The diversification of life can be thought of as an enormous, ever-branching tree, as Darwin pointed out 150 years ago in The Origin of Species. Darwin also suggested that all living things are related, so you could follow any lineage back down the tree of life to its origins many millions of years ago. Subsequent work has confirmed this observation and we now know that the first living species appeared some 4,500 million years ago, just a few million years after the planet was formed. But despite this apparent conformity of views among palaeontologists, there are two key questions that remain: first, whether life expanded in a continuous, perhaps exponential manner, and second, whether the main controls on speciation have been biological or physical. To take the first of these, the fossil record suggests that, through the past 600 million years at least, the diversity of species has expanded in fits and starts from rather low levels to the present huge figures of many millions. The pattern of expansion seems to have been close to exponential, with new species appearing at an ever-increasing rate, and with no sign of an end to the expansion. This is especially true of life on land where, during the past 100 million years in particular, some groups such as insects and flowering plants have diversified seemingly explosively.
Some palaeontologists, however, argue that these patterns of explosive diversification are artificial, and that much of the ‘expansion’ depends on the fact that the quality of the fossil record gets poorer as one goes further back in time. Ancient times seem to have low diversities of life because fossils are not so commonly preserved, and the apparent rapid rise in diversity towards the present could be, they argue, merely an artefact of the improving rock record.
These two interpretations require much further work and many palaeontologists are interested in exploring the true, empirical pattern, and the effects of biases. For the present, I argue that more of the pattern is biological than geological, thus life has taken a long time to reach its present diversity, and might well continue to diversify to ever-higher levels.
The other major issue concerns the drivers of diversification. A classic view is that species originate and become extinct largely as a result of interactions with other species; so competition between two species for a limited resource might lead to the disappearance of the weaker competitor. Likewise, the loss of a competitor – on an island, for example – might provide an opportunity for a new species to arise. Overall diversity might then be limited to some equilibrium level, or not, as the case may be, but here the key drivers of diversification are biological (competition and predation). This has sometimes been called the ‘Red Queen’ view of evolution, after the Red Queen in Lewis Carroll’s Alice through the Looking-Glass, who said that “it takes all the running you can do, to keep in the same place”. In this model, evolution is driven by competitive predator-prey interaction, but the relative ecological positions of the two organisms remain similar as their biotic interactions evolve.
New species are appearing at an ever-increasing rate, and with no sign of an end to the expansion
The alternative view, termed the ‘Court Jester’ model, recalls the capricious behaviour of the licensed fool of medieval times. In this model, evolution, speciation and extinction rarely happen except in response to unpredictable changes in the physical environment. Much recent study of climate and diversity changes through long time spans tends to support this view. External physical factors such as global temperature, sea levels and the relative positions of continents and oceans often correlate well with changes in biodiversity.
So how to resolve these two seemingly quite distinct models of large-scale evolution?
I follow certain previous analysts who have suggested that evolution should be viewed at different levels. This doesn’t mean that evolution is necessarily hierarchical in process, but that different processes dominate at different levels of analysis. So, for example, there is no question that the classic Darwinian picture of competition and predation shaping and honing adaptations and species is correct at the ecosystem level and over short time-scales. But at time-scales of millions of years, much larger regional- or global-scale processes overwhelm the scurrying in the undergrowth. A major glaciation or rise in sea level has a large and profound effect; such physical processes may kill species, but in turn that provides opportunities for many new species to arise.
In conclusion, I would argue that when viewed close up, evolution is all about biotic interactions in ecosystems (Red Queen model), but from further away the large patterns of biodiversity are driven by the physical environment (Court Jester model).