Molecular Evolution Flashcards
Define Evolution
Change inheritable characteristics of biological populations over successive generations.
Natural Selection
The effects of a wide range of factors on the frequency of heritable changes in a species.
Fitness
How well a species is able to reproduce in its environment
What is selected for in terms of fitness?
Anything that increases fitness is selected for, anything that decreases fitness is selected against.
Other neutral changes will vary randomly.
If the environment is changed, fitness can change.
What is the modern synthesis?
The idea that evolution can be unified with genetics to explain molecular processes underlying evolution. Genetic variation is the main source of heritable changes in a species.
What affects frequencies of genetic variants?
Selection:
- Genetic variants that confer a positive advantage will be selected for and vica versa.
Mutation:
- Variation in the genome aries.
- Large numbers of genomic variants are already in the population and their frequency depends on selection and when they first arose.
- A rare variant (SNV) may have arisen very recently or be deleterious and being selected against or both.
Migration:
- The physical movement of people from a different population results in new pools of variants being introduced to an existing population. This is called genetic admixture. Population frequencies of specific variants can change purely due to admixture and not be disease related.
Genetic drift:
- This is how the frequency of a variant changes in a population due to change. Not all organisms in a population will pass on their genetic variants. Random change in variant allele frequency. Mechanisms such as recombination will also result in not all variants being passed on.
What is sequence conservation?
The conservation of a DNA sequence that is vital to the survival of an organism so does not normally show much evidence of variation.
What happens to variants in the sequence conservation region?
Most variants in these regions will be selected against as they are likely to have a strongly deleterious effect. There is some flexibility for variation in the third base of codons as the genetic code is degenerate.
Describe sequence conservation in genes
High conservation - coding regions (not exons - as these contain non-coding regions)
Intermediate conservation - promoter, 5’UTR, 3’UTR, terminator
Low conservation - introns, 3rd base of condons and terminator
What is cross-species comparison used for?
It can be used to generate an evolutionary profile for a gene or gene family.
Give an example of conservation
NAMPT - nicotinamide phosphoribosyltransferase is conserved with