The Kingdom of Rarities Read Online Free PDF Page A

Ocean from the Juan Fernández Islands: the huge island of New Guinea. Here we can clearly observe how geologic events and evolutionary mechanisms influence rarity; indeed, they help to create it. New Guinea’s array of fabled birds of paradise and tree kangaroos illustrates how such episodes and processes lead to the separation of populations and the surfacing of new species, a number of which earn the moniker “rare.”
    Geologic events are harbingers of evolution. By this I mean that they often create the conditions that divide previously continuous populations into subpopulations that are isolated from one another, the condition that enables them to evolve separately. In New Guinea, for example, the formation of rugged mountain ranges isolated populations of the same species from one another by distances so great that individuals no longer dispersed between them. Over many generations, through genetic mutation and adaptation to differences in the environments on the isolated ranges, the populations may diverge enough in important characteristics that if members of the divergent groups are reunited, they can no longer interbreed. Inthis way new species are born—a process called speciation—with such new species often starting their existence as rare forms with a narrow range and low numbers. For this process to occur for birds and mammals, the isolated populations must persist long enough for the required genetic changes to accumulate. Speciation in birds and mammals does not happen on small islands because distances are not great enough to isolate populations for a long enough time. In contrast, amphibians do speciate on small islands, owing to their reduced mobility. But New Guinea is large enough and rugged enough for its lofty mountain ranges, surrounded by tropical lowlands unsuitable for mountain animals, to have witnessed many bouts of speciation. Although much of the seminal work on speciation used islands as the model, speciation is hardly restricted to islands and occurs on large landmasses as well. So, a critical insight about rarity is that island life per se is not the key to frequent evolution of new species and new rarities; rather, it is isolation, which can be provided by island archipelagoes or large islands.
    If all islands are physically isolated from a mainland, are all islands, by the very nature of their limited range, repositories of rarities? Not necessarily, because several factors influence evolutionary processes on islands. Among them are distance from a mainland, length of time the island has been isolated, and size and diversity of habitats on the island. Islands that were once connected to the mainland are called continental islands. Some of them separated from a mainland when the ancient continent of Gondwanaland split up, beginning about 200 million years ago. These include Madagascar, New Zealand, the Seychelles, New Guinea, and New Caledonia, off the northeastern coast of Australia. They are loaded with ancient endemic species, many rare, and most quite different from the closest mainland flora and fauna. Other continental islands separated from adjacent mainland only a few thousand years ago when rising sea levels caused by melting glaciers severed their lowland connections. For example, Sri Lanka was connected to India only a few thousand years ago, and Trinidad was connected toVenezuela until about 11,000–15,000 years ago. On each of these islands, the flora and fauna are quite similar to those of the continent nearby because insufficient time has elapsed for much evolutionary change to happen.
    Other island groups, such as the Hawaiian and Juan Fernández Islands, are oceanic islands formed by volcanic action. They were never connected to a continental landmass. Thus, all organisms living on them must have dispersed across oceanic barriers. This is why the Juan Fernández Islands, although formed 1 to 6 million years ago, have few endemic vertebrates and