Chapter 7: Mendelian Genetics in Populations II: Migration, Genetic Drift, and
(Pages 223 – 228, 232 – 242 and 264 – 276)
• It is the movement of alleles between populations
• To evolutionary biologists, evolution refers to the gene flow: the transfer of
alleles from the gene pool of one population to the gene pool of another
• Mechanisms behind this can vary from the occasional long-distance dispersal
of juvenile animals to the transport of pollen, seeds, or spores by wind, water,
• The actual amount of migration among populations in different species varies
enormously, depending on the mobility of individuals in the population &
stresses placed on the population.
Adding Migration to the Hardy-Weinberg Analysis: Migration as a Mechanism of
• To explain this, the text uses the “one-island” model of migration found on
page 225, figure 7.4
o Imagine two populations: one on a continent and the other on a small
island (e.g. Africa and Madagascar)
o Since the island pop. is small relative to the continent population,
migration from the mainland will be minor for allele and genotype
frequencies on the continent
o Now consider a single locus with two alleles A and1A . 2
Can migration from the continent to the island upset the Hardy-
Weinberg equilibrium of the island? Yes.
Assume the frequency of A on 1he island is 1.0. Therefore the
frequency of the A allele is 0.
• When gametes in a gene pool combine at random to make
zygotes, the genotype frequencies for A A w1ll1be 1.0, 0 for
A1A2 and 0 for A A2 2
• Also consider that there are 800 zygotes which will become
• Assume that the continental population has a frequency of
1.0 for A2and 0 for A (1pposite of the island’s frequencies)
and consider that 200 individuals from the continent
migrate to the island.
• Now there are 1000 individuals in total of which 80% is the
original population (A 1 and 20% is the migratory
population (A 2
• The A 1requency is now 0.8 (original was 1.0) and the A 2
frequency is now 0.2 (original was 1.0)
• Thus the migration of the new population, threw the Hardy-
Weinberg Equilibrium out of balance on the island.
Empirical Research on Migration as a Mechanism of Evolution
• Read the research on water snakes in Lake Erie on pages 227 and 228. • Migration of individuals from the mainland to islands appears to be preventing
the divergence of island versus mainland populations of Lake Erie water
Migration as a Homogenizing Evolutionary Process across Populations
• The general effect of migration is that it tends to homogenize allele
frequencies across populations
o However, in most cases this homogenizing effect is opposed by selection
• How far would homogenization go if selection did not oppose it?
o Gene flow from a continent to an island will eventually drive the allele
frequency on the island to a value exactly equal to what it is on the
• Read example on pages 230 and 231 on the variation in alley frequencies
among populations of red bladder campion.
o The results indicate that there is less variation in allele frequencies
among populations of intermediate age than among young and old
populations. The low diversity among intermediate populations probably
reflects the homogenizing influence of gene flow.
o The higher diversity of young and old populations probably represents
7.2. Genetic Drift
• This mechanism is absolutely random
• Genetic drift does not lead to adaptation, but it does lead to changes in allele
• In the Hardy-Weinberg model, genetic drift results from violation of the
assumption of infinite populations
A Model of Genetic Drift
• Read pages 232 and 233 for a model showing how genetic drift works.
• Selection is differential reproductive success that happens for a reason;
genetic drift is differential reproductive success that just happens.
Genetic Drift and Population Size
• Genetic drift is essentially the result of finite population size
• It is most important in small populations
o As genetic drift continue