Download Population genetics

By: Mandelli Luca, Italiano Valentina, Monti Elena, Pirovano Giada, Strimone Sara
Population genetics is the study of allele frequency
distribution and changes under the influence of the five main
evolutionary processes: natural selection, genetic drift,
mutation and gene flow and not random mating. It also takes
into account the factors of recombination, population
subdivision and population structure. It attempts to explain
such phenomena as adaptation and speciation.
Population genetics was a vital ingredient in the emergence
of the modern evolutionary synthesis.
It is based on three key words:
1) population:all the organisms of a species which live in
a certain place and in a particulary time.
2)genetic pool: is the complete set of alleles for a gene in a
single population.
3)fitness:is the reproductive success of an organism.
Population genetics is the study of the frequency and
interaction of alleles and genes in populations.
Let’s start with an example: all of the moths of the same
species living in an isolated forest are a population. A gene
in this population may have several alternate forms, which
account for variations between the phenotypes of the
organisms. An example might be a gene for coloration in
moths that has two alleles: black and white. The allele
frequency for an allele is the fraction of the genes in the pool
that is composed of that allele (for example, what fraction of
moth coloration genes are the black allele). Evolution occurs
when there are changes in the frequencies of alleles within a
population; for example, the allele for black color in a
population of moths becoming more common.
Biston betularia f. carbonaria is
the black-bodied form of the
peppered moth.
Biston betularia f. typica is the whitebodied form of the peppered moth.
Natural selection will only cause evolution if there is
enough genetic variation in a population. Before the
discovery of Mendelian genetics, one common hypothesis
was blending inheritance. But with blending inheritance,
genetic variance would be rapidly lost, making evolution
by natural selection implausible. The Hardy-Weinberg
principle provides the solution to how variation is
maintained in a population with Mendelian inheritance.
According to this principle, the frequencies of alleles
(variations in a gene) will remain constant in the absence
of:
The Hardy-Weinberg "equilibrium" refers to this stability of
allele frequencies over time.
A second component of the Hardy-Weinberg principle
concerns the effects of a single generation of random mating.
In this case, the genotype frequencies can be predicted from
the allele frequencies. For example, in the simplest case of a
single locus with two alleles: the dominant allele is denoted A
and the recessive B and their frequencies are denoted by p
and q; freq(A) = p; freq(B) = q; p + q = 1. If the genotype
frequencies are in Hardy-Weinberg proportions resulting
from random mating, then we will have freq(AA) = p2 for the
AA homozygotes in the population, freq(BB) = q2 for the BB
homozygotes, and freq(AB) = 2pq for the heterozygotes.
Mutations are changes in a genomic sequence. They are
errors during meiosis or replication. They can be sudden and
spontaneous. Mutations are caused by radiation, viruses and
mutagenic chemicals. Mutations can change the result of the
gene or can prevent that the gene from functioning properly.
In every new human being, with about 30 000 genes, there
are only two new mutations. So the number of new mutations
to every generations of a specific population is very high.
Mutations are the material of evolutionary changes.
Some examples of
chromosomal mutations
In population genetics, gene flow (also known as gene
migration) is the transfer of alleles or genes from one
population to another.
Migration into or out of a
population may be responsible
for a marked change in allele
frequencies. Immigration may
also result in the addition of
new genetic variants to the
established gene pool of a
particular species or
population.
There are a number of factors that affect the rate of
gene flow between different populations. One of the
most significant factors is mobility, as greater mobility of
an individual tends to give it greater migratory
potential. Animals tend to be more mobile than plants,
although pollen and seeds may be carried great
distances by animals or wind.
Maintained gene flow between two populations can
also lead to a combination of the two gene pools,
reducing the genetic variation between the two groups. It
is for this reason that gene flow strongly acts against
speciation.
Genetic drift, is a change in the gene pool which
occurs by the action of the case.
It has an important role in determining the course of
evolution of populations.
The phenomenon of genetic drift is crucial in the case
and the effect of founder bottleneck.
A population bottleneck is when a population
contracts to a significantly size over a short period
of time due to some random environmental event.
The bottleneck can result in radical changes in allele
frequencies, completely independent of selection.
There have been many known cases of population
bottleneck in the past. In Illinois the population of the great
prairie chickens in 1900 were about 100 millions, in 1990 it
was about 50 birds.
The decline of population resulted from hunting and habitat
destruction.
When very few members of population migrate to form a
new separate population, the founder effect occurs.
Some members of the
group of the Old Order
Amish
The founder effect is a special case of a population bottleneck,
occurring when small group in a population splinters off from the
original population and form a new one.
