Climate interacts with local abiotic features to influence where and how organisms live, but these are not the only factors determining where organisms are found. Evolutionary history—where and when species originated and diverged—is key to determining their biogeography.

Until European naturalists traveled the globe in the nineteenth century, they had no way of knowing how organisms were distributed in other parts of the world. Alfred Russel Wallace, who along with Charles Darwin advanced the idea that natural selection could account for the evolution of life on Earth (see Key Concept 20.1), was one of those global travelers. Wallace spent seven years in the Malay Archipelago, where he noticed some remarkable patterns in the distributions of species. For example, he described the dramatically different species that inhabited the adjacent islands of Bali and Lombok. He pointed out that the differences could not be explained by the physical environment, because Bali and Lombok are only 24 kilometers apart.

Wallace saw that, based on the distributions of plant and animal species, he could draw a line that divided the Malay Archipelago into two distinct halves. He correctly deduced that the dramatic differences in flora and fauna were related to the depth of the channel separating Bali and Lombok. This channel is so deep that it would have remained full of water, and thus would have been a barrier to the movement of terrestrial animals, even during the glaciations of the Pleistocene epoch, when sea level dropped more than 100 meters and Bali and the islands to the west were connected to the Asian mainland.

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With these insights, Wallace established the conceptual foundations of biogeography. In The Geographical Distribution of Animals, published in 1876, he detailed the factors known at the time that influence the distributions of animals, including past glaciation, land bridges, deep ocean channels, and mountain ranges. He earned some measure of scientific immortality in that the Malay discontinuity that first piqued his curiosity is known to this day as “Wallace’s line” (see Figure 53.14 below).

Question

One would expect speciation to increase as land masses separated because species would become reproductively isolated from one another, thus increasing the chance that they could follow different evolutionary trajectories. The separation of species in this way is known as vicariance.

The biotas of different parts of the world differ enough to allow us to divide Earth into many continental-scale areas called biogeographic regions, each containing characteristic assemblages of species. The boundaries of the biogeographic regions in Figure 53.14 were originally proposed by Wallace, and represent assemblages of species that change dramatically, often over short distances. A major process controlling the formation of these biogeographic regions is *continental drift. For example, we now know that over the course of the Triassic and Jurassic periods, the supercontinent Pangaea separated into two great land masses, Laurasia and Gondwana (see Figure 24.14), which subsequently separated into the continents we know today. After the land masses broke up, the descendants of the organisms widely disturbed across Pangaea evolved independently, forming new species and new species assemblages. The legacy of these continental movements can be found in several existing taxonomic groups and in the fossil record. For example, the modern southern beeches—trees of the genus Nothofagus—are found in both the Neotropical and the Australasian biogeographic regions. Evidence of fossilized Nothofagus pollen from 55 to 34 million years ago has also been found in Australia, New Zealand, western Antarctica, and South America, suggesting that beeches originated in Gondwana during the Cretaceous period and were geographically separated by the breakup of that land mass 100 million years ago (see Figure 53.14).

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The evolutionary separation of species can be attributed to two basic processes, vicariance and dispersal. Vicariance occurs when a physical barrier prevents dispersal and divides a species into two or more discontinuous populations. Dispersal occurs when the members of a species cross an existing barrier and establish a new population elsewhere.

Given that the processes of vicariance and dispersal both influence distribution patterns, how can biogeographers determine the role of each process when reconstructing the evolutionary history of a particular species? As you saw in Chapter 21, taxonomists have developed powerful molecular methods of reconstructing the phylogenetic relationships among organisms that can be used to understand how organisms came to occupy their present-day distributions. Phylogenetic trees can be used to discover whether the distribution of an ancestral species was influenced by a vicariant event, such as continental drift or a change in sea level, or is simply the result of a dispersal event.