BIO152H5 Chapter Notes - Chapter 25: Gene Flow, Founder Effect, Wild Type

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31 Jan 2013

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Key Concepts
- The Hardy-Weinberg principle acts as a null hypothesis when researchers want to test
whether evolution or nonrandom mating is affecting a particular gene.
- There are four evolutionary mechanisms, and each has different consequences. Only natural
selection produces adaptation. Genetic drift causes random fluctuations in allele
frequencies. Gene flow equalizes allele frequencies between populations. Mutation
introduces new alleles.
- Inbreeding changes genotype frequencies but does not change allele frequencies.
- Sexual selection leads to the evolution of traits that help individuals attract mates. It usually
affects the traits of males more strongly than those of females.
- The four mechanisms that cause evolution are natural selection, genetic drift, gene flow,
and mutation.
- Natural selection increases the frequency of certain alleles.
- Genetic drift causes allele frequencies to change randomly.
- Gene flow occurs when individuals immigrate into or emigrate from a population. Allele
frequencies may change when gene flow occurs.
- Mutation modifies allele frequencies by continually introducing new alleles.
25.1 Analyzing change in Allele Frequencies: The Hardy-Weinberg Principle
- To study how the four evolutionary processes affect populations, in 1908 G. H. Hardy and
Wilhelm Weinberg developed a mathematical model to analyze the consequences of
matings among all of the individuals in a population.
- To do this, they imagined that all of the gametes produced in each generation go into a
single group called a gene pool and then combine randomly.
- Their calculations predict the genotypes of the offspring that the population would produce,
as well as the frequency of each genotype.
The Hardy-Weinberg Principle
- They started with the simplest situation, a gene with two alleles, A1 and A2.
- The frequency of A1 is represented by p and the frequency of A2 is represented by q.
Because there are only two alleles, p + q = 1.
- In this situation, three genotypes are possible: A1A1, A1A2, and A2A2. What will the frequency
of these genotypes be in the next generation?
- The model predicts that the frequency of the A1A1 genotype in the new generation will be
p2, that of the A2A2 genotype will be q2, and that of the A1A2 genotype will be 2pq.
- Because all individuals in the new generation must have one of the three genotypes, the
sum of the three genotype frequencies must equal 1 (100% of the population): p2 + 2pq + q2
= 1. This is the Hardy-Weinberg equation.
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Notes From Reading
- When allele frequencies are calculated for this new generation, the frequency of A1 is still p
and the frequency of A2 is still q.
- The Hardy-Weinberg principle makes two fundamental claims:
- (1) if the frequencies of alleles A1 and A2 in a population are given by p and q, then the
frequencies of genotypes A1A1, A1A2, and A2A2 will be given by p2, 2pq, and q2 for generation
after generation;
- (2) when alleles are transmitted according to the rules of Mendelian inheritance, their
frequencies do not change over time. For evolution to occur, some other factor or factors
must come into play.
The Hardy-Weinberg Principle: Important Assumptions
- The Hardy-Weinberg principle holds when the following five conditions are met with respect
to the gene in question:
- (1) no natural selection
- (2) no genetic drift or random allele frequency changes
- (3) no gene flow
- (4) no mutation
- (5) random mating
Hardy-Weinberg Principle: A Null Hypothesis
- The Hardy-Weinberg principle serves as a null hypothesis for determining whether evolution
is acting on a particular gene in a population.
- When genotype frequencies do not conform to Hardy-Weinberg proportions, evolution or
nonrandom mating is occurring in that population.
- Lets look at two examples of the use of the Hardy-Weinberg principle as a null hypothesis.
Are MN Blood Types in Humans in Hardy-Weinberg Equilibrium?
- Most human populations have two alleles for the MN blood group.
- The genotype of a person can be determined from blood samples.
- Analysis to determine if the Hardy-Weinberg principle holds requires four steps:
- (1) estimate genotype frequencies by observation (or testing),
- (2) calculate observed allele frequencies from the observed genotype frequencies,
- (3) use the observed allele frequencies to calculate the genotypes expected according to
the Hardy-Weinberg principle, and
- (4) compare the observed and expected values.
- The observed and expected MN genotype frequencies were almost identical.
- Since the genotypes at the MN locus are in Hardy-Weinberg proportions, evolutionary
processes do not currently affect MN blood groups, and mating must be random with
respect to this trait.
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