BIOGEOGRAPHY

We have referred to pattern and process throughout different sections of this course. These concepts are central to the study of biogeography which, in turn, incorporates many of the topics in evolutionary biology. Biogeography often leads us to infer process from pattern.

Biogeography is the study of the distributions of organisms in space and time. It can be studied with a focus on ecological factors that shape the distribution of organisms, or with a focus on the historical factors that have shaped the current distributions. Certain regions of the world have "Mediterranean climates" where ocean current and wind patterns hit the west coast of N and S continents (Medit. region, California coast, Chile coast, SW Africa coast). Similar climate has lead to convergent , but unrelated (by definition) types of plants. To make sense of these types of ecological patterns we require a phylogenetic (historical) perspective: we need to focus on monophyletic groups.

The importance of a geographic scale was certainly appreciated by Darwin: the Galapagos finches were morphologically distinct and geographically distinct and there must be a connection. Moreover, the general view that speciation is a central phenomenon in evolution, and that most speciation is allopatric speciation assumes that geography plays a central role: some geographic feature divides a species range in two or more parts and over time speciation is achieved (details in later lectures).

These sorts of observations were made by early biogeographers who recognized certain types of distributions of organisms. Some species are restricted to a certain region and are referred to as endemic species. Endemism needs to be defined with relation to the taxonomic group: all life forms we know are endemic to the planet earth; the genus Geospiza (Darwin's finches) are restricted to the Galapagos islands; Geospiza fortis is endemic to specific islands; the spotted owl is endemic to the old-growth forests of the pacific northwest. Cosmopolitan species have a world wide distribution. They may be restricted to specific habitats, but occur on most continents.

In addition to endemism, another important pattern that needed to be explained were examples of disjunct distributions where clearly related species (or even the same species) are found in different areas. Marsupials are found in Australia and South America. Ratite birds (Ostrich, Emu&Cassowary, Rhea) are found in Africa, Australia and South America, respectively.

Alfred Russell Wallace noticed that different regions of the world had congruent patterns of endemic species and he drew up six biogeographic realms (see fig. 18.2, pg. 510; nearctic, neotropical, holarctic, ethiopian, oriental and australian). Wallace worked primarily in Malaysian region and had noticed a clear break between Australian fauna and the fauna on the islands to the northwest. This break has come to be known as Wallace's line (also a line between the Australian and the Asian biogeographic zones). These patterns described long before continental drift was an issue.

Different biogeographic areas can be quantified for levels of similarity in their biota (biota=general term for flora+fauna, includes microbes). N₁ = number of species (or other taxonomic unit) in one region, N₂ = number in another region (N₁ < N₂) and C = number of same species. Index of Similarity = C/N₁. For Australia: New Guinea, I.S. = 0.93 (93%), while Australia: Philippines, I.S. = 0.50 (50%). See table 18.1, pg 511. This provides a simple quantification of Wallace's Line.

How do we account for these patterns? Early biogeographers tended to invoke dispersal (prior to knowledge about continental drift). Potential problems: ad hoc, could pull dispersal out of a hat whenever you needed to explain a peculiar distribution. Leads to many wild scenarios of "gravid females" (pregnant, or inseminated females carrying eggs) making there way to distant regions. Muddyfooted duck carrying propagules in its feet; land bridges invoked connecting disjunct regions. Criticized by many as unscientific: cannot falsify the dispersal hypothesis because it is something we'll never know for sure, thus is of no explanatory power.

Nevertheless, all these types of events probably have occurred at some point. The Bering land bridge is well documented as an avenue of dispersal; the Opossum (a marsupial) in North America clearly dispersed here from South America via the Isthmus of Panama (see below); oceanic islands have life on them and it must have gotten there by dispersal. Several modes of dispersal can be described: Corridors between two regions on the same land mass, Filter bridges as selective connections between two areas, Sweepstakes as rare chance events (e.g. muddyfooted duck).

