Topic 7: Genetics, populations, evolutions and ecosystems

Types of selection

Selection is the process by which individuals that are better adapted to their environment are more likely to survive and breed. This means that they can pass on their advantageous alleles. Every living organism is subject to natural selection determined by the conditions which they are living in.
Directional selection - occurs when the environmental factors change and the phenotypes best suited to the new conditions are more likely to survive. As a result these individuals will breed and produce offspring. Overtime the mean of the population will move in the direction of these individuals. An example of this is bacteria being resistant to antibiotics. A single bacteria will have had a mutation that meant it was not killed by penicillin. As a result it was able to grow and populate, and the frequency of the allele that enabled this increases in the population.
Stabilising selection - the phenotypes with successful characteristics are preserved and those of greater diversity are reduced. This selection doesn't occur due to changes in the environment. If the environment stays the same then the individual closest to the mean are favoured because they have the alleles with the survival advantage.
Disruptive selection - this is the opposite of stabilising selection and in this case both extremes of the normal distribution are favoured over the mean. An example of this is where certain large mammals are adapted to surviving long period without food due to increase body fat, whilst small mammals of the same species survive due to their decreased need for food.

Populations
A Population is a group of organisms of the same species occupying a particular space at the particular time that can potentially interbreed. The total numbers of alleles that are present in a population is known as the gene pool whilst the allelic frequency expresses as a decimal or percentage the proportion of a certain allele in a gene pool.
The Hardy-Weinberg Equation can be used to estimate the frequency of alleles in a population and to see whether a change in allele frequency is occurring in a population over time. Assumptions made:

No mutations occur to create new alleles.
there is no movement of alleles into or out of the population by migration.
the population is large.
there is no selection, so every allele has an equal chance of being passed to the next generation.
Mating is random

The formulae for the Hardy-Weinburg principle is:
P + Q = 1.0 and P2 + 2PQ + Q2 = 1
P = the frequency of the dominant allele.
Q = the frequency of the recessive allele

Inheritance

Evolution and natural selection

Niche - the role of a species within its environment. Species which share the same niche compete with each other and the better adapted survive. The idea that better-adapted species survive is based on natural selection.

Many organisms have a unsustainablly large number of offspring. Darwin suggested that the reason many offspring are produced is so that there is greater competition( within the species (intraspecific competition**) and therefore only those that have alleles best suited to the environment survive long enough to grow and reproduce passing the alleles on to the next generation.

The variation in genotypes and phenotypes within a population increases the chance that a species will survive in a habitat that is changing.

Process of evolution by natural selection:

There is a variety of phenotypes within a population.
An environmental change occurs and as a result of that the selection pressure changes. Some individuals possess advantageous alleles which give them selective advantage and allow them to survive and reproduce.
The advantageous alleles are passed on to their offspring.
Over time, the frequency of these alleles in a population changes and this leads to evolution.

Chi squared test
The chi squared test is a statistical test which can be used to establish whether the difference between observed and expected results is small enough to occur purely due to chance. A null hypothesis is one which the results of a scientific investigation will produce no statistical significance e.g there is no difference in the number of times a flipped coin will land on heads or tails.

The sample size must be sufficiently large enough, that is over 20.
Used only for data that falls into discrete categories.
Only raw counts and not percentages

Monohybrid inheritance
When a phenotype or trait is controlled by a single gene. For instance, cystic fibrosis is where the individual with double recessive alleles is affected.

Dihybrid inheritance
When two characteristics are studied and is determined by two different genes that are present on two different chromosomes at the same time.

Codominance and multiple alleles
Codominance is when two phenotypes are expressed such as when birds have feathers that are both black and white.

Sex Linkage
Sex linkage is the expression of an allele dependant on the gender of the individual as the gene is located on a sex chromosome. Humans have 22 pairs of autosomal chromosomes that are not involved in sex determination, and one pair of sex chromosomes, called either X or Y.

XY - Males
XX - Females
Most sex linked traits are located on the X chromosome with there being no equivalent locus on the Y chromosome. Therefore females carry two alleles of the sex linked gene, but males will only carry a single allele.
An example of a sex-linked disease is haemophilia which is a disease in which the blood clots slowly and there may be slow and persistent internal bleeding, especially around the joints. It is caused by a change to the DNA sequence resulting in a faulty protein being created.
As a male can only obtain the Y chromosome from his father, it means that the X chromosome must come from his mother. As this disease is not found on the Y chromosome it means that this disease is always inherited from the mother in the males. If the mother doesn't suffer from the disease but is heterozygous with the alleles then she is a carrier.

Autosomal Linkage
In an autosomal linkage two or more genes are on the same autosomal chromosome.

Epistasis
In epistasis the interaction of different loci on the gene, one gene locus affects the other gene locus. One gene loci can either mask or suppress the expression of another gene locus. An example is the two genes in mice that control fur colour. Genotypes AA or Aa have black bands in their fur, while genotype aa has solid black fur. Gene B controls the expression of gene A. Genotypes BB or Bb will allow the expression of gene A, but genotype bb will not. Mice with genotype bb are all white, called albino.

Recessive epistasis occurs when the presence of a recessive allele prevents the expression of another allele at a second locus. Recessive epistasis gives the ratio of 9:3:4
Dominant epistasis is when a dominant allele at one locus completely masks the alleles at a second locus. Dominant epistasis gives a ratio 12:3:1.

Formula
x2= the sum of (observed - expected)2/expected
The value obtained is compared to the critical value. In chi-square, the critical value is p=0.05. Where the value obtained is equal or greater than the critical value, the null hypothesis is accepted as the difference due to chance is not significant. To read the value the degrees of freedom need to be known. These are simply the number of categories minus one.

Whereas in a case where the value is less than the critical value, the null hypothesis is rejected meaning that the difference between observed and expected results is not due to chance, as is significant.

Variation in phenotype
Variations in the alleles of genes for members of the same species is due to:

Random fertilisation - the gametes that are carrying different alleles will join together randomly.
Meiosis - meiosis is the nuclear division that creates gametes and means that the alleles will be assorted in the gametes at random.
-Mutation - the mutation of an allele can go on to lead to the creation of another new allele which can then be passed to the next generation.

Environmental influences
They may also be environmental influences, with the majority of phenotypic traits being influenced by environmental factors. An example is two plants that possess the same alleles for the flower colour. However, an environmental factor of one plant growing in a soil that lacks a certain mineral may mean the pigment in one will not develop so will have a different colour. If a large enough sample is taken out of this trait then a normal distribution will be shown.

Types of selection

Selection is the process by which individuals that are better adapted to their environment are more likely to survive and breed. This means that they can pass on their advantageous alleles. Every living organism is subject to natural selection determined by the conditions which they are living in.
Directional selection - occurs when the environmental factors change and the phenotypes best suited to the new conditions are more likely to survive. As a result these individuals will breed and produce offspring. Overtime the mean of the population will move in the direction of these individuals. An example of this is bacteria being resistant to antibiotics. A single bacteria will have had a mutation that meant it was not killed by penicillin. As a result it was able to grow and populate, and the frequency of the allele that enabled this increases in the population.
Stabilising selection - the phenotypes with successful characteristics are preserved and those of greater diversity are reduced. This selection doesn't occur due to changes in the environment. If the environment stays the same then the individual closest to the mean are favoured because they have the alleles with the survival advantage.
Disruptive selection - this is the opposite of stabilising selection and in this case both extremes of the normal distribution are favoured over the mean. An example of this is where certain large mammals are adapted to surviving long period without food due to increase body fat, whilst small mammals of the same species survive due to their decreased need for food.