MICROEVOLUTION
AND THE GENETICS
OF POPULATIONS
The Scale of Evolution
Microevolution
Macroevolution
• Microevolution occurs
over a relatively short
period of time within a
population or species.
The Grants observed this
level of evolution in
Darwin’s finches which
will be discussed later in
this lesson.
• Macroevolution occurs
over geologic time above
the level of the species.
The fossil record reflects
this level of evolution. It
results from
microevolution taking
place over many
generations and will be
discussed in the next
lesson of this chapter.
Microevolution Explained
• Microevolution is how individual
traits within a population change over
time.
• In order for a population to change,
some things must be assumed to be
true.
• In other words, there must be some
sort of process happening that causes
microevolution.
• The five ways alleles within a
population change over time are
natural selection, migration (gene
flow), mating, mutations, or genetic
drift.
• One common misconception about
evolution is the idea that individuals
can evolve. Individuals do not evolve.
• SPECIES EVOLVE…
Genes in Populations
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Genes do not change over time.
Individual adaptations help species survive in the environment.
Evolution takes a long time, spanning several generations, to happen
Mutations or adaptations do not equal evolution.
There are no species alive today that have individuals that live long enough to see all of
evolution happen to its species. A
new species may diverge from an existing species’ lineage, but this was a buildup of new
traits over a long period of time and did not happen spontaneously in an instant.
So if individuals cannot evolve, then how does evolution happen?
Populations can evolve.
The unit of evolution is the population.
A population consists of organisms of the same species that live in the same area and can
interbreed.
Populations of individuals in the same species have a collective gene pool in which all future
offspring will draw their genes from.
This allows natural selection to work on the population and determine which individuals are
more “fit” for their environments.
The aim is to increase those favorable traits in the gene pool while weeding out the ones that
not favorable.
Natural selection cannot work on a single individual because there are not competing traits in
the individual to choose between.
Gene Pool
• A population consists of many
genotypes; together they make up
the population’s gene pool.
• The gene pool consists of all the
available genes of all the members of
the population that are able to be
passed down from parents to
offspring.
• For each gene, the gene pool includes
all the different alleles for the gene
that exist in the population.
• The more diversity there is in a
population of a species, the larger the
gene pool.
• The gene pool determines which
phenotypes are visible in the
population at any given time.
• The gene pool can change in an area
due to migration of individuals into or
out of a population.
• If individuals that have certain traits
are the only ones in the population
and they emigrate to a different
population, the gene pool shrinks and
those traits are no longer available to
be passed down to offspring.
• If individuals that have different traits
immigrates into a population, they
increase the gene pool and a new
type of diversity can be seen within
the population in that area as they
interbreed with the others who
already life there.
• The size of the gene pool directly
affects the evolutionary trajectory of
that population.
• The Theory of Evolution states
that natural selection acts on a
population to favor the desirable
traits for that environment while
simultaneously weeding out the
unfavorable characteristics.
• As natural selection works on a
population, the gene pool changes.
• Favorable adaptations become more
plentiful and the less desirable traits
become fewer or even disappear
from the gene pool completely.
Allele Frequencies
• Allele frequency or genetic
variation is how often an allele
occurs in a gene pool relative to
the other alleles for that gene.
• Gene allele
frequencies determine genetic
variation and the distinct traits
that can be passed on from
parents to offspring.
• Gene variation is important to
the process of natural selection.
• Natural selection is the result of
the interactions between genetic
variations in a population and the
environment.
• The environment determines
which variations are more
favorable. More favorable traits
are thereby passed on to the
population as a whole.
• Genetic variation occurs mainly
through DNA mutation, gene
flow (movement of genes from
one population to another)
and sexual reproduction.
• Due to the fact that
environments are unstable,
populations that are genetically
variable will be able to adapt to
changing situations better than
those that do not contain genetic
variation.
Hardy and Weinberg and
Microevolution
• The focus of Hardy's and Weinberg's work was on very small
changes at a gene level either due to chance or other circumstances
that changed the gene pool of the population.
• The frequency at which certain alleles appeared changed over
generations.
• This change in frequency of the alleles was the driving force behind
evolution at a molecular level, or microevolution.
• Since Hardy was a very gifted mathematician, he wanted to find an
equation that would predict allele frequency in populations so he
could find the probability of evolution occurring over a number of
generations.
• Weinberg also independently worked toward the same solution. The
Hardy Weinberg Equilibrium Equation used the frequency
of alleles to predict genotypes and track them over generations.
The Hardy Weinberg
Equilibrium Equation
• In order for this equation to work, it is assumed that all of the following
conditions are not met at the same time:
(1) Mutation at a DNA level is not occurring. Therefore, no new alleles are
being created.
