BIOLOGY 1M03 Chapter Notes - Chapter 25: Stabilizing Selection, Genetic Drift, Archaea

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Evolutionary Processes
Key Concepts
The Hardy-Weinberg principle acts as a model for generating predictions consistent with a
hypothesis when researchers want to test whether one among 5 factors is affecting a particular
gene.
Four factors affect allele frequencies directly, and each has different consequences.
o Natural selection produces adaptation.
o Genetic drift produces stochastic fluctuations in allele frequencies between populations.
o Gene flow (migration) equalizes allele frequencies between population
o Mutation introduces new alleles.
is defined as “...individuals from the same species that live in the same are and
interbreed.” (p.527)
Inbreeding, as exemplifies most-conspicuously in its extreme-through selfing, 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 more strongly the traits of members in the gender that invests ledd in offspring and
benefits more from being promiscuous that traits of the gender that invests more in offspring
and benefits more from being choosy.
Inbreeding and sexual selection are the most-intensively studied phenomena for the fifth factor,
biased mating, which still could effect evolutionary change.
Introduction
The four factors that tend to change allele frequencies are natural selection, genetic drift, gene
flow and mutation.
o Natural Selection: increases the frequency of certain alleles.
o Genetic Drift: causes allele frequencies to change stochastically.
alleles may increase in frequency
o Gene Flow: occurs when individuals immigrate into or emigrate from a population.
Allele frequencies may change when gene flow occurs.
o Mutation: modifies allele frequencies by continually introducing new alleles, even
neutral or deleterious ones.
The Hardy Weinberg Principle
To study how the four factors 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 imagine that all of the gametes produced in each generation go into a single
group called a gene pool and then combine.
o Gene pool: all of the alleles of all the genes in a certain population.
Their calculations predict the genotypes of the offspring that the population would produce, as
well as the frequency of each genotype
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
o Frequencies really are proportions
In this situation, three genotypes are possible:A1A1, A1A2 and A2A2.
(Figure 25.1)
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.
Evolutionary Processes
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.
When allele frequencies are calculated for this new generation, the frequency of A1 is still p and
the frequency of A2 is still q.
(Figure 25.2)
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 assumptions are met with respect
to the gene in question:
1. No natural selection
2. No genetic drift (stochastic allele frequency changes)
3. No gene flow
4. No mutation
5. No biased mating
Population Genetics
Population genetics is a subject area in evolutionary biology in which alleles in populations are
considered as primary and evolution is reduced to changes in allele frequencies.
Population genetics involves the notion ‘gene pool’ wherein alleles in one generation may be
considered as analogous to an infinitely large marble collection that is ‘jumbled thoroughly’
before sampling pairs to be put into an empty urn to constitute the next gene pool.
The Hardy Weinberg-(Castle) Equilibrium Principle is predicated on the 5 assumptions listed in
the previous frame.
The urn analogy renders intuitive what a population geneticist should expect when an
assumption is violated...
Implicit in the analogy is the assumption that genotypes encode phenotypes.
Hardy-Weinberg Principle & Hypotheses
The Hardy Weinberg principle serves as a model for generating predictions consistent with a
hypothesis for determining whether a factor is acting ultimately on a particular gene in a
population.
When genotype frequencies do not conform to Hardy-Weinberg proportions, a factor is
affecting that population.
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 assuming that
the population had been adhering to the Hardy Weinberg principle.
4. Compare the observed and expected values.
(Figure 25.1)
Evolutionary Processes
The observed and expected MN genotype frequencies were almost identical.
Since the genotypes at the MN locus are in Hardy-Weinberg proportions, the factors do not
currently affect MN blood groups.
Are HLA Genes in Humans in Hardy-Weinberg Equilibrium
The HLA genes code for proteins that are important in the function of the immune system. To
test the hypothesis that heterozygotes for the HLA-A and HLA-B genes might be more fit that
homozygotes, researchers used the genotypes of 125 Havasupai tribe members to estimate
population allele frequencies.
The Expected genotype frequencies did not match the observed frequencies.
Therefore, at least one of the Hardy-Weinberg assumptions must have been violated for these
alleles in this population.
(Figure 25.2)
Mutation, migration, and genetic drift are negligible in this case.
These are two possible explanations for this result:
o Mating is biased with respect to the HLA genotype
o Heterozygous individuals have higher fitness
Overdominace, for one locus
Research continues on this with no answer apparent yet.
Patterns of Natural Selection
Natural selection occurs in a wide variety of patterns.
For example, heterozygote advantage (over dominance, for one locus) is a pattern of natural
selection in which heterozygous individuals have higher fitness that homozygous individuals do.
This pattern maintains genetic variation in a population.
Genetic variation: refers to the number and relative frequency of alleles that are present in a
particular population.
Maintaining genetic variation is important because lack of variation can make populations less
able to respond successfully to changes in the environment.
Directional Selection
Directional Selection: is a pattern of natural selection that increases the frequency of one allele
by operating on a phenotype that is encoded at least partially by that allele.
This type of selection reduces a population’s genetic diversity over time.
If directional selection continues over time, the favoured alleles eventually reach a frequency of
1.o or 100%. Disadvantageous alleles will reach a frequency of 0.0
Alleles that reach a frequency of 1.0 are said to be fixed. Alleles that are no longer found in the
population are said to be lost.
(Figure 25.3)
Stabilizing Selection
Stabilizing Selection: a pattern of natural selection that occurs when individuals with
intermediate traits reproduce more than others, thereby maintaining intermediate phenotypes
in a population.
An example is the percentage of newborns with various birth weights compared with their
mortality rates, as shown in the figure. Those with birth weights in the middle of the range were
most likely to survive.
Stabilizing selection reduces a population’s genetic variation over time but does not change the
average value of a trait over time.
Multigene trait like size, encoded at 7 loci with three alleles at each (s,m,l); s and l alleles should
disappear over time)
(Figure 25.4)