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Long ago, the curious distributions of species between continents, seemingly inexplicable in terms of dispersal over vast distances, led biologists, especially Wegener (1915), to suggest that the continents themselves must have moved. This was vigorously denied by geologists, until geomagnetic measurements required the same, apparently wildly improbable explanation. The discovery that the tectonic plates of the earth’s crust move and carry with them the migrating continents, reconciles geologist and biologist (Figure 1.14). Thus, whilst major evolutionary developments were occurring in the plant and animal kingdoms, populations were being split and separated, and land areas were moving across climatic zones.

Figure 1.14 Continental drift means that continents that are now separate were once joined to one another. (a) The ancient supercontinent of Gondwanaland began to break up about 150 million years (Myr) ago. (b) About 50 Myr ago (early Middle Eocene) recognisable bands of distinctive vegetation had developed, and (c) by 32 Myr ago (early Oligocene) these had become more sharply defined. (d) By 10 Myr ago (early Miocene) much of the present geography of the continents had become established but with dramatically different climates and vegetation from today; the position of the Antarctic ice cap is highly schematic.

Source: After Norton & Sclater (1979), Janis (1993) and other sources.

placental and marsupial mammals

The drift of large landmasses over the face of the earth explains many patterns in the distribution of species that would otherwise be difficult to understand. A classic example is provided by the placental and marsupial mammals. Marsupials arrived on what would become the Australian continent about 90 million years ago (in the Cretaceous period), when the only other mammals present were the curious egg‐laying monotremes (now represented only by the spiny anteaters (Tachyglossus aculeatus) and the duckbill platypus (Ornithorynchus anatinus)). An evolutionary process of radiation then occurred that in many ways paralleled that of placental mammals on other continents (Figure 1.15). The subtlety of the parallels in both the form of the organisms and their lifestyle is so striking that it is hard to escape the view that the environments of placentals and marsupials provided similar opportunities to which the evolutionary processes of the two groups responded in similar ways. Because they started to diversify from a common ancestral line, and both inherited a common set of potentials and constraints, we refer to this as parallel evolution (as opposed to convergent evolution, where structures are analogous (similar in superficial form or function) but not homologous (i.e. not derived from an equivalent structure in a common ancestry), such as the wings of birds and bats). The important point here, though, is that the marsupials are found where they are not simply because they are the best fitted to those particular environments but also because of an accident of history – in this case, geological history.

Figure 1.15 Parallel evolution of marsupial and placental mammals. The pairs of species are similar in both appearance and habit, and usually (but not always) in lifestyle.