Biogeographic distributions

I. Three important principles:
How do these principles support descent with modification?
A. Environment cannot account for either similarity or dissimilarity, since similar environments can harbor entirely different species groups
B. "Affinity" (=similarity) of groups on the same continent (or sea) is closer than between continents (or seas)
C. Geographical barriers usually divide these different groups, and there is a correlation between degree of difference and rate of migration or ability to disperse across the barriers.

Disjoint locations for the same extant species: Is this evidence for creation? Note that Evolution proposes Single Centers for the origins of species, so Discontinuous Distributions need to be explained.
A. this means that a method of dispersal must be proposed.
1. Changes in climate or geology must have affected migration (i.e., by first allowing migration and then preventing migration)
2. Darwin designed tests of a priori assumptions
3. Although "accidental", dispersal is not really random (and thus allows very specific predictions about distributions in some cases)

B. Case study: Similarity of flora and fauna at mountain summits (is this evidence for independent creations or something else?)
1. Evidence is clear for recent glaciation
2. Migrations are easily visualized in the gradual advances and retreats of glaciers
3. Because mountain tops retain a colder climate, some cold-adapted, northern species would be retained on mountain tops (and thus isolated during glacial retreat)
4. Also explains why such mountain-top species are most closely related to species living due north
5. Isolation poses an opportunity for change, esp. if it means a change in its interspecific associations
6. Assumption of the scenario: Circumpolar distribution is uniform (presently the case)
7. Secondary assumption: Similar situation for subarctic species

C. Many difficulties remain to be solved, esp. the very distinct, but distantly related forms in the Southern hemisphere (e.g., marsupial versus placental mammals)
1. These species are too distinct to be explained by the recent glaciation
2. Darwin postulates an earlier glaciation, because he did not know about plate tectonics
3. With plate tectonics, many (if not all) of these kinds of problems are soluble.

Fresh water distributions
Because freshwater is isolated, you might expect restricted ranges, however, this is not the case just the opposite, they often have distributions even broader than terrestrials: How can this be explained? Three cases to consider:
A. Distribution of Fish
B. Distribution of Shells (molluscs)
C. Distribution of Plants (often very wide ranges)
In all cases, dispersal of freshwater organisms depends largely on animal (esp. bird) transport
Distribution of species on oceanic islands
Darwin considered this evidence as especially strong in its support of descent with modification
A. The total number of species on oceanic islands is small compared to the number on an equal area of continent
B. Proportion of endemic species is very high
C. Oceanic islands are missing entire Classes
D. Endemic species often possess characters that are adaptive elsewhere, but are useless characters on the island
E. Endemic species often show (new) adaptive traits not possessed by any of their relatives
F. Batrachians are universally absent (except one frog in New Zealand)
G. Terrestrial mammals are not found on any island >300 miles from mainland
H. But arial mammals are found on such islands, and many of these are endemic
I. Also a correlation between the depth of the sea separating islands inhabited by mammals and the degree of "affinity" (classification) between these species
J. "The most striking and important fact" (p. 397) is the affinity of these island species to those of the nearest mainland, without being actually the same species
K. Within an archipelago, species are more closely related to each other than to those on the mainland (but still distinct from each other)
L. The principle applies widely that island inhabitants are most closely related to the inhabitants of a region from which colonization is possible
M. According to this principle, it must be the case that at some former time, a single parental species covered both ranges (i.e., the migration event itself)N. Darwin draws a parallel between Time and Space in the "Laws of Life"

Ecological Biogeography

Unlike historical biogeographers, ecological biogeographers make extensive use of current population information. They study the ways in which species develop and interact in the presence of other species and different environments. Many ecological biogeographers mimic Darwin: they study island communities as a type of experimental system to test hypotheses about species development.

Much of ecological biogeography is concerned with species richness, the number of different species an area supports. In specific, ecological biographers have developed the species richness equilibrium model.

