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Lecture Outline
I.
Mechanisms of Evolutionary Change
A. Evolution is defined as a change in allele frequencies over time.
II.
B.
Natural selection is the major force that alters the frequencies of alleles.
1. Natural selection acts on individuals, but evolutionary change occurs in
populations.
2. Natural selection increases the frequency of alleles that contribute to improved
reproductive success.
3. Natural selection is the only mechanism that results in adaptation and leads to
increased fitness.
C.
Other Mechanisms That Shift Allele Frequencies in Populations
1. Mutation modifies allele frequencies by continually introducing new alleles.
2. Gene flow produces allele frequency changes when:
a. New individuals enter an existing population.
b. Individuals leave an existing population for elsewhere.
3. Genetic drift changes allele frequencies randomly.
Analyzing Change in Allele Frequencies: The Hardy-Weinberg Principle
A. Biologists take a two-pronged approach when studying how the four evolutionary
processes affect populations.
1. Create mathematical models that track the fate of alleles over time.
2. Collect data to test the predictions made by these models.
B.
One of the most powerful of these models is the Hardy-Weinberg principle.
1. Acts like a control treatment, or null hypothesis in an experiment on evolution.
2. Lets biologists predict how allele frequencies change when the four forces are
not operating.
3. Provides a contrast to models and data that explore the effects of mutation,
migration, drift, and selection.
C.
Development of the Hardy-Weinberg Principle
1. Hardy and Weinberg began by assuming that there are just two alleles of a
particular gene in a population, A1 and A2.
2. Thus, at this locus, three genotypes are possible: A1A1, A1A2, or A2A2.
3. What will be the frequencies of the two alleles and three genotypes in the next
generation if the alleles follow Mendel’s rules of independent segregation and
assortment?
a. The easiest way to analyze the fate of alleles is to think about the gametes
produced.
b. Imagine that all of the gametes produced in each generation go into a
single bin called a gene pool.
(1) Each gamete in the pool contains one allele for each locus.
(2) Frequency of the A1 allele is p, and the frequency of A2 is q.
c.
d.
e.
f.
(3) Since p and q are frequencies, they can have any value between 0 and
1, and since there are only two alleles, p + q = 1.
To illustrate random mating, imagine picking two gametes (alleles) at
random and joining them to form a zygote.
If the allele frequencies of the parent generation are known, the genotype
frequencies in the offspring generation can be predicted using this model.
(Fig. 24.1)
(1) The allele frequencies of the parents were p (for A1) and q (for A2).
(2) The genotype frequencies in the offspring are: p2 (for A1A1), 2pq (for
A1A2), and q2 (for A2A2).
(3) The genotype frequencies in the offspring generation must total 1.
The frequencies of the alleles in the offspring generation can then be
calculated from the genotype frequencies (just as they were calculated for
the parent generation in b, above).
Allele frequencies in the offspring and parental generations can be
compared to determine if evolution is occurring.
(1) If allele frequencies are the same in both generations, no evolution is
occurring, because allele frequencies do not change over time when
they are transmitted according to the rules of Mendelian inheritance.
(2) If allele frequencies are different between the generations, then
evolution is occurring via some other factor (selection, mutation, drift,
or migration).
D.
What does the Hardy-Weinberg principle assume?
1. For a population to conform to the Hardy-Weinberg principle, there must be:
a. No selection
b. No genetic drift or random allele frequency changes
c. No gene flow via immigration or emigration
d. No mutation
e. Random mating within the population
2. This principle tells us what to expect if none of the evolutionary forces are
acting on the population.
E.
How Does the Hardy-Weinberg Principle Serve as a Null Hypothesis?
1. A null hypothesis: Humans do not choose their mate based on the type of blood
they have.
a. M or N proteins are found on the surface of red blood cells.
b. An individual can be MM, MN, or NN for this gene (codominant alleles).
c. For every human population surveyed, genotypes at the MN locus are in
Hardy-Weinberg proportions. (Table 24.1)
d. The assumptions of the Hardy-Weinberg model are valid for this locus.
e. The M and N alleles are not affected by the four evolutionary mechanisms.
f. Null hypothesis accepted: Mating is random with respect to this locus.
2.