A well documented example is found in the Amish migration to
Pennsylvania in 1744. Two members of the new colony shared the
recessive allele for Ellis-van Creveld syndrome.
Members of the colony and their descendants tent to be religious
isolated and remain relative insular. As a result of many
generations of inbreeding, Ellis-van Creveld syndrome is now much
more prevalent among the Amish then in the general population.
Change the equilibrium of Hardy-Weimberg can
also be produced by non-random mating. For
example, the snow goose can be either blue or
white. This is a type of variation called
polymorphism, in which two or more phenotypically
distinct forms coexist in the same population.
The snow geese tend to mate with white geese, white
and blue ones with blue geese. So only two alleles
are concerned and therefore there will be a
decrease in the frequency of heterozygotes and an
increased frequency of homozygotes.
Sexual reproduction is the creation of news organism by combing the
genetic material of two organisms.
Asexual reproduction is a kind of reproduction by which offspring
arises from a single parent, and inherits the genes of that parent only,
by processes of mitosis and cytokinesis.
The sexual reproduction allows to maintain the genetic variability
thanks to 3 different processes:
through independent assortment during meiosis;
by crossing over and genetic recombination;
by the combination of two different genomes.
The advantage of the asexual reproduction is the double speed
reproduction of the offspring if compared to sexual reproduction.
Sexual reproduction
Asexual reproduction
The heterozygote advantage describes the case in which the
heterozygote genotype has a higher fitness than the
homozygote dominant or homozygote recessive genotype.
An example is the sickle-cell anemia. This disease is very
common in Africa. An organism who suffers from anemia
should die before reproducing. So in a period of time this
recessive gene is expected to disappear from the gene pool
of the population. This doesn’t happen. In the end scientists
discovered that the recessive allele of anemia remains in the
gene pool thanks to the heterozygote genotype. This
genotype is selective in the homozygote dominant and
recessive genotype because it is more resistant to the malaria
and the heterozygote women are more fertile.
Some red blood
cells of an
individual
soffering from
falciform anemy
which are an
example of the
heterozygote
advantage
The key concept of the theory of Darwin is that of
the natural selection. According to him, it is nature
itself that chooses the individuals with the most
favorable hereditary features. Thanks to these such
individuals will have more possibility of leaving a
greater number of descendants.
There are 5 types of natural selection:
1-Stabilizing
2-Divergent
3-Directional
4-Frequently-dependent
5-Sexual
The stabilizing selection is responsible for the
elimination of the individuals with extreme
characters. The example is that of the male deer.
The male fights to have the possibility of dominating
and has horns to fight. If the horns become too large
and heavy, this can become a problem, so selection
will promote the individuals with the right size horns.
A deer with small horns
A deer with big horns
A second type of selection is the divergent selection, where
the frequency of extreme characteristics of a population
increases at the expense of the intermediate forms.
A particularly clear example of divergent selection has been revealed in studies
on plants grown in soil contaminated by waste products of mineral extraction.
Another case of divergent selection, studied more recently, also can be considered
an example of frequency-dependent selection is the case of the silver salmon.
The silver salmons that live in the north-western Pacific lay eggs in freshwater,
where they spend their first year of life before heading to the ocean where they’ll
become sexually mature.
After reaching maturity, these fish return to their river of origin reproduce and die.
The females of this species reach sexual maturity and return to the river at the age
of three years, while males can be sexually mature at the age of two or three
years. The males of two years are known as “jacks” and the male of three years
are called “hooknose”.
Studies have shown that divergent selection promotes the smallest males jacks and
the biggest hooknoses.
When a female lays her eggs, male hooknoses fight to remain in the vicinity of the
deposition and, usually, they win. On the contrary, the jacks hide among the rocks.
The biggest jacks and smallest hooknoses are able to mate and in the population
the two extreme forms of male are preserved.
In population genetics, directional selection is a mode of
natural selection in which a single phenotype is favored,
causing the allele frequency to continuously shift in one
direction. Under directional selection, the advantageous
allele increases in frequency independently of its
dominance relative to other alleles; that is, even if the
advantageous allele is recessive, it will eventually
become fixed.
Human interaction can also speed up directional
selection. Hunters most often kill the biggest individuals
of the population for their meat or other large
ornamental or useful parts.
Therefore, the population tends to skew towards the
smallest individuals. This causes the directional selection
bell curve to shift to the left.
Frequency-dependent selection is the term given to an
evolutionary process where the fitness of a phenotype is
dependent on the frequency to other phenotype of the same
population. It can be:
Positive: the fitness of a phenotype increases, becoming more
common;
Negative: the fitness of a phenotype decreases, becoming
rarer.
