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.