The rise and fall of plant empires
Version of Record online: 20 DEC 2001
Journal of Evolutionary Biology
Volume 14, Issue 6, page 992, November 2001
How to Cite
Van Dijk, H. (2001), The rise and fall of plant empires. Journal of Evolutionary Biology, 14: 992. doi: 10.1046/j.1420-9101.2001.00357.x
- Issue online: 20 DEC 2001
- Version of Record online: 20 DEC 2001
The Origin, Expansion and Demize of Plant Species. By Donald A. Levin. Oxford University Press, 2000. ISBN 0-19-512729-3, 230 pp.
People who lived in Rome in the first century AD were generally not aware that the empire they lived in was but temporary. For the same reason species always have given us the impression of being stable entities. However, we now know that they, too, come and go. Speciation has been a frequent subject in scientific literature and recently there are an increasing number of books dealing with their extinction (or on how to avoid extinction: conservation biology). The originality of the present book is that it deals with all aspects of the species life cycle: not only how they arise, but also how they expand, disintegrate and finally become extinct, sometimes splitting off daughter species which in their turn may go through the above cycle.
Of course, a species definition has to be adopted, inparticular for distinguishing between daughter and parental species. The author uses the ‘ecogenetic’ species concept, with two criteria for species being different: there has to be both ecological and genetic isolation. The ecological and the genetic transition are discussed in two chapters, respectively. It may look as if the author is presenting two different processes but this is a misunderstanding. In both chapters the ecological transition is primary, but results in genetic differences being brought about by differential selection. One chapter deals with the changes at the phenotypic level, closely connected with the environment, and in the other chapter the underlying genetic aspects are discussed. The first step in differentiation can also be genetic, e.g. through chromosomal changes or the formation of polyploids. But also in such cases an ecological transition is necessary, according to the ecogenetic species concept, which takes us back to the former process.
In the next chapter the geographical scale of speciation is analysed. In the classical textbook model a species area is split in two by barriers of whatever kind, followed by differential evolution of the two parts (the basic model of allopatric speciation). Levin argues and illustrates with examples that the birth of a new species is rather a local phenomenon, and that the scenario mentioned is unrealistic. Crucial is that species distribution areas are usually discontinuous and that especially the geographically or ecologically marginal populations are susceptible to evolutionary change. However, populations normally are too short-lived to be the unit of speciation. Instead, the metapopulation seems to be the ideal compromise.
New species either become extinct immediately or expand. Usually there is a lag phase prior to a period of exponential growth. It is not explained why, but there may be selection for greater dispersal ability: later in the chapter a decrease of this ability is reported, which makes this a logical conclusion. Long-distance seed dispersal plays a dominant role in the expansion enabling the rapid spreads that are sometimes recorded, especially after the last ice age. The author emphasizes that in this phase the dispersal to similar habitats elsewhere is the main mechanism and that genetic differentiation is of minor importance: moving in ‘geospace’ is far easier than moving in ‘ecospace’. The latter, implicating ecological differentiation, may even be counteracted if there is massive gene flow coming from the main area. In the long term, however, genetic differentiation will gain effect. We then almost come to the end of the species life cycle and meet again the speciation events, but now from the parental species’ point of view. Here ecological, geographical and chromosomal races are discussed, together with an extensive analysis of hybrid zones. The cycle finishes with reproductive isolation and character displacement.
It is not quite clear what happens to the main (central) parts of the parental species, which are relatively insensitive to evolutionary change. The chapter in question deals with the decline and demise of species, but this is when they certainly have already reached a vulnerable position. Has their habitat changed, are they incapable to adapt to this change, or are they surpassed by one of their daughter species or other competitors? This book clearly does not provide an answer to all the questions that arise, and certainly not to the more theoretical or philosophical queries. Its strength lies in its rich source of examples from practice, illustrating what actually is known. Therefore, this book is to be considered as a very useful contribution to both research and education.