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Chapter 16
Chapter 16-1
 Population genetics is the study of evolution from a
genetic point of view (evolution= change over time)
 A population consists of a collection of individuals of
the same species that interbreed.
 A population is the smallest unit in which evolution
can occur
•Members of a species
can interbreed & produce
fertile offspring
•Species have a shared
gene pool
•Gene pool – all of the
alleles of all individuals
in a population
 A bell curve is a bell-shaped graph that shows the
standard distribution of a trait across a species
 A bell curve illustrates that most members of a
population have similar values for a given, measurable
trait. Only a few individuals display extreme variations
of the trait
 Variations are influenced by environmental factors,
such as the amount or quality of food available to an
organism. Variation is often influenced by heredity.
Usually, both play a role.

1.
2.
Variations in genotype arise in three main ways:
mutation- results from flawed copies of
individual genes
Recombination- the reassociation of genes in a
diploid individual. Recombination occurs during
meiosis by the independent assortment of genes
on non homologous, or different, chromosomes
and by crossing-over between genes located on
homologous chromosomes
3. The random fusion of gametes- a game of chance
played by individual gametes. Often there are
hundreds of millions of sperm involved in a meeting.
The one that actually fertilizes an egg is largely a
matter of chance.
*these processes ensure that offspring are not carbon
copies of their parents
•
TODAY’S theory on evolution
• Recognizes that GENES are responsible for the inheritance
of characteristics
• Recognizes that POPULATIONS, not individuals, evolve due
to natural selection & genetic drift
• Recognizes that SPECIATION usually is due to the gradual
accumulation of small genetic changes
• Changes occur in gene pools due to mutation, natural
selection, genetic drift, etc.
• Gene pool changes cause more VARIATION in individuals in
the population
• This process is called MICROEVOLUTION
• Example: Bacteria becoming unaffected by antibiotics
(resistant)
 Population geneticist use the term gene pool to
describe the total genetic information available in a
population.
*Remember a gene is a segment of DNA that contains
coding for a polypeptide or protein; a unit of
hereditary information. An allele is an alternative form
of a gene.
 If you could inventory a gene pool and know the
alleles that are present, then you could apply a
simple set of rules based on the probability theory
to predict expected genotypes and their
frequencies for the next generation.
 Allele frequency is determined by dividing the
number of a certain allele by the total number of
alleles of all types in the population.( Remember
that gametes are haploid and carry only one form
of an allele)
 Suppose that there are two forms of a hypothetical
allele, A & a, in a set of 10 gametes.
 If half the gametes in the set carry the allele A, we
would say that the allele frequency of the A allele is?
Allele Frequencies Define Gene Pools
500 flowering plants
480 red flowers
320 RR
160 Rr
20 white flowers
20 rr
As there are 1000 copies of the genes for color,
the allele frequencies are (in both males and females):
320 x
(80%)
160 x
(20%)
2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8
R
1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2
r
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*remember that the phenotype is the external
appearance of an organism that is determined by the
individual’s genotype.
The phenotype frequency is a ratio stating the number
of times a specific phenotype occurs in a population in
a single generation
 To calculate the phenotype frequency take the number
of individuals with a particular phenotype and divide
by the total number of individuals in the population.
 For example, 4 pink plants divided by a total of 8
plants equals a phenotype frequency of 0.5 pink
*Read the section titled Allele frequencies and Gene
Pool in your text pg. 300 - 302
•
Favors heterozygotes (Aa)
• Maintains both alleles (A,a) instead of removing less
successful alleles from a population
• Sickle cell anemia
> Homozygotes exhibit severe anemia, have
abnormal blood cell shape, and usually die before
reproductive age.
 > Heterozygotes are less susceptible to malaria
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 A German physician, Wilhelm Weinberg, and a British
Mathematician, Godfrey Hardy, independently
showed that allele frequencies in a population tend to
remain the same from generation to generation unless
acted on by outside influences.
 Genetic equilibrium is based on a set of assumptions
about an ideal hypothetical population that is not
evolving
Conditions…
1. No net mutations occur; that is allele
frequencies do not change overall because of
mutation
2. Individuals neither enter nor leave the
population
3. The population is large (infinitely)
4. Individuals mate randomly
5. Selection does not occur
*true genetic equilibrium is a theoretical state