(2) Natural Selection is not occurring. Thus, all members of the population
have an equal chance of reproducing and passing their genes to the next
generation.
(3) The population is infinitely large.
(4) All members of the population are able to breed and do breed.
(5) All mating is totally random. This means that individuals do not choose
mates based on genotype.
(6) All individuals produce the same number of offspring.
(7) There is no emigration or immigration occurring. In other words, no one
is moving into or out of the population.
THE EQUATION
Genotype Frequencies in a Hardy-Weinberg Equilibrium Population (for
one gene with two alleles, A and a)
-------------------------------------------------------------------------------------------------Genotype
Genotype Frequency
-------------------------------------------------------------------------------------------------AA
p2
Aa
2pq
aa
q2
-------------------------------------------------------------------------------------------------(p = frequency of A, q =frequency of a): p2 + 2pq + q2 = 1.
p+q=1
• If p = 0.4, what is the frequency of the AA genotype using the HardyWeinberg Equilibrium Equation: p2 + 2pq + q2 = 1?
THE SOLUTION
Genotype Frequencies of the AA genotype when p = 0.4 using the HardyWeinberg Equilibrium Equation (for one gene with two alleles, A and a)
----------------------------------------------------------------------------------------------------------Genotype
Genotype Frequency
----------------------------------------------------------------------------------------------------------AA
p2= 0.16 or 16%
Aa
2pq
aa
q2
----------------------------------------------------------------------------------------------------------(p = frequency of A, q =frequency of a): p2 + 2pq + q2 = 1.
p+q=1
• CHECK YOUR WORK:
• Since p + q = 1 and p= 0.4; q = 0.6
• p2 + 2pq + q2 = 1
• (0.4)2 + 2(0.4 x 0.6) + (0.6)2 = 1
The Hardy-Weinberg
Theorem
• Hardy-Weinberg Equilibrium Equation is commonly referred to
as the Hardy-Weinberg Theorem and is regarded as the
founding principle of population genetics.
• Its mathematical equation shows that allele frequencies do not
change in a population if certain conditions are met and the
population remains in genetic equilibrium, and under these
conditions evolution cannot occur.
• Such a population is said to be in Hardy-Weinberg equilibrium.
• However, if the conditions are not met then evolution can
occur.
Forces of Evolution
• From the theorem, we can infer factors that cause allele
frequencies to change.
• These factors are the forces of evolution.
• There are five such forces:
(1) natural selection
(2) migration (gene flow)
(3) mating
(4) mutations
(5) genetic drift
Natural Selection
Natural selection is the main mechanism for microevolution.
Alleles that produce favorable adaptations are more likely to be passed down to future
generations because they allow those individuals to live long enough to reproduce.
As a result, unfavorable adaptations are eventually bred out of the population and those
alleles are removed from the gene pool.
Over time, changes in allele frequency become more apparent when compared to
previous generations.
TYPES OF NATURAL SELECTION
(1) Directional Selection
Directional because of the shape of the approximate bell curve
that is produced when all individuals' traits are plotted.
Instead of the bell curve falling directly in the middle of the axes
on which they are plotted, it skews either to the left or the right
by varying degrees.
In other words one of two extreme phenotypes is selected for or
favored over another.
This phenomena is usually seen in environments that have
changed over time. Changes in weather, climate, or food
availability lead to directional selection.
Directional selection curves are most often seen when one
coloring is favored over another for a species. This could be to
help them blend into an environment, camouflage themselves
from predators, or to mimic another species to trick predators.
Other factors that may contribute to one extreme being selected
for over the other include the amount and type of food available.
(2) Disruptive Selection
Disruptive selection is also named for the way the bell curve skews when
individuals are plotted on a graph.
To disrupt means to break apart and that is what happens to the bell curve
of disruptive selection. Instead of the bell curve having one peak in the
middle, disruptive selection's graph has two peaks with a valley in the
middle of them.
The shape comes from the fact that both extremes are selected for during
disruptive selection, in other words the phenotypes in the middle of the
range are selected against.
The median is not the favorable trait in this case. Instead, it is desirable to
have one extreme or the other, with no preference over which extreme is
better for survival. This is the rarest of the types of natural selection.
Disruptive selection can lead to speciation (the appearance of new
species), and form two or more different species in areas of drastic
environmental changes.
Like directional selection, disruptive selection can be influenced by human
interaction. Environmental pollution can drive disruptive selection to
choose different colorings in animals for survival.