Dispersal hypotheses often associated with arguments about centers of origin: those regions with the greatest species (or higher rank) diversity. Greater diversity should be due to presence in that region longer (more time for speciation), hence should be the region where the group originated and from which dispersal events took place. Assumes that extant diversity has not moved from origin of diversity. Possible, but not guaranteed for all taxa.

Alternative to Centers of Origin and subsequent Dispersal as a way to explain the current distribution of species is vicariance where some barrier to genetic exchange causes the separation of the related taxa. With the acceptance of continental drift, vicariance biogeography became a discipline in which one could test hypotheses (see below). See models in fig. 18.6, 18.8 and 18.9, pgs. 518-521.

As with most dichotomies in science: often need to invoke Both vicariance and dispersal to account for distributions (not always in the same instance). Example: Galapagos finches had to have dispersed to the archipelago from the mainland and in so doing imposed a vicariance event on themselves. South American land bridge when sea levels dropped in the Pliocene the isthmus of Panama rose and served as an avenue of dispersal for terrestrial mammals (the "Great American Interchange" where unique N.American mammals dispersed to S. America and unique S. Amer. mammals moved north, 3 mil. years ago; see fig. 18.14, tables, 18.2, 18.3, pgs. 528-529), but served as a vicariance event for marine life that was distributed in the region. Lead to the formation of Geminate species (species pairs on either side of the isthmus who are each other's closest relative and were probably one species before the sea level dropped).

Pleistocene refugia nicely illustrate how vicariance and dispersal may need to be invoked to explain current distributions. Glacial ice sheet forced species to new distributions (vicariance event), after glacial retreat, the separated forms dispersed to previous regions (or wider distribution). Relative roles of dispersal and vicariance in determining species distributions can vary widely with a given species dispersal abilities (see fig. 18.3, 18.4 pg. 514-515). Essential to realize that dispersal has two components: the ability to move and the ability to become established. These two properties may not be "optimized" in the same organism.

Continental Drift as source of vicariance events. Evidence for continental drift provided by disjunct fossil specimens: Mesosaurus in South America and Africa. Illustrates the space and time component of biogeography since the strata reflect the same time (old) but are widely separated in space. Continents must have moved. (Fig. 18.5, pg. 517).

Major stages of the split-up of continents: Pangaea formed in Permian (> 250 MyBP) and began to break up in the Triassic (200 MyBP). Laurasia and Gondwana separated at the Tethys seaway (135 MyBP). Tropical corals, sea grasses and mangroves are related in Americas and old world tropics reflecting earlier Tethyan distribution. Gondwana began to break up about 80 MyBP and the major continents were separated by late Cretaceous (65 MyBP). India smashed into Asia crating the Himalayas. As the continents separated vicariance events abounded and the fauna of various continents became increasingly Provincialized. The South American mammals had many unique forms with respect to the North American Fauna. Marsupials in Australian zone are distinct form of mammal.

Testing biogeographic hypotheses with cladistic analysis. Brundin's midges (fig. 18.7, 18.8, pg. 519-520) a classic in vicariance biogeography. Sibley and Ahlquist's Ratites and the Gondwana breakup. Testing hypotheses about the sequence of vicariance events with cladograms from several species. Validity of biogeographic hypothesis can be supported by congruence of independent cladograms from unrelated species (see Cracraft, 1983, American Scientist vol. 71: pg273). By considering the relationships of organisms and their geographic distributions, the most parsimonious combination of the species cladograms can lead to an hypothesis of vicariance events, a so-called area cladogram which presents the sequence of splitting events.

Using cladistic methods, one can test biogeographic hypotheses by asking whether area cladograms for other, unrelated taxa are congruent. If different taxa all have similar area cladograms (i.e., are "congruent"), then the sequence of vicariance events is supported. If one taxon is represented in a region where none of the other taxa are found, then one might be forced to invoke dispersal to account for the disjunct distribution. The strength of this approach is that hypotheses are testable and one need not resort to ad hoc explanations that should be taken on faith. Biogeography can be practiced in a scientific manner despite its historical nature.