Biogeography: Analysis of spatial distributions of organisms
I. Major types of biogeographic distributions
A. Cosmopolitan (not necessarily entirely cosmopolitan):
e.g., Drosophila melanogaster, Caenorhabditis elegans, common rock pigeon
B. Endemic: restricted to particular regions
1. Alfred Russell Wallace (and other early biogeographers) realized that many endemic taxa had approximately congruent distributions (e.g., in particular regions of Indonesia), forming "Biogeographic Realms"
2. Although a particular type of habitat might occur in several widely scattered places throughout the world, species in one habitat are more closely related to nearby species in other habitats than to species in the same habitat elsewhere (in other realms)
3. Despite this, species in corresponding habitats often have convergently similar adaptations
C. Disjunct: separated
1. The major problem that Darwin raised in explaining the origin of species by evolution (because descent requires that different species have evolved from common ancestors; how can common ancestors be inferred at a common point of origin if descendants are completely separated geographically?)
2. Disjunct distributions usually fall into one of a fairly small number of patterns...
II. Historical causes of disjunct biogeographic distributions
1. A priori assumptions can be tested for the hypothesis that many species can disperse great distances through environments that could not support survival or reproduction
2. Results of recent dispersals can be observed (e.g., 1883 Krakatoa eruption killed all life, but within 50 years, the island was covered with forest inhabitants that clearly came from Java and Sumatra)
3. Several types of dispersal mechanisms (e.g., corridors, filter bridges, sweepstakes; according to Simpson)
4. Ability to disperse varies greatly from group to group (e.g., bats are the only mammals native to New Zealand and Hawaii)
B. Vicariance (splitting of a taxon's range)
1. Hypothesize disjunct patterns result from changes imposed on an originally continuous distribution (e.g., by changes in the distribution of land, by continental or tectonic shifts, mountain-building, etc.)
For example, the breakup of Gondwanaland in the Mesozoic can explain the distribution of some ancient groups like ratites and marsupials
2. Vicariant hypotheses also include explaining disjunct distributions by extinctions of intervening populations
3. Both dispersal and subsequent vicariance may be required to explain a particular pattern, but the pattern is vicariant if the dispersal resulted in a continuous distribution (e.g., present distribution of Camelidae is explained by vicariance because it was originally a continuously-ranging group that was fragmented by extinctions)
III. Evidence for historical biogeography
1. If the fossil record is good for a taxon, it is very useful for explaining distribution
2. In fact, it is sometimes critical in determining when a group arose, and thus if its distribution could have been fragmented by plate tectonics
3. That is, the past distribution of a group can indicate whether its present distribution arose by dispersal or by vicariance
B. Systematics (i.e., phylogeny)
1. Congruence between "area phylogeny" and organismal phylogeny
a. If the phylogeny matches the geological / geographical history, the distribution pattern was probably due to vicariant events (i.e., vicariance makes a good null hypothesis)
b. If the phylogeny is not congruent with the area phylogeny, then dispersal generally needs to be invoked
2. Congruence among relationships of several taxa are consistent with vicariance
C. An example of the interplay between continental drift and dispersal: the Great American Interchange
1. Biotic interchange occurs when two previously separate faunas come into contact, often resulting in enormous changes in biodiversity: e.g., 3 MYA when the isthmus of Panama arose, connecting N. & S. America
2. Over the previous 50 Myr, many modern orders of mammals originated in N. America, Africa and Europe, but S. America did not have these forms, and evolved its own distinctive fauna (e.g., forms of marsupials, armadillos, sloths, anteaters, ungulates)
3. Many of these are now extinct (e.g., saber-toothed carnivore marsupials, giant ground sloths)
4. Sometime before the continuous land bridge formed, some N. American mammals dispersed to S. America, like rodents (and S. America evolved peculiar S. American forms)
5. Additional mammals started migrating ~8-9 Myr ago (e.g., procyonids = racoons)
6. 3 Myr ago, the bridge formed, and there was Savanna habitat on both sides; organisms adapted to this habitat could cross both ways (e.g., mustelids (skunks), canids, felids, equids, ursids, camelids came down from the north; dasypodids (armadillos), didelphids (opossums), and edentate anteaters came up from the south; ~10% migration from each continent, but there greater species diversity in the north
7. This interchange resulted in a fairly large extinction of S. American forms, but few N. American forms
a. There were more species from the North, and they apparently speciated more rapidly when they came south than the S. American spp. did in the north
b. N. American species may have lived a more "competitive" life in a larger continent with more species; the "arms race" may have progressed further in the North (?)
c. Environmental factors were also changing (drying) the landscape as the Andes were pushing up-maybe this helped open habitats for the N. American species (?)
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- A geologist has reconstructed the following historical events related to the different areas. Originally, the 4 areas were part of a major land mass which was broken up in a series of tectonic shifts. First, D split off from the rest of the land mass, then C split off, and then A and B split. What is the "area cladogram"?
- A systematist determines that the species in taxon I are related according to the following cladogram that is presented here in "parenthetical notation" (where hierarchical groupings are indicated with nested series of parentheses):
(((1,2),3),4). Is this phylogeny congruent with the area cladogram? Are these data consistent with a vicariant or a dispersal hypothesis for the observed distribution? Reconstruct a historical scenario.
- A different systematist is working on taxon II, and reconstructs the phylogenetic relationships of those species as: (5,(6,(7,8))). What is the most parsimonious hypothesis to explain the observed distribution of these species? Why?
- A third systematist who works on both taxa comes up with a different idea of how the species are related: (((1,5),(2,6)),((3,7),(4,8))). Is this phylogeny congruent with the "area cladogram"? In what portions is it congruent? Infer scenarios that are most consistent with these data.
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