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BIOB11H3 Lecture Notes - Zygosity, Gene Flow, Mendelian Inheritance

Biological Sciences
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Chapter 7: Mendelian Genetics in Populations II: Migration, Genetic Drift, and
Nonrandom Mating
(Pages 223 – 228, 232 – 242 and 264 – 276)
7.1 Migration
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,
or animals.
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-islandmodel of migration found on
page 225, figure 7.4
oImagine two populations: one on a continent and the other on a small
island (e.g. Africa and Madagascar)
oSince 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
oNow consider a single locus with two alleles A1 and A2.
Can migration from the continent to the island upset the Hardy-
Weinberg equilibrium of the island? Yes.
Assume the frequency of A1 on the island is 1.0. Therefore the
frequency of the A2 allele is 0.
When gametes in a gene pool combine at random to make
zygotes, the genotype frequencies for A1A1 will be 1.0, 0 for
A1A2 and 0 for A2A2
Also consider that there are 800 zygotes which will become
Assume that the continental population has a frequency of
1.0 for A2 and 0 for A1 (opposite 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 (A1) and 20% is the migratory
population (A2)
The A1 frequency is now 0.8 (original was 1.0) and the A2
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.
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