It usually is the result of interactions between species. For
example between pray and predator: the predator attacks
the pray with a phenotype (for ex. the color) more common.
Later that phenotype will become rarer, and so the predator
will attack the pray with the new common phenotype.
A lioness following a gazelle
The term "sexual selection" usuallyrefers to competition for mates.
Sexual selection can be intrasexual, as in cases of competition
among individuals of the same sex in a population, or intersexual, as
in cases where one sex controls reproductive access by choosing
among a population of available mates. Most commonly, intrasexual
selection involves male–male competition and intersexual selection
involves female choice of suitable males, due to the generally
greater investment of resources for a female than a male in a single
offspring. However, some species exhibit sex-role reversed behavior
in which it is males that are most selective in mate choice; the bestknown examples of this pattern occur in some fishes of the family
Syngnathidae , though likely examples have also been found in
amphibian and bird species.
Some features that are confined to one sex only of a particular
species can be explained by selection exercised by the other sex
in the choice of a mate, for example, the extravagant plumage
of some male birds.
Similarly, aggression between members of the same sex is
sometimes associated with very distinctive features, such as the
antlers of stags, which are used in combat with other stags.
More generally, intrasexual selection is often associated with
sexual dimorfism, including differences in body size between
males and females of a species.
A second type of physical adaptation is the ecotype.
A species that occupies many different habitats may
appear slightly different in each of these environments.
An example of ecotype is the plants of Potentilla
glandulosa, a plant similar to the strawberry. Plants
located at differents altitudes have different features.
The Potentilla glandulosa reproduces asexually; plants
placed in the same areas were genetically similar and it
was demonstrated that many phenotypic differences
between the plants of different areas were due to
genetic variations.
Two plants of Potentilla
Glandulosa, grown in two
different places :
The colours of the plant’s
flowers are different
Sometimes the phenotypic variation within a species
depend on the geographic distribution and it is
correlated with changes in temperature or humidity,
an example is the CLINE.
Cline is the variation of a feature or set of features
within the same species.
An example are the sparrows household, they tend
to have smaller dimensions in warm regions and
larger sizes in colder regions to disperse less heat,
because of the decrease of the relation between the
surface and the volume.
A big dimension sparrow
A small dimension sparrow
In biology we have coevolution when two or more
species having a close ecological relationship evolve
together so that one species adapt to the changes of the
other, by affecting each other's evolution.
Coevolution can occur at microscopic level, the genes of
interdependent species are found to evolve together or
at macroscopic level, the traits of interdependent species
co-vary over time. For example is the coevolution of
insects and flowers they pollinate or the coevolution
between predators and their prey.
An example of coevolution is the following: flowers and
their insect pollinators are a classic example of
coevolution, as flowers arose some insects evolved to eat
the nectar while incidentally carrying pollen from flower
to flower.
The flowering plants
evolved to be more
attractive to the insect
pollinators. All
unconsciously, due to
random mutation that
erected variations in
both flowers and
plants that were
subject to natural
selection.
Another example is the case of the Monarch Butterflies and
Butterfly Weed, a type of milkweed, have coevolved as plant
and pollinator. This means that they both rely on one another to
survive. Milkweed is the primary source of nutrition for monarchs.
Monarchs only eat Asclepias tuberosa a particular species of
Milkweed.
The monarch relies on toxins in
the milkweed to fend off
predators such as birds. The
toxic tendencies of the milkweed
plants caused the government to
eradicate the plant along
roadsides and in cow pastures.
.This
has caused a major decline in the population of milkweed,
which is also endangering monarchs. Milkweed relies on the
monarch to pollinate; it so that it can reproduce.
Batesian mimicry is the ability to appear to be or to imitate something
other than what you really are. The use of mimicry is prevalent throughout
nature and is a prime example of evolution by natural selection.
Butterflies use it as a protection mechanism in their larva stage and in the
final adult stage. Either to trick predators into thinking they are an
inedible species or perhaps an entirely different organism all together.
Moreover, the intention of mimicry is to attract attention. This is usually
achieved, but not always, by advertising your presence with bright colours
and is known as "aposematism". These bright colours are probably easier
for predators to learn and therefore likely to reduce the number of
casualties necessary before the predator learns the pattern to avoid and
providing the mimic with protection.
On the other hand mullerian mimicry is the reality, species reveal for
what they are.
An example of batesian mimicry: a wasp (on the left) and a
butterfly (on the right); the two insects have the same colours.