC, and New Zealand Marsden Fund.
References (46)
Among-site rate variation and its impact on phylogenetic analyses
Trends Ecol. Evol.
(1996)- et al.
Cambrian lobopodians—ancestors of extant onychophorans?
Zool. J. Linn. Soc.
(1995) - et al.
The Cambrian evolutionary `explosion': decoupling cladogenesis from morphological disparity
Biol. J. Linn. Soc.
(1996) - Simpson, G.G. (1944) Tempo and Mode of Evolution, Columbia University...
- Hecht, M.K. et al., eds (1985) The Beginnings of Birds, Jura Museum,...
A theory of evolution above the species level
Proc. Natl. Acad. Sci. U. S. A.
(1975)Burgess Shale type faunas in the context of the `Cambrian explosion': a review
J. Geol. Soc. London
(1992)Explosive radiation in Tertiary birds and mammals
Science
(1995)- McMenamin, M.A.S. and McMenamin, D.L.S. (1990) The Emergence of Animals; the Cambrian Breakthrough, Columbia University...
Mammalian phylogeny: shaking the tree
Nature
(1992)