IB 31, Sp. 2008
Jan. 29, 2008
Caldwell, Evolution and Natural Selection I
Central to almost every aspect of Modern Biology is Evolution based on the concept of Natural
Selection, as formulated by Darwin and Wallace, and substantiated and modified by later scientists.
understand modern behavioral biology, it is essential that you understand these processes.
This will be
old hat for some of you, but it is so important to understanding behavior that I want to make sure that the
class starts on a solid foundation.
Therefore, if you have had this before, please bare with me.
are having difficulty with evolutionary concepts, try logging onto our UC Berkeley website,
as we now understand it, is a change in the genetic composition of a population.
(For our purposes, a population is a group of potentially interbreeding individuals.)
changes we are concerned with are in the relative frequencies of particular forms of a gene.
Different forms of a gene are called ALLELES.
For example, in humans, a particular gene may be one of many involved in the production of the
skin pigment melanin.
The normal genetic instructions at this locus promote melanin production.
This is called the “normal” allele.
A defective set of genetic instructions at this same locus might
block pigment production and produce albinism.
If you inherited two copies of this allele, you
would be an albino.
(Remember from your basic biology, we have two sets of genetic instructions
carried on pairs of autosomal chromosomes.
Each chromosome of a pair has one copy (allele) of a
particular gene found at a particular location on that chromosome.)
One normal allele is sufficient
to produce melanin.
Typically, the frequency of albino alleles is very rare in human populations - much less than 1%,
and there are very few albinos.
In fact, there may be some selection against this condition due to
UV exposure causing skin and eye cancer.
However, if for some reason albinos in a population
were favored and were more likely to reproduce, the allelic frequency of the albinism form of the
gene would increase (Cuna Indians of the San Blas – moon children), there would be more albinos
and we would say that the population had evolved - there was a change in gene frequency!
Evolution can also occur when there are changes in the structure of chromosomes and the
arrangement of genes on them, affecting how genes are expressed and inherited or how genes are
switched on and off, but for our purposes, we will be primarily concerned with changes in gene
Lets look at one well-documented case of evolution - a rare instance where we have been able to
observe the process in action - INDUSTRIAL MELANISM.
This is a classic example used in
many text books, but I want to concentrate on how we deduce the mechanisms underlying