Thursday, March 6, 2008 (BASC 201: Lecture 16)
THIS IS AN
VERSION OF THE NTC.
The NTC is currently under review by the editor, and so the edited version will be posted
The principles applied to organic evolution can be applied to many other things. This lecture has
been divided into: Part 1: Evolutionary Principles, and Part 2: Evolutionary Applications.
Part 1: Evolutionary Principles:
The Polanyi Urn problem is well known for its deceitful simplicity. The problem is as follows:
Imagine there are two balls in an urn, one red and one blue. You pick a ball at random, and then
replace the ball with a second ball of the same colour (e.g. if you pick red, you put the original
back in plus another red ball). If you repeat this process indefinitely, what is the expected value
or the distribution of the frequency of blue? For example, is it a 50-50 frequency, or two-thirds
of one colour (maybe the colour that was picked first)? We can use a computer program to solve
this problem, which can give us either the frequency of the blue balls when the frequency has
settled at a constant, or the frequency of the red balls. The computer simulation shows that the
relative numbers do tend towards a definite limiting value. What is interesting is that this limiting
value is different every time the simulation is run. The value cannot be predicted and the answer
never recurs. This is a good example to show that even simple problems sometimes do not have
But we’re usually interested in more complex structures, like trees and animals. Thus, we want to
know how complex and highly integrated structures come into being. There have been three
classic answers to this problem: a supernatural power, Lamarckian evolution, and Darwinian
The idea of a supernatural power was prevalent in almost all societies until about 200
years ago. This idea is that there must be a power that plans and executes plans to come up with
complex and integrated structures.
Lamarckian evolution, although plausible, doesn’t seem to be the system that occurs in
genes. Lamarckian evolution posits that natural principles provide guidance, and organisms
change to become better at what they do. Therefore, adaptation is guided by nature, and these
adaptations are transferred through inheritance (directed variation + inheritance
As stated, although this is plausible, things just don’t seem to work this way.
Darwinian evolution, then, is the last possibility. Darwinian evolution states that variation
is random, some is inheritable, and selection favours those that happen to be better than others.
The variation is not directed by nature, but once a variation occurs, if it happens to be superior to