Organic Evolution - Lecture 9/april 23 - Biogeography Ii

Front 1

The many means of dispersal (2)

Back 1

Active: walking, swimming, flying

Passive: carried by wind, currents, other organisms, etc.

Front 2

Species-area curve

Back 2

Alexander von Humboldt (1807) noted that the larger the geographic area sampled, the greater the number of species.

Front 3

Theory of Island Biogeography

Back 3

-Robert MacArthur and E. O. Wilson, 1960’s
-model of island biogeography incorporating key determinants of species diversity on an island with a given set of physical characteristics

Front 4

Theory of Island Biogeography (oceanic vs continental islands)

Back 4

Focused on dispersal to oceanic islands

Continental islands: once joined to a greater land mass and became islands as the sea level rose. Java was once part of the East Asian mainland and Britain was once joined to Europe, consequently they are surrounded by water usually less than 200 m deep.

Oceanic islands: usually surrounded by much deeper water, have never been connected to a mainland, usually formed by volcanism, buildup of coral reefs. Examples: Hawaii, Galapagos

Front 5

Theory of Island Biogeography

Back 5

Consider a newly formed oceanic island that receives colonizing species from a distant mainland.

Two variables determine the number of species (species richness) that will inhabit the island:
the rate at which new species immigrate to the island and
the rate at which species become extinct on the island.

And two physical features of the island affect immigration and extinction rates: its size and its distance from the mainland.

Front 6

The effects of size and distance on immigration to an island (6)

Back 6

Small islands generally have lower immigration rates because potential colonizers are less likely to reach a small island.

Small islands also have higher extinction rates, as they generally contain fewer resources and less diverse habitats for colonizing species to partition.

A closer island generally has a higher immigration rate than one farther away.

And because of their higher immigration rates, closer islands also have lower extinction rates, as arriving colonists help sustain the presence of a species on a near island and prevent its extinction

“S” also affected by the number of species already present. As the number of species on the island increases, the immigration rate of new species decreases, because any individual reaching the island is less likely to represent a species that is not already present.

At the same time, as more species inhabit an island, extinction rates on the island increase because of the greater likelihood of competitive exclusion.

*MAKE SURE YOU LOOK AT SLIDES 13-15

Front 7

Experiments on mangrove islands (Simberloff & Wilson, 1969)

Back 7

1. Tiny mangrove islands off the Florida Keys were surveyed for numbers and types of arthropod species
2. Islands were completely enclosed by plastic tents and fumigated with methyl bromide to kill all arthropods
3. Tents were removed and periodic surveys were carried out on numbers of recolonizing arthropod species
4. Number of recolonizing species reached a size similar to pre-fumigation number
5. Prediction confirmed

Front 8

Island equilibrium model is probably an oversimplification, but…

Back 8

Predictions of species richness may apply in only a limited number of cases and over relatively short time periods, where colonization is the main process affecting species composition.

Over longer time periods, abiotic disturbances such as storms, adaptive evolutionary changes, and speciation generally alter the species composition and community structure on islands.

A consequence of M-W model is stimulation of research on the effects of habitat area on species diversity.
vital importance for conservation biology

Front 9

Phylogenetic Tests in Historical Biogeography

Back 9

Phylogenetic trees can help us distinguish between vicariance and dispersal histories for organisms.

How we do this depends on whether we know the geologic history of the area we are studying.

Front 10

I. If The Geologic History of the Area IS Known (2)

Back 10

The geologic history gives us a pattern that would have to be reflected in the phylogenetic tree if vicariance were true.

Two points need to be understood:

1. It is not possible to falsify dispersal because any phylogenetic tree is congruent with some dispersal model.

2. It is possible to falsify vicariance - if the members of a genus living in an area are not a monophyletic group, some dispersal must have occurred. Therefore we start by trying to falsify vicariance.

*LOOK AT SLIDES 20-22 FOR EXAMPLE

Front 11

II. A Second Test for Vicariance (4)

Back 11

Look for repeated patterns or area relationships.
1. First construct cladograms for as many different taxa as possible.

2. Substitute the name of the area where the species lives for the name of the species. This creates an area cladogram.

3. If the same area relationships are found over and over again, then the cause is probably vicariance, because it is unlikely that several different kinds of animals and plants would disperse in exactly the same pattern.

4. If all of several area cladograms for several different kinds of organisms give the same pattern except one, then vicariance is most likely for the majority of species. Dispersal explains best the reason for the one exception.

*LOOK AT SLIDE 24, 25 FOR EXAMPLE

Front 12

Even though these cladograms are all different, they do agree on one basic pattern:

Back 12

Because they agree and it is not likely that these very different organism dispersed in exactly the same pattern, vicariance is probably the underlying cause.