Selection And Evoloytion Flashcards
Continuous variation
Quantities differences such as mass or height. Don’t fall into distinguishable categories
Discontinuous variation
Qualitative difference which fall into clearly distinguishable categories with no intermediated. For example, blood groups
The genetic basis of discontinuous variation
Different allele at a single gene locus have a large effect on the phenotype. Different genes have quite different effects on the phenotype.
The genetic basis of continuous variation
The small effect of different alleles of one gene in the phenotype and the additive effect of different genes on the same phenotypic character
Environmental effects on phenotype; growth
Lack of nutrition may decrease potential growth based on the genotypic contribution to growth. Plants may not grow as tall in poor light conditions and soil with low nutrition
Environmental effects on the phenotype; cats
One example of the effect of environmental factors is the development of dark tips to the ears, paws and tail of Siamese cats. The colouring is caused by an allele which allows the formation of the dark pigment at low temperatures, which is normally the extremities
Using the T-test to compare the variation of two different populations
Used to assess whether or not the means of two sets of data with roughly normal distribution, are significantly different from one another.
Why is genetic variation important in selection
Genetic variation provides the raw material on which natural selection can act, so in selective breeding it is important to know how much of the phenotypic variation is genetic and how much is environmental. There is no point in selecting parents for a breeding programme on the basis of environmental variation as they would not be able to pass on these characteristics to their offspring
Natural selection
Natural selection occurs as populations have the capacity to produce many offspring that compete for resources. Populations tend to remain fairly constant. Meaning that some organisms die as a result of both abiotic (water supply) and biotic (predation) factors. Variation within a population mean that some will have features which will give them an advantage in the, “struggle for existence.” Only the individuals that are best adapted survive to breed and pass on their alleles to the next generation.
Selection pressures
Selection pressures increase the chance of some alleles being passed on to the next generation and decreases the chance of others. For example the cold may favour a thick coat
Stabilising selection
When the selection pressure acts as against the two extremes, and favours the values nearest the mean
Directional selection
When the range of variation shifts towards one of the extremes, such as being big
Disruptive selection
When selection favours the two extremes but not the values in between
Example of directional selection
Antibiotic resistance. When a disease is being treated with antibiotics there is a chance that one of the bacteria will have a mutation giving it antibiotic resistance. This gives the bacteria a selective advantage as the bacteria without this allele will be killed as antibiotics are a selection pressure, while those bacteria with resistance can survive and reproduce. Bacteria also reproduce rapidly meaning that a very large population of bacteria will be produced which are resistant to antibiotics. If the resistance is on the plasmid it can be transferred between different bacterial species. The more we use antibiotics the greater the selection pressure we exert on bacteria to evolve resistance to them
Example of disruptive selection
In moths the normal speckled colouring is produced by the recessive allele c, whilst the black colour is caused by the dominant allele C. The allele C increased in more industrial areas as the black moth can be camouflaged by the darker trees in these polluted areas, meaning that birds will not kill them. This selection pressure causes a change in allele frequency with there being more C alleles. In rural areas the speckled moths are camouflaged by the lichen growing on the trees so will not be predated. As we have become less industrial speckled moths have increased in frequency
Example of stabilising selection
Sickle cell anaemia. There is a selection pressure against people who are homozygous for the sickle cell allele HbsHbs because they become seriously anaemic and are unlikely to survive and reproduce. Selection against people who are homozygous HbAHbA is very strong as they are more likely to die of malaria. In areas where malaria is common, heterozygous HbAHbs has a strong selective advantage as they wont die from malaria or sickle cell anaemia so will be able to pass on their genes to their offspring.
Genetic drift
This is a change in allele frequency which occurs by chance because only some of the organisms of a population reproduce. It is most noticeable when there is only a small population as they do not have a large allele frequency. Further genetic drift in a small population will alter the allele frequency more and evolution will occur in a different direction.
Founders effect
When genetic drift occurs in a isolated small population it has a more profound effect
Selection
When there is a new environmental factor (selection pressure), the allele which favours survival in this environment will be more likely to be passed on. For example if we entered an ice age the allele for white fur would increase as this helps with camoflage. There may be a mutation which gives rise to a new allele which is favourable for survival. Over multiple generations the allele frequency would increase
The Hardy-Weinberg principle equations
p+q =1
p^2 + 2pq + q^2 = 1
You use these equations to calculate genotype frequency. For example of you know aa is 0.01 you can square root it to find out the frequency of P, take it away from 1 then square to find AA. You then take AA and aa away from 1 to find Aa
When does the Hardy-Weinberg principle not occur
When there is significant selective pressure against one of the genotypes. When there is migration of individuals carrying one of the two alleles in or out of the population. Non-random mating
Artificial selection
When humans purposely apply selection pressures to a population
The artificial selection of cows
Docility, fast growth rate and high milk yields have been selected for. Individuals with these characteristics have been bred together with the alleles being passed on. Over many generations the desirable alleles increase in frequency.
