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. typica is the white-bodied form of the peppered moth.

Biston betularia f. carbonaria is the black-bodied 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.

http://en.wikipedia.org/wiki/File:Hardy-Weinberg.svg

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-Stabilizing2-Divergent3-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 best-known 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.