Another null hypothesis: Heterozygotes for the HLA-A and HLA-B proteins
have no reproductive advantage over homozygotes at that locus.
a. These codominant HLA alleles code for proteins in the immune system that
recognize different pathogens.
b.
c.
d.
e.
Thomas Markow and colleagues collected data on the HLA genotypes of
122 individuals from the Havasupai tribe in Arizona.
From observed genotype frequencies, the frequency of each allele was
calculated.
Observed and expected values did not match Hardy-Weinberg
predictions.
(1) There were too many heterozygotes and not enough homozygotes.
(Table 24.2)
(2) A Hardy-Weinberg assumption was being violated; but mutation,
drift, and migration are negligible in a population this small.
Two possible hypotheses to explain which assumption is being violated:
(1) Mating may not be random with respect to the HLA genotype.
(2) Heterozygous individuals may have stronger immune systems, are
healthier and are more reproductively successful (heterozygote
advantage).
III. Natural Selection and Sexual Selection
A. The average fitness of a population does not increase because of mutation,
migration, or drift.
B.
Selection is the only evolutionary mechanism that leads to nonrandom changes in
allele frequencies.
1. Individuals with certain phenotypes survive better or reproduce more than
other individuals do.
2. Alleles associated with a favored phenotype increase in frequency, while other
alleles decrease.
C.
Directional Selection
1. A type of natural selection in which allele frequencies change in one direction
(Fig. 24.2)
2. Tends to reduce the genetic diversity in populations by favoring one extreme
over another (drug resistance in certain bacteria)
3. Can be reversed if environmental conditions warrant such a change (cliff
swallows)
a. This is known as oscillating directional selection.
b. It can help maintain genetic variation in a population.
D.
Stabilizing Selection
1. A type of natural selection in which both extremes in a population are reduced
and the average phenotype is favored (human birth weights; Fig. 24.3)
2. It has two important consequences:
a. There is no change in the average value of a trait over time.
b. Genetic variation in the population is reduced.
E.
Disruptive Selection
1. A type of natural selection in which both extremes in a population are favored
and the average phenotype is selected against (Fig. 24.4)
2. Relatively rare, but maintains overall variation in a population in which it
occurs.
F.
Sexual Selection
1. Another form of nonrandom mating, in which females prefer males with
certain characteristics
a. Occurs when individuals within a population differ in their ability to
attract mates
b. A special type of natural selection in which there is selection for enhanced
ability to obtain mates
2. The theory of sexual selection
a. Bateman was the first to understand how sexual selection works.
b. His theory of the fundamental asymmetry of sex contains two elements:
(1) A pattern component
(a) Sexual selection usually acts on males much more strongly than
on females.
(b) Traits that respond to sexual selection are more highly elaborated
in males than in females.
(2) A mechanism component
(a) Explains the pattern: “Eggs are expensive, but sperm are cheap.”
(b) Females usually invest much more in their offspring than do
males.
(c) The energetic cost of producing a large egg is enormous
compared to the production of sperm.
3. Testing the theory of sexual selection
a. Sexual Selection via Female Choice
(1) Female birds invest a lot of energy in laying eggs and rearing their
young. (Fig. 24.5)
(2) Because of this large energy investment, the female must be choosy
about the male with which she mates.
(3) Bright beaks and feathers in males are a result of a good diet and
excess carotenoids and indicate good health. (Figs. 24.6, 24.7)
(4) Female birds (of many species) preferentially mate with males with
brightly colored feathers.
b. Sexual Selection via Male-Male Competition
(1) Le Boeuf and colleagues tested this theory during a long-term study of
an elephant seal population.
(2) Males fight over the ownership of patches of beach occupied by
congregations of females; large males win more battles. (Fig. 24.9a)
(3) The larger males that win battles monopolize matings with the
females residing inside their territories.
(4) Territory-winning males pass on their alleles to the next generation.
(Fig. 24.9b)
(5) Because the alleles for large body size have a definite fitness
advantage, large male size has evolved in this population.
(6) Male elephant seals weigh four times more than do females.
4. What Does Sexual Selection Produce? (Fig. 24.10)
a. Sexual selection often results in sexual dimorphism.
b. Sexually selected traits are involved in mate choice and often differ sharply
between the sexes.
c.
Sexual dimorphism refers to any trait, other than genitalia, that differs
between males and females.