Problems with the artificial selection of cows
The animals are large and take a long time to mature. The gestation period is long and the number of offspring is small. A bull can not be assessed for milk production and instead we have to look at the offspring
Artificial selection for disease resistant wheat
Breeding for resistance to fungal diseases, such as head blight because of the loss of yield resulting from such infections. The wheat genetic improvement network aims to bring together research workers and commercial plant breeders. It screens seed collections for plants with traits such as disease or climate resistance. Any plants with suitable traits are grown in large numbers and passed on to commercial breeders
Allopatric speciation
Allopatric speciation is caused by a physical barrier. As the two groups become separated and reproductively isolated as a result, the gene flow is reduced. Each group experiences a different selection pressure as the environment they live in is different. Over time, the frequency of alleles changes through natural selection and the two parts of the population can no longer interbreed and become separate species.
Founders effect
Decrease in genetic diversity which occurs when the population descends from a small number of ancestors
Genetic bottleneck
The rapid reduction in population size which has an effect on the population size and genetic variation in future generations
Evolution via natural selection
There’s 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 a selective advantage and allow them to survive and reproduce. The advantageous alleles are passed on to their offspring. Over time, the frequency of alleles in a population changes and this leads to evolution
Artificial selection for rice
The international rice research institute holds the rice gene bank and work with rice farmers. The yield of rice can be reduced by bacterial diseases such as blight. Researchers are hoping to use selective breeding to produce varieties of rice that are resistant to the deceases.
Dwarf plants
Plants have shorter stems now as it makes them easier to harvest and they have higher yields because they use more of their energy to produce seeds then grow tall. Most of the dwarf varieties carry mutant alleles of two reduced height genes. These genes code for DELLA proteins which reduce the effect of gibberellins on growth. The mutant alleles cause dwarfism by producing more of these transcription factors. Another mutant allele has its dwarfing effect because the plant cells do not have receptors for gibberellins
Inbreeding and hybridisation in maize
If maize plants are inbred (crossed with plants with similar genotypes), the plants become smaller and weaker after every generation. However, if they are outbred at random there will be a lot of variety in the plants, making them difficult to harvest. To combat this, farmers buy seeds from companies that use inbreeding to produce homozygouse plants and then crossing them. There are lots of different homozygous maize varieties and different crosses produce different hybrids, suited for different conditions
Species
One definition of species is a group of interbreeding organisms that have similar morphology (structural features), physiology (how their body works), biochemistry (sequence of bases in DNA) and behavioural features
Speciation
In order for a new species to be produced, the two groups within the species must undergo reproductive isolation, so that they can evolve to be different and are no longer able to reproduce to produce fertile offspring
Pre-zygotic isolating mechanisms
Individuals not recognising each other as potential mates or responding to mating behaviour. Animals being physically unable to mate. Incompatibility of pollen and stigma in plants. Incompatibility of male gametes to fuse with female gametes
Post-zygotic isolating mechanisms
Failure of cell division in the zygote, non-viable offspring, sterile offspring
What does both post-zygotic and pre-zygotic isolating mechanisms result in
A group of interbreeding organisms into the evolution of a new species.
Sympatric speciation
When a new species arises without the original population being separated by a physical barrier
Autopolyploid
Occurs through polyploidy, in which an offspring will be produced with twice the normal number of chromosomes. Where a normal individual has two copies of each chromosome (diploidy), these offspring may have four copies (tetraploidy). A tetraploid individual cannot mate with a diploid individual, creating reproductive isolation as it is unable o produce gametes as the chromosomes are unable to pair up during meiosis. But it can reproduce asexually if it is a plant. Occasionally it will be able to produce gametes which fuse with normal gametes to form a triploid zygote with 3 sets of chromosomes, it will be sterile.
Definition of autopolyploids
Contains four sets of chromosomes from the same species
Definition of allopolyploids
When they contain chromosomes from two different species
Allopolyploids
A polyploid organism, usually a plant, that contains multiple sets of chromosomes derived from different species. Hybrids are usually sterile, because they do not have sets of homologous chromosomes and therefore pairing cannot take place. However, polyploids can be formed from two sets of chromosomes from one species and two sets of chromosomes from another. This type of tetraploid is known as an allotetraploid; as it contains two sets of homologous chromosomes, pairing and crossing over are now possible.
