Saturday, July 05, 2014

What We're Reading: New Natural History

The subject of The Monkey's Voyage is biogeography, including historical biogeography. Biogeography, in colloquial terms, is the science that seeks to explain why particular plants and animals are found in different geographical locations, why they occur where they are, what scientists term “disjunct distributions.” More narrowly speaking, a disjunct distribution is any discontinuous distribution in which some part of the species or larger group is separated by some physical barrier from the same species or a closely related species. This book summarizes for a popular audience the history of biogeography, tracing the various answers that have been suggested for disjunctive distributions since Darwin’s time, and in particular, analyzing the effect that major modern scientific discoveries in geology and molecular biology have had on the field in the twentieth century.  De Queiroz concludes by summarizing the current paradigm shift he thinks is underway in the field, and suggests what the implications of that change are for our understanding of the history of life and how evolution works. 

For the most part, the science is not difficult for the lay reader. The chapter on DNA sequencing and polymerase chain reactions, and understanding how molecular clocks work might be a bit of a challenge. The argument de Queiroz makes is clear and logical. He presents the history, foundations, and biological evidence for his case in an artfully constructed sequence of chapters.
De Queiroz devotes much of his exposition here to countering what has been called the "vicariance" model that has dominated biogeography in the last thirty years or so.  Vicariance biogeography became prominent because of one of the revolutionary discoveries of the 20th century--plate tectonics--a theory that gained credence from  the growing body of  evidence that the continents had existed in different configurations in geological history and that their current arrangement was the result of splitting and continental drift. Vicariance is the splitting of the continuous range of a group into two or more parts by the development of some sort of barrier to dispersal. It refers to the fragmentation of the range of a species, and is a mechanism whereby one species becomes two or more species as a species continues to evolve in a disjoined location.

Vicariance biogeographers offered an explanation for the distribution of species in the dictum “Earth and life evolve together.” They argued that another 20th century science, cladistics, which represented a more methodical way of determining the sequence and evolutionary tree of closely related species, supported their view.

The continental breakup central to the vacariance view is the breakup of the southern landmass known as Gondwana about 160 million years ago. It broke up under tectonic pressures into what today are the discrete continents of Africa, South America, Australia, and Antarctica, and the island nations of New Zealand, New Caledonia, and other islands. India, once attached to Australia, Antarctica, and Africa, famously detached and wandered north, plowing into Asia and creating the Himalayas. Vacariance biogeographers argued that the various species in these present day areas have a common origin in Gondwana.

The biota of New Zealand, in particular, has been much studied as an exemplar of the vacariance theory of disjunct distributions. Vacariance biogeographers are notably contemptuous of the pre-continental drift theories that were used to explain disjunctions of species, those that posited (often with little evidence) the prior existence of land bridges, or those that explained similar species existing on separate continents as the result of chance dispersals over water, either by flight or by ocean dispersal (for land animals natural “rafts” were invoked). 
Great Spotted Kiwi

The problem with this model of biogeography, as de Queiroz points out, is that many of the cases that have long been the classic exemplars of the vicariance model (for example the class of  large flightless birds known as ratites) have become problematical. Recent discoveries in geology, the fossil record, and especially studies that have used refined techniques involving molecular clocks, have shown that the age of branching between many of the different species found on the fragments of Gondwana are too young to have diverged from each other at the times of continental fragmentation. They often prove to have a closer evolutionary association with existing species on other landmasses. So how did they get there? For de Queiroz the answer is that they somehow travelled there, that chance dispersals by flight and by water over long distances have been critical in evolutionary history and explain the character of the particular plant and animal life in different locations. His examination of how the biota of islands develop is a convincing argument, and as he points out, it is difficult to explain how certain fragments of the Gondwanan continent, like New Caledonia and Chatham islands can trace a continuous line of origin from Gondwana when it now appears that for a long period of their history they had been completely submerged. 
      The Capybara, largest member of the rodent family

The broad import of de Quieroz’s  argument of a dispersal model to explain disjunct distributions becomes apparent to us when he discusses the origins of some of the major faunal groups of South America.  He has built up to this, but the breadth of his argument is still likely to catch the reader by surprise, for we suddenly become aware of the huge impact of just a handful of unlikely and chance dispersals. He believes that the ancestors of all the monkeys, caviomorphs (rodents), and sigmodontines (also rodents) were a few individuals that arrived on the continent by chance (swimming or rafting), and were the progenitors of the huge radiation of species that now characterize the faunal life of the South American continent. These chance dispersal events may be infrequent, but they become more probable given the millions of years in which they had opportunity to occur, the stretch that has been dubbed geological "deep time," and their impact can be simply tremendous.

The Monkey’s Voyage gives us a fascinating look at how science works as de Queiroz shows us how new discoveries and evidence can revolutionize a particular field. It is the story of how a new understanding and a new scientific paradigm comes into being. More importantly, though, he leaves us with a new understanding of the history of life. That history and our own arrival on the planet are ultimately the history of chance, and the course of the development of life on the planet is more unpredictable than we have ever thought. Vicariance biogeographers have long scorned dispersal as improbable, unlikely, even miraculous rather than scientific, as an explanation for the course of evolution and the distribution of plants and animals on the globe, but de Quieroz concludes that such unlikely events must now be seen as inevitable, that “In the long history of this living world, the miraculous has become the expected."

The Spider Monkey, one of the "New World" monkeys. 
His ancestors probably arrived on the South American continent
on a natural raft from Africa


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