world has not been constructed by someone taking each species in turn, testing it against each environment, and moulding it so that every species finds its perfect place. It is a world in which species live where they do for reasons that are often, at least in part, accidents of history. We illustrate this first by considering continental drift, a process that operates over a timescale of tens of millions of years.
1.4.1 Movements of landmasses
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.
1.4.2 Island history
Hawaii provides another remarkable example of a historical process that depends on the movement of a tectonic plate, but in this case in relation to volcanism and in a restricted geographic area. The Hawaiian chain of islands is volcanic in origin, having been formed gradually over the last 40 million years, as the centre of the Pacific tectonic plate moved steadily over a volcanic ‘hot spot’ in a south‐easterly direction. Thus, Niihau and Kauai are the most ancient of the islands, and Hawaii itself the most recent.
Hawaiian Drosophila
The Drosophila ‘fruit‐flies’ of Hawaii provide an especially spectacular example of species formation and endemism on islands. There are several thousand species of Drosophila worldwide (not all named yet) of which up to 1000 are found only in the Hawaiian Islands (Kang et al., 2016). Of particular interest are the 120 or so endemic species of ‘picture‐winged’ Drosophila, very few of which occur on more than one island. The majority of the picture‐winged species are specialised to oviposit and develop in the decaying bark of native trees in particular families. The lineages through which these species have evolved can be traced by analysing their DNA sequences, in this case using five nuclear genes with a total of 4260 nucleotides, to produce a comprehensive phylogeny of 93 of the species (Magnacca & Price, 2015). The evolutionary tree that emerges is shown in Figure 1.16, with each estimated species divergence date lined up alongside the island on which it occurred.
Figure 1.16 An evolutionary tree linking 93 species of picture‐winged Drosophila on the Hawaiian Islands, traced by the analysis of DNA sequences on five nuclear genes, with species groups indicated. The shaded areas denote the time periods when the labelled island was the youngest available for colonisation. Species’ divergence dates are lined up against the island on which they occurred. The islands of Maui, Molokai and Lanai are grouped together because they were recently linked by a land bridge. The earliest split (with separation of the adiastola group) occurred prior to the emergence of the mature Kauai, with subsequent splits into the other three species groups occurring on the ancient Kauai, and later dispersal to the younger islands as these emerged and matured. Speciation often involved specialisation on particular host plant families, indicated by different colours in the tree (‘sap flux’ is fermenting sap as it oozes from trees; ‘minor hosts’ are individual plant species, not in the families in the list, and used by only one or two species).
Source: From Magnacca & Price (2015).
The standard view for the biogeographic evolution of Hawaiian taxa is a ‘progression‐rule pattern’, with the most basal species occurring on ancient Kauai and each lineage dispersing to younger islands as these emerged and matured to a deeply eroded topography with forests
