The prevailing explanation for the observed distributional patterns and areas of endemism of tropical forest organisms is the Pleistocene refuge hypothesis, which proposes that wide-ranging ancestral taxa were isolated into forest refuges during certain glacial periods, and that this isolation provided them with the opportunity to speciate. John Endler has recently argued that two predictions of the refuge hypothesis-that contact zones between vicars should be between refuges and that contact zones of rapidly reproducing butterflies should be wider than those of more slowly reproducing birds-are not borne out by the evidence. Endler therefore rejects the refuge hypothesis. We show that the data available are far too imprecise to permit any conclusions regarding contact zone widths and that, according to our reanalysis of the African bird data used by Endler, all the contact zones between vicars do indeed occur between refuges, exactly where they are expected. Additional strong support for the refuge hypothesis comes from the existence of many taxa endemic to the particular forest areas which have been postulated as refuges and from fragmented taxa which are still allopatric, never having come into secondary contact.

The prevailing explanation for the observed distributional patterns and areas of endemism of tropical forest organisms is the Pleistocene refuge hypothesis, which proposes that wide-ranging ancestral taxa were isolated into forest refuges during certain glacial periods, and that this isolation provided them with the opportunity to speciate. John Endler has recently argued that two predictions of the refuge hypothesis-that contact zones between vicars should be between refuges and that contact zones of rapidly reproducing butterflies should be wider than those of more slowly reproducing birds-are not borne out by the evidence. Endler therefore rejects the refuge hypothesis. We show that the data available are far too imprecise to permit any conclusions regarding contact zone widths and that, according to our reanalysis of the African bird data used by Endler, all the contact zones between vicars do indeed occur between refuges, exactly where they are expected. Additional strong support for the refuge hypothesis comes from the existence of many taxa endemic to the particular forest areas which have been postulated as refuges and from fragmented taxa which are still allopatric, never having come into secondary contact.

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O'Hara, R. J. 1998. Population thinking and tree thinking in systematics.-Zool. Scu. 26: 323-329.Two new modes of thinking have spread through systematics in the twentieth century. Both have deep historical roots, but they have been widely accepted only during this century. Population thinking overtook the field in the early part of the century, culminating in the full development of population systematics in the 1930s and 1940s, and the subsequent growth of the entire field of population biology. Population thinking rejects the idea that each species has a natural type (as the earlier essentialist view had assumed), and instead sees every species as a varying population of interbreeding individuals. Tree thinking has spread through the field since the 1960s with the development of phylogenetic systematics. Tree thinking recognizes that species are not independent replicates within a class (as earlier group thinkers had tended to see them), but are instead interconnected parts of an evolutionary tree. It lays emphasis on the explanation of evolytionary events in the context of a tree, rather than on the states exhibited by collections of species, and it sees evolutionary history as a story of divergence rather than a story of development. Just as population thinking gave rise to the new field of population biology, so tree thinking is giving rise to the new field of phylogenetic biology.

Accounts of the evolutionary past have as much in common with works of narrative history as they do with works of science. Awareness of the narrative character of evolutionary writing leads to the discovery of a host of fascinating and hitherto unrecognized problems in the representation of evolutionary history, problems associated with the writing of narrative. These problems include selective attention, narrative perspective, foregrounding and backgrounding, differential resolution, and the establishment of a canon of important events. The narrative aspects of evolutionary writing, however, which promote linearity and cohesiveness in conventional stories, conflict with the underlying chronicle of evolution, which is not linear, but branched, and which does not cohere, but diverges. The impulse to narrate is so great, however, and is so strongly reinforced by traditional schemes of taxonomic attention, that natural historians have more often abandoned the diverging tree than they have abandoned the narrative mode of representation. If we are to understand the true nature of the evolutionary past then we must adopt "tree thinking", and develop new and creative ways, both narrative and nonnarrative, of telling the history of life.