BEAD MODEL OF SPECIATION:
A DEMONSTRATION
This is a graphic depiction of a model of an evolutionary branching process described by Martin L. Weitzman in Biodiversity Loss: Economic and Ecological Issues, edited by Charles Perrings et. al. (Cambridge University Press, 1995), pp. 24-30. The model has been adapted for use in an introductory course in biogeography. These pages are to be used in conjunction with a class exercise.
Step 1: Evolution in prototype common ancestor species, A0.
A. The ancestor species A0 is represented as a string of beads, each bead representing a bundle of genetic information. At time t0, Ancestor A0, the prototype for the evolutionary lineages in the exercise, looks like:
B.
From an infinite source or pool of beads, a new bead is added to the top of its string at a rate of one per unit evolutionary time
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And a bead is lost from the bottom of the string at a rate of one per unit evolutionary time.
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C. As a result, after one unit of evolutionary time has passed, Ancestor A0 has changed into Ancestor A1. The string of beads looks like this:
Step 2: Reproductive Isolation occurs between two populations of Ancestor A1
Step 3: Independent evolution of isolated populations and speciation
Biogeography Home Page
Created By S. Woodward, 7/20/99.