The model begins with an uninhabited "island" that can be either a literal island or an area of like habitats completely surrounded by unlike habitats. All species available to colonize the new area are called the "species pool." As more and more new species enter the new area, the species pool becomes smaller and smaller, and the immigration rate (the probability that any given species moving into the area will be a new species) decreases.

At the same time, the island becomes more and more crowded and supplies become scarce, causing the extinction rate to increase. The point at which the extinction rate and the immigration rate balance is called the equilibrium point. The model predicts that changes in extinction and immigration rates will tend toward the equilibrium point, which is different for every island, depending on resources and degree of separation from other areas. This is shown graphically in the figure below.

Historical biogeographers

Historical biogeographers also make use of a tool called an area cladogram. This diagram is made by taking a taxonomic tree, which shows various species and their relatedness, and replacing the species names with the geographic location in which those species are found. This new tree allows scientists to determine how the differences in environments have effected the evolutionary history of different species of common origin. A sample area cladogram is shown below:

Biogeorgraphic Distributions

The geographic distributions of species can be of a number of types
Consider the distribution of three species of toucans in the genus Ramphastos.
• Endemic distributions
Two of the species, R. vitellinus and R. cluminatus , have endemic distributions: they are limited to a particular area. Endemic distributions can be more or less widespread.
• Cosmopolitan distributions
The extreme case of species that are found on all continents of the globe are called cosmopolitan. The pigeon, for example, is found on all continents except Antarctica; on a strict definition, the pigeon (pictured opposite) might not be allowed to be cosmopolitan, but the term is usually intended less strictly - and the pigeon is called a cosmopolitan species.
• Disjunct distributions
Other species, like R. ariel , are not confined to a single area, but are distributed in more than one region with a gap between them: these are called disjunct distributions.
Maps can be drawn for a taxonomic group at any Linnaean level: just as species have geographic distributions, so too do genera, families, orders. Biogeography aims to explain the distributions of the higher taxa too, in addition to those of species; and different explanatory processes are often appropriate at different levels.
Short-term movements of individuals influence the distributions of populations and species, whereas slower acting geological processes may control the biogeography of higher taxa.

BioGeorgraphy and Range Limitations?

What factors limit the geographic range of a species?
Ecological factors
The distributional limits of a species are set by its ecological attributes:

• Fundamental niches
If a species is able to tolerate a certain range of physical factors such as temperature, humidity, and so on and it has the capacity in theory live anywhere within these tolerance limits, this is its fundamental niche.

• Realized niches - Competing species
However, if there are competing species that occupy part of this range the competition may be too strong to permit both species to exist. The near extinction of the red squirrel in Britain due to competition from the grey squirrel is a good example. Each species' realized niche will be smaller than its physiology makes possible. In other words each species will occupy a smaller range than it otherwise would in the absence of competition.

BioGeography and Evolution

Biogeography was central to Darwin's logic when he summarized his findings from five years of collecting evidence around the world as a passenger on the HMS Beagle. He realized that animal and plant species, though diverse, were more similar to each other on the same continent.

So what is BioGeography? It is the study of the distributions of plants and animals over the surface of the Earth spatially and temporally. The spatial component describes and explains the distributions of one or more species over the world. The temporal component is used to explain the changing distributions of organisms over time, either in the short term or over geological time.

For example Australian species were more similar to each other than they were to South American species. But such geographic diversity also played out on local island groups such as those of the Archipelago Galapagos in the South Pacific. The famous Darwin finches were his prime exhibit in formulating the theory of evolution.

How does this idea work?
Species will change over time in go in a different direction if they are isolated from each other over long periods of time. Now remember that time periods in evolution are very, very long. They are measured in geological time, for example in MYA or million years ago.
Fossil records together with the theory on plate tectonics and continental drift support the idea of speciation, which come from a long lasting period of geographic isolation.

What is speciation? Speciation is the process of evolving two different species from a founder species as the result of an event that caused separation of the founder population into two isolated populations. Consequently, individuals from one population cease to reproduce with individuals from the other population. Their similarities will continue to exist, but their differences will start to become apparent.