Lecture #7 Biogeography

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Lecture #7 Biogeography

Lecture #7—Biogeography

Biogeography is the study of the distribution of organisms on Earth and how they got there. Alfred Russel Wallace (yes, the same guy who was a co-discover along with Darwin of natural selection as the driving force of evolution) is the founder of this field. He identified key areas of the world where the organisms were the same throughout the zone.

Biogeography provides important evidence for evolution because the Creationists’ believed there was a single center of creation and that species had been created with adaptations to their particular climate. This made no sense to Wallace because there was no single biogeographical center (rather there were several zones) and he could find similar climatic regions with very different animals in them. E.g. Africa and South America had similar climates but very different animals and plants. This became a powerful argument for evolution: 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. So the job of evolutionists became one of trying to explain how organisms got to where they were found and how they adapted to their new environment.

The Hypothesis of Evolution suggests that related species should live near one another and unusual distribution patterns should be explained by the geological record—overcoming barriers and dispersal of species. Recall that Darwin did find that in South America species living close together are more similar than those living farther apart; exactly what an evolutionist would predict. And the unusual animals of the Galapagos could be explained by the ancestors having accidently dispersed from the mainland (overcoming an ocean barrier) and their spread onto different islands and their subsequent adaptations to different environments and food supplies.

How do organisms get to where we find them?

  1. Created (evolved) there.
  2. Active Immigration: under their own power (walking, swimming, flying).
  3. Passive immigration carried by water, air, or land movement.

a. floating rafts of debris.

b. birds carrying seeds

c. insects carried by air currents

d. tectonic plates

e. humans carrying domestic animals & plants

General Types of Distribution Patterns:

  1. Cosmopolitan distribution—found all over the world (e.g. rats)
  2. Continuous distribution—found all over a very wide range (e.g. starlings in North America & Europe).
  3. Endemic Distribution (highly restricted to one area)

e.g. lemurs are restricted to Madagascar

  1. Discontinuous (disjunct) distribution—found in a few separate areas. The challenge here is for biologists to find out how these populations got separated. Here are some possibilities:
  1. Dispersal—a population becomes spread from its center because the organisms fly, swim, or get carried elsewhere by wind, water, or animals including humans. (1883 the volcano on the island of Krakatoa erupted & killed all life, but within 50 years, the island was covered with forest inhabitants that clearly came from Java and Sumatra)

Galapagos species from the mainland –flew, swam or got carried by birds, debris rafts, water or air currents from the mainland.

  1. Vicariance—a once continuous distribution gets split up because of continental drift, glaciers, mountain building, and rivers producing the extinction of intervening populations. (Speciation will then occur over time)

(E.g. squirrels on each side of the Grand Canyon became separated once the Colorado River cut through the country side and became different species.)

A large number of unusual distributions can be explained by continental drift (e.g. Mesosuarus fossil reptile in Africa and S. America.)

Mountain building and human hunting have produced the scattered distribution of the Big horn sheep.

Patterns of Interchange between Biotic Zones:

Corridors, Land Bridges, Filter Bridges, Sweepstakes Route.

Island Biogeography—Islands have an unusual distribution of species.

So we can explain island biogeography on the basis of immigration into islands and extinction.

Immigration depends upon the distance from colonizing source (e.g. mainland). The more isolated the islands, the fewer species.

Extinction depends on the area available for speciation and niche specialization (The greater the island area, the more species can specialize and survive.)

Wallace’s line is a deep water channel that separates the Asian fauna from the Australian fauna in Indonesia. Here is an area that few animals have passed because their dispersal mechanisms are too limited.

Terms/Concepts to Define:

Alfred Russel Wallace

Biogeographic zone

Cosmopolitan distribution

Continuous distribution

Endemic Distribution

Discontinuous (= disjunct) distribution

Dispersal

Vicariance

Corridor

Land Bridge

Filter Bridge

Sweep stakes route

Mesosaurus

Convergent evolution

Wallace’s Line

Can you answer these questions?

  1. What was Alfred Russel Wallace’s contribution to the field of Biology?
  2. What do biologists mean when they speak of biogeographical zones?
  3. What do we mean when we say that the Kiwi is endemic to New Zealand?
  4. What do we mean when we say that Drosophila is a cosmopolitan genus?
  5. The Creation hypothesis posits that there was a single place of creation and that organisms spread out from there. If that were the case then what would you expect the distribution of organisms on the planet to look like?
  6. The climates of Africa and South America are very similar, yet there are no mammals, birds or reptiles that they have in common. How does a biogeographer explain this?
  7. Darwin in South America found species living close together are more similar than those living farther apart. Why did this turn out to be an important clue for evolution?
  8. Here is the distribution pattern for moose. How do you think this distribution pattern came about? And what do you think keeps it somewhat stable?


  1. Lemurs are endemic to Madagascar and there are ~100 species on the island. Where would you expect their nearest ancestors to be and how did this unusual distribution occur?
  2. The earliest horse fossils occur in North America yet we find horses species all over the world today. How can we explain this distribution pattern?
  3. Why don’t we find members of the elephant family in South America when their fossils are all over North America?
  4. Small islands have fewer species than large. Why is that?
  5. Marsupials are largely confined to Australia and South America. Why is this the case? Does it surprise you to learn that in 1982 fossil marsupials bones were found in the Antarctic? Why or why not?
  6. How do we explain Wallace’s Line?