Ch. 16-2

1.
2.
Any violation of the five conditions necessary for
Hardy-Weinberg equilibrium can result in
evolution
Mutation- can produce totally new alleles for a
trait
Migration- movement of individuals into a
population or the movement of individuals out
of the population *adding immigrants’ alleles to
the gene pool of a population changes the
relative abundance of alleles just as removing
emigrants’ alleles from the gene pool changes
the relative abundance of alleles
3. Genetic Drift- is the phenomenon by which allele
frequencies in a population change as a result of
random events or chance. In a small population, a
particular allele may disappear completely over a few
generations (about 45) If we assume that we started
with two alleles for a trait, then only one allele is left &
every individual is homologous for the remaining
allele. Once this happens, the danger of becoming
extinct because of no variation for natural selection to
act on. For example, a new disease could wipe out the
entire population.
• Population bottleneck an event in which a population’s
size is greatly reduced. When this happens, genetic
drift may have a substantial effect on the population.
• In other words, when the population size is radically
reduced, gene frequencies in the population are likely
to change just by random chance and many genes may
be lost from the population, reducing the population’s
genetic variation.
• Bottleneck Effect
- a drastic reduction in population (volcanoes, earthquakes,
landslides …)
- Reduced genetic variation
- Smaller population may not be able to adapt to new
selection pressures
• Founder Effect
- occurs when a new colony is started by a few members of the
original population
- Reduced genetic variation
- May lead to speciation
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 An example of a bottleneck:
Northern elephant seals have reduced genetic
variation probably because of a population bottleneck
humans inflicted on them in the 1890s. Hunting
reduced their population size to as few as 20
individuals at the end of the 19th century. Their
population has since rebounded to over 30,000—but
their genes still carry the marks of this bottleneck:
they have much less genetic variation than a
population of southern elephant seals that was not so
intensely hunted.
 Another Example – Cheetahs
• Founder effect changes in gene frequencies that
usually accompany starting a new population from a
small number of individuals. The newly founded
population is likely to have quite different gene
frequencies than the source population because of
sampling error (i.e., genetic drift). The newly founded
population is also likely to have a less genetic variation
than the source population.
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 For example, the Afrikaner population of Dutch
settlers in South Africa is descended mainly from a few
colonists. Today, the Afrikaner population has an
unusually high frequency of the gene that causes
Huntington’s disease, because those original Dutch
colonists just happened to carry that gene with
unusually high frequency. This effect is easy to
recognize in genetic diseases, but of course, the
frequencies of all sorts of genes are affected by founder
events.
4. Nonrandom mating- the fourth requirement of
genetic equilibrium is random matings without regard
to genetic makeup. Many species do not mate
randomly. Mate selection can be influenced by
geographic proximity or physical characteristics.
5. Natural Selection- an ongoing process in nature, and
it is the single most significant factor that disrupts
genetic equilibrium
 Stabilizing selection is a type of natural selection
in which the average form of a trait causes an
organism to have an advantage in reproduction
 Directional selection is a type of natural selection
in which a more extreme form of a trait causes an
organism to have an advantage in reproduction
 Disruptive selection is a type natural selection in
which either extreme variation of a trait causes
those organisms to have an advantage in
reproduction
 Stabilizing selection favors intermediate phenotypes
 Directional selection favors one extreme phenotype
 Disruptive selection favors both extreme phenotypes
 A form of natural selection in which traits that
increase mating success are favored
 A good example of sexual selection can be seen in
many species of birds, the males are brightly colored &
often heavily plumed, like the peacock. Females tend
to choose the males they mate with based on certain
traits.
Chapter 16-3
 Remember that a species is a group of organisms of
a single type are capable of producing fertile
offspring in the natural environment & speciation
is the formation of a new species
 Morphological concept of species – a species is
defined primarily according to its structure and
appearance. This way of making species
designations is convenient, but has limitations.
There can be phenotypic differences among
individuals in a single population
 Biological species concept- proposed by German-
born, American biologist Ernst Mayr; a species is a
population of organisms that can successfully
interbreed but cannot breed with other groups.
The biological species concept does not provide a
satisfactory definition for species of extinct
organisms, whose reproductive capability cannot
be tested
*The modern definition of species includes
components of both the morphological and
biological species concepts
•
TODAY’S theory on evolution
• Recognizes that GENES are responsible for the
inheritance of characteristics
• Recognizes that POPULATIONS, not individuals,
evolve due to natural selection & genetic drift
• Recognizes that SPECIATION usually is due to the
gradual accumulation of small genetic changes
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• Changes occur in gene pools due to mutation,
natural selection, genetic drift, etc.
• Gene pool changes cause more VARIATION in
individuals in the population
• This process is called MICROEVOLUTION
• Example: Bacteria becoming unaffected by
antibiotics (resistant)
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 The physical separation of members of a population.
Once the subpopulations become isolated, gene flow
between them stops. Natural selection and genetic
drift cause the two subpopulations to diverge,
eventually making them incompatible for mating.

1.
2.
Reproductive isolation results from barriers to
successful breeding between population groups in
the same areas. There are two broad types of
reproductive isolation:
Prezygotic isolation- which occurs before
fertilization
Post zygotic isolation- which occurs after fertization.
* If two potentially interbreeding species mate and
fertilization occurs, success is measured by the
production of healthy, fully fertile offspring. But
this may be prevented by one of several types of
postzygotic isolation. The offspring of
interbreeding species may not develop completely
and may die early, or, if healthy, they may not be
fertile. From an evolutionary standpoint, if death
or sterility of offspring occurs, the parent
organisms have wasted their gametes producing
offspring that cannot, in turn, reproduce.
 In the punctuated equilibrium model of speciation,
species arise abruptly and are quite different from the
root species. These species then change little over
time.
 In the gradual change hypothesis species evolve
gradually at a stable rate. ( this can sometimes require
millions of years)