IV. Genetic Drift
A. Genetic drift is defined as any change in the allele frequencies in a population that is
due to chance. (Table 24.3)
1. This process causes alleles frequencies to drift up and down randomly over
time.
2. It can be simulated through the flipping of a coin.
B.
Three Important Points about Genetic Drift
1. Genetic drift is random with respect to fitness. Allele frequency changes are not
adaptive.
2. Genetic drift is much more pronounced in small populations.
3. Over time, genetic drift can lead to the random loss or fixation of alleles.
C.
Experimental Studies of Genetic Drift
1. Kerr and Wright studied the long-term consequences of continued drift within
a population.
a. They used fruit flies that contained a genetic marker (specific alleles that
cause a distinctive phenotype) for normal or forked leg bristles.
b. Each starting population contained equal amounts of individuals with
normal leg bristles and forked leg bristles.
c. The only evolutionary process operating was genetic drift.
(1) Natural selection was not a factor, because the trait observed does not
affect fitness.
(2) No migration occurred between the populations.
(3) Mutations between normal and forked are extremely rare.
2. After 16 generations, the 96 populations studied fell into three distinct groups:
a. In 29 populations, forked leg bristles were found on all individuals.
b. In 41 populations, all individuals had normal leg bristles.
c. In 26 populations, both alleles were still present.
3. Therefore, in 73% of the populations, genetic drift had reduced allelic diversity
to zero.
D.
Genetic Drift in Natural Populations
1. How do founder effects cause drift?
a. When a group of individuals emigrate to a new geographic area and
establish a new population.
b. If the new population is small, the allele frequencies will likely be different
from the original population (founder effect).
2. How do population bottlenecks cause drift?
a. When disease outbreaks, natural catastrophes or other events cause a
sudden reduction in population size.
b. The remaining individuals likely have different allelic frequencies than do
individuals in the original population.
V.
Gene Flow
A. In an evolutionary sense, gene flow refers to the movement of alleles between
populations via migration.
B.
Gene flow tends to equalize allele frequencies among populations. (Fig. 24.11)
C.
Human migration across continents is homogenizing allelic frequencies in human
populations.
VI. Mutation
A. Because mutation occurs constantly, the first assumption of the Hardy-Weinberg
model is almost certain to be violated.
1. Mutations can occur during DNA replication.
2. Mutation constantly introduces new alleles into all populations at all loci.
3. Mutation increases genetic diversity in populations.
B.
Mutation is not an important cause of evolutionary change.
1. It does not occur often enough.
2. Most mutations give rise to deleterious alleles that decrease fitness.
C.
What Role Does Mutation Play in Evolutionary Change?
1. Lenski and Travisano evaluated the role that mutation plays at many loci over
many generations in E. coli. (Fig. 24.12)
a. Mutation was the only source of genetic variation in these populations.
b. Competition experiments proved that relative fitness increased
dramatically over time, but in a stair-step pattern.
c. Each jump in fitness was caused by a novel mutation that conferred a
fitness benefit.
d. Mutation is the ultimate source of genetic variability.
2. Although mutation rates are low for individual loci, they may be high
throughout entire genomes.
3. When considered across entire genomes, and combined with natural selection,
mutation becomes an evolutionary force.
VII. Inbreeding
A. The Hardy-Weinberg model assumes that random mating is occurring within any
particular population.
B.
In nature, matings between individuals are seldom, if ever, random.
C.
How Inbreeding Affects Populations
1. This is a type of nonrandom mating.
2. Suppose that individuals do not produce gametes that go into the gene pool,
but self-fertilize instead. (Fig. 24.13)
a. Homozygous parents produce all homozygous offspring.
b. Heterozygous parents produce both homozygous and heterozygous
offspring in a 1:2:1 ratio.
3. As a result, the proportion of homozygotes increases with each generation,
while the heterozygote proportion is halved.
4.
5.
Inbreeding depression is the loss of fitness that takes place when homozygosity
increases. (Fig. 24.14)
a. This occurs because the frequency of detrimental homozygous recessive
alleles increases.
b. This prediction has been verified in a wide variety of species, including
humans. (Table 24.5)
Therefore, many species have mechanisms that prevent or avoid inbreeding.
a. Example: Plants with “self-incompatibility” loci
b. Example: Taboos and laws against inbreeding in human societies