(3) Stabilizing Selection
The most common of the types of natural selection is stabilizing
selection. In stabilizing selection, the median phenotype is the one
selected for during natural selection. This does not skew the bell curve
in any way. Instead, it makes the peak of the bell curve even higher
than what would be considered normal.
Stabilizing selection is the type of natural selection that human skin
color follows. Most humans are not extremely light skinned or
extremely dark skinned. The majority of the species fall somewhere in
the middle of those two extremes. This creates a very large peak right
in the middle of the bell curve. This is usually caused by a blending of
traits through incomplete or codominance of the alleles.
Diversity in a population is decreased due to stabilizing selection.
However, this does not mean that all individuals are exactly the same.
Often, mutation rates in DNA within a stabilized population are
actually a bit statistically higher than those in other types of
populations. This and other kinds of microevolution keep the
population from becoming too homogeneous.
Migration (Gene Flow)
• Migration is the movement of individuals into or out of a
population. Just like the migration of birds from the north to
the south in the winter, organisms will sometimes change
their locations and therefore change the gene pool of that
population. Gene flow occurs when individuals move into or
out of a population. If the rate of migration is high, this can
have a significant effect on allele frequencies.
• During the Vietnam War in the 1960s and 1970s, many
American servicemen had children with Vietnamese women.
Most of the servicemen returned to the United States after the
war. However, they left copies of their genes behind in their
offspring. In this way, they changed the allele frequencies in
the Vietnamese gene pool. Was the gene pool of the American
population also affected? Why or why not?
Mating
• Many species are not selective when it comes to
mating. Asexual reproduction essentially clones the parent by
copying its alleles without any sort of mating between
individuals. Some species that use sexual reproduction will
choose any available individual that is available as a partner
with no regard for which characteristics they show. This keeps
the alleles that are being passed down from generation to
generation random.
• However, many animal species are selective when finding a
mate. These individuals look for particular traits in a mate that
will translate to an advantage for their offspring. Since this
mating is no longer random, many undesirable alleles are bred
out of the population over several generations. This makes the
gene pool shrink and fewer traits available for the next
generation, causing microevolution.
Mutations
• Mutations change alleles by changing the actual DNA of the
organism, thus creating genetic variation in a gene pool.
• It is how all new alleles first arise. There are several types of
mutations that can occur and have varying degrees of change that
accompany them.
• Alleles may not necessarily change if the change in DNA is small, like
a point mutation, but could be lethal to organisms if it has a
profound change, like a frame shift mutation.
• Cells are equipped with a system of checkpoints to make sure
mutations do not occur and are corrected if they do. Mutations
within populations that actually change the gene pool are rare.
However, mutations provide the genetic variation needed for other
forces of evolution to act. Mutations result in genotypic variations
within organisms and these variations in genotypes sometimes lead to
variations in the phenotypes on which evolutionary forces act.
Genetic Drift
• Unlike natural selection, genetic drift is a random, chance
event that depends solely on statistical chance instead of
desirable traits being passed down to offspring.
• Genetic drift changes allele frequencies in small populations
with small gene pools.
• The smaller a population, the more likely it is to see significant
microevolution related differences over generations.
There are two special conditions under which genetic drift
occurs. They are called bottleneck effect and founder effect.
Bottleneck Effect
• The bottleneck effect occurs when a larger population shrinks
significantly in size in a short amount of time. Usually, this
decrease in population size is generally due to a random
environmental affect like a natural disaster or spread of
disease. This rapid loss of alleles makes the gene pool much
smaller and some alleles are completely eliminated from the
population.
Founder’s Effect
• Another cause of genetic drift is called founders effect. The
root cause of Founders Effect is also due to an unusually small
population.
• However, instead of a chance environmental effect reducing
the numbers of available breeding individuals, the founders
effect is seen in populations who have chosen to stay small
and do not allow breeding outside of that population.
Lesson Summary
• Microevolution occurs over a short period of time in a
population or species. Macroevolution occurs over geologic
time above the level of the species.
• The population is the unit of evolution. A population’s gene
pool consists of all the genes of all the members of the
population. For a given gene, the population is characterized
by the frequency of different alleles in the gene pool.
• The Hardy-Weinberg theorem states that, if a population
meets certain conditions, it will be in equilibrium. In an
equilibrium population, allele and genotype frequencies do
not change over time. The conditions that must be met are no
mutation, no migration, very large population size, random
mating, and no natural selection.
• There are five forces of evolution: natural selection, migration
(gene flow),mating, mutation, and genetic drift. Natural
selection for a polygenic trait changes the distribution of
phenotypes. It may have a stabilizing, directional, or disruptive
effect on the phenotype distribution.