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
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
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.