Download Genes Within Populations

AP® Biology Review:
Genes Within Populations
Hardy-Weinberg Equilibrium
&
Types of Selection
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Gene Variation is the Raw Material
of Evolution
• evolution—widely used to refer to how an entity
changes through time
• The first five editions of Darwin’s book, On the
Origin of the Species, never used the term!
• Rather, he used the phrase “descent with
modification”.
• It still captures the essence of evolution
– all species arise from other, pre-existing species
– however, through time they accumulate differences
such that ancestral and descendant species are NOT
identical
Natural Selection is an Important
Mechanism of Evolutionary Change
• Darwin was not the first to propose a
theory of evolution
• Darwin proposed natural selection as the
mechanism of evolution
• Natural selection produces evolutionary
change when, in a population, some
individuals possessing certain inherited
characteristics produce more surviving
offspring than individuals lacking these
characteristics
Natural Selection is an Important
Mechanism of Evolutionary Change
• The end result—the population will
gradually come to include more and more
individuals with the advantageous
characteristics
• the population evolves and becomes
better adapted to its local circumstances
Lamarck and the Inheritance of
Acquired Characteristics
• A second proposed [albeit wrong]
mechanism of evolution
• Individuals pass on to offspring body and
behavior changes acquired during their
lives
• ancestral giraffes with short necks tended
to stretch their necks to feed on tree
leaves
• this extension of the neck was passed on
to subsequent generations leading to
long-necked giraffe
Darwin’s take on giraffe
• By contrast, Darwin’s theory used the idea
that preexisting genetic differences among
individuals made them more fit for their
environment thus they thrive, reproduce
and introduce more of their kind into the
population.
Speaking of freak mutants…
• Natural selection is not the only process
that can lead to the genetic makeup of
populations
• Allele frequencies can also change as the
result of repeated mutations from one
allele to another and from migrants
bringing alleles into a population
• In small populations, the frequencies of
alleles can change randomly as the result
of chance events
Gene Variation in Nature
• Evolution within a species may result from
ANY process that causes a change in the
genetic composition of a population
• Start by looking at the genetic variation
present among individuals within a species
• this is the raw material available for the
selective process
Measuring Levels of Genetic
Variation
• Blood groups
– 30 blood group genes exist in humans in
addition to the ABO locus
– 1/3 are routinely found in several alternative
allelic forms
– more than 45 variable genes encoding other
proteins in human blood cells and plasma
which are not considered blood groups
– total: 75 genetically variable genes in this one
system alone
Measuring Levels of Genetic
Variation
• Enzymes
– use electrophoresis to determine alternative
alleles of genes specifying particular enzymes
– About 5% of the enzyme loci of a typical
human are heterozygous
Enzyme Polymorphism
• Many loci in a given population have more than
one allele at frequencies significantly greater
than would occur from mutation alone
• polymorphic—many forms; refers to a locus
with more variation than can be explained by
mutation
• most populations of insects and plants are
polymorphic at more than ½ of their enzymecoding loci
Enzyme Polymorphism
• vertebrates are somewhat less polymorphic
• heterozygocity—the probability that a randomly
selected gene will be heterozygous for a
randomly selected individual—about 15% in
Drosophila and other invertebrates and between
5% and 8% in vertebrates and around 8% in
outcrossing plants [don’t memorize this stuff!]
• Point being—lots of raw material for evolution!
Population Genetics
• The study of the properties of genes in
populations
• Darwin had no knowledge of meiosis!
• The theory of blending inheritance—where
offspring were expected to be phenotypically
intermediate relative to their parents—was
widely accepted.
• WRONG—the effect of any new genetic variant
would quickly be diluted to the point of
disappearance in subsequent generations.
The Hardy-Weinberg Principle
• Following the rediscovery of Mendel’s research,
two people, independently [hence the hyphen]
solved the puzzle of why genetic variation
persists
• G. H. Hardy, an English mathematician
• G. Weinberg, a German physician
• They pointed out that the original proportions of
the genotypes in a population will remain
constant from generation as long as you meet
the five assumptions
Hardy-Weinberg Equilibrium
Assumptions:
The allele frequencies with a population will
remain constant as long as…
• The population size is very large
• Random mating is occurring
• No mutation takes place
• No Immigration takes place
• No selection occurs
Hardy-Weinberg Equilibrium
•
•
•
•
•
Dominant alleles do NOT replace recessive
ones
The proportion of dominant to recessive alleles
does NOT change in a H-W equilibrium
p represents the frequency of the dominant
allele
q represents the frequency of the recessive
allele
frequency is the fraction of the allele’s
abundance in a population [part/whole or a
percentage expressed as a decimal value]
Hardy-Weinberg Equilibrium
•
p+q=1
•
p2 + 2pq + q2 = 1
•
•
•
p2 = the number of homozygous dominant individuals
q2 = the number of homozygous recessive individuals
2pq = the heterozygotes
•
the above equation is a binomial expansion of (p + q)2
which is equal to
12 since p + q = 1
A Feline Example
• Consider a population of 100 cats
• 84 black [frequency of black is then 84/100
= 84% or .84]
• 16 white [frequency of white is then 16/100
= 16% or .16]
• Assume the white cats are homozygous
recessive, designated bb
• The black cats are either BB or Bb
A Feline Example
• Start with the white cats
• q2 = homozygous recessive = bb = .16
• that means that the frequency of b = q and is
the square root of q2
.16  0.4
• Now, if q =0 .4 and the sum of p + q = 1, then
1 - 0.4 = p which equals 0.6
A Feline Example
• If p = B = 0.6 and q = b = 0.4, then
p2 = the number of homozygous dominant
= 0.62 = 0.36 of the population
= 36 homozygous, BB, black cats
2pq
= the number of heterozygotes
= 2(0.6)(0.4) = .48 of the population
= 48 heterozygote, Bb, black cats
This population was easy since
there were 100 individuals!
A Punnet Square May Prove Useful
A Human Example
• Cystic fibrosis patients are homozygous
for the recessive allele that causes the
disease.
• In North American Caucasians, the allele
is present at a frequency of about 22 per
1,000 individuals
• Therefore, q = .022
What proportion of North American
Caucasians is expected to express
this trait?
• If q = 0.022, then q2 represents the
number of afflicted individuals
• 0.0222 = 0.000484 of the population
• 0.000484 = which is about .0005 or
5/10,000 which reduces to 1/2,000
• So one of every 2,000 individuals in that
population is expected to have Cystic
Fibrosis
What proportion is expected to be
heterozygous carriers?
• This is where the real risk lies!
• If q = 0.022, the 1-0.022 = p = 0.978
• 2pq = the heterozygous carriers
= 2(0.022)(0.978) = 0.043032
• Which is about 0.043 which is about
43/1,000 or 1 of every 23 people.
• If two carriers mate [think Punnet square]
there is a 25% chance the child will
express Cystic Fibrosis.
Why Do Allele Frequencies
Change?
• They won’t change from generation to
generation as long as H-W conditions are
observed
– no mutation
– no gene flow [immigration & emigration]
– no selection
– NO CHANCE!!
Why Do Allele Frequencies
Change? 5 reasons
• mutation—the ultimate source of variation!
Individual mutations occur so rarely that
mutation alone does not change allele frequency
much
• gene flow—a very potent agent of change.
Populations exchange members
• nonrandom mating—Inbreeding is the most
common form. Artificial selection—It does not
alter allele frequency but decreases the
proportion of heterozygotes.
Why Do Allele Frequencies
Change? 5 reasons
• genetic drift—statistical accidents;
usually occurs only in very small
populations
• selection—the only form that produced
adaptive evolutionary changes
• Selection is the only agent that depends
on the nature of the environment. The
other 4 are independent of the
environment.
A bit more about genetic drift
• In small populations the allele frequencies
may change drastically by chance alone
• Since these changes occur randomly, as if
the frequencies were drifting, we call it
genetic drift
• There are two related causes of decreases
in a population’s size
– founder effects
– bottleneck effect
Founder Effects
• Sometimes one or a few individuals
disperse and become the founders of a
new, isolated population at some distance
from their place of origin
• Since these individuals may not represent
all of the alleles present in the original
population, alleles are lost and frequencies
of alleles changed
• Darwin’s finches
• the blue people of Kentucky
The Bottleneck Effect
• Even without movement, populations can
be drastically reduced in size
• Flooding, drought, epidemic disease, other
natural forces or progressive changes in
the environment are all causes
• the few survivors represent a random
genetic sample of the original population
• the resultant loss of genetic variability has
been termed the bottleneck effect
The Bottleneck Effect
Examples
• Northern elephant seal, breeds on the
western coast of North America and
nearby islands
• Hunted to near extinction in the 19th
century
• Reduced to a single population of 20
individuals on the Island of Guadalupe off
the coast of Baja, California
• 10s of thousands now, but very little
genetic variation
Examples
• Cheetahs—almost extinct
• Today’s cheetahs are so genetically
identical that skin taken from one cheetah
and grafted onto another is not rejected
Selection
• natural—nature decides! The
environmental conditions determine which
individuals in a population produce the
most offspring
• artificial—a breeder decides! The breeder
makes purposeful crosses between
animals or plants to produce a set of
desired characteristics
Natural Selection’s 3 Conditions
• Variation must exist among individuals in a
population. Some traits are favored over
others—no variation, no natural selection
• Variation results in differences in number of
offspring surviving in the next generation.
The essence of natural selection! Because of
phenotype or behavior, some individuals are
more successful in producing offspring, thus
passing on their genes
• Variation must be genetically inherited.
Necessary for natural selection to result in
evolutionary change
Natural Selection ≠ Evolution
Natural selection is a process.
Evolution is the historical record of change
through time. It is an outcome, not a
process.
Only if variation is genetically based will
natural selection, the process, lead to
evolution, the outcome.
Selection to Avoid Predators
• Camouflage—green caterpillars blend into the
leaves upon which they feed making it more
difficult for birds to see them.
• Industrial Melanism [where darker individuals
predominate over light] and the peppered moth
• The trees in England are covered in light bark
and lichens. During the industrial revolution,
soot covered the trees making them dark. There
was a complete swap between the frequencies
of light, peppered moths and their dark, near
black, cousins.
• Birds are quite adept at picking out individuals
that are not adapted to their backgrounds.
The Peppered Moth
• Before 1850 the trees
were light and although
black coloration is
dominant, it was rare
• After the industrial
revolution that situation
reversed
• Sometimes, it pays to
blend in!
Pesticide resistance in Insects
• Rapid evolution of more than
400 resistant species
• Some mutants were born with
fewer receptor sites on their
cell membranes
• This decreases the binding
ability of the insecticide
• Other alleles in other mutants
enhance the ability of the
insects’ enzymes to identify
and detoxify insecticide
molecules.
Heterozygote Advantage
• Selections that favor individuals with copies of
both alleles give heterozygotes an advantage
• Sickle Cell Anemia is a striking example
• Abnormal red blood cells are irregular, or sickle
shaped
• Particularly common among African Americans
3/1,000
• Often fatal—before treatments developed, all
affected individuals died as children
Heterozygote Advantage
• So why didn’t the allele become “extinct”?
• In Africa, where the frequencies are MUCH
higher than here in the states, malaria plays a
role.
• The plasmodium parasite that causes malaria
invades blood cells. When it invades a red
blood cell of a heterozygote it causes it to sickle.
These sickled cells are then quickly swept up by
the spleen thus eliminating the parasite.
• The spleen’s filtering effect is what leads to
anemia in homozygotes with the disease
Heterozygote Advantage
• This means that being a heterozygote
carrier for sickle cell has an added bonus
for a natural immunity to malaria
• These carriers have an advantage over
the homozygous dominant individuals that
have no protection against Malaria.
• Selection is acting to eliminate the allele in
the African American population where
Malaria is no threat, thus the
heterozygotes have no advantage.
Forms of Selection
• Disruptive Selection—intermediate types
are eliminated
• African fire-bellied seedcracker finch
– Populations of these birds contain individuals
with large and small beaks while very few
individuals have intermediate-sized beaks.
– Why? The seeds they feed upon come in two
sizes, small and large
– Intermediate beaks are clumsy with small
seeds and not strong enough to crack large
seeds.
Forms of Selection
• Directional
selection—acts to
eliminate one extreme
from an array of
phenotypes
• Selection has changed
the population in the
direction of lower light
attraction read this
Forms of Selection
• Stabilizing
selection—selection
acts to eliminate both
extremes; it favors an
optimum condition
and increases the
frequency of the
already common
intermediate type,
making it even more
common by
eliminating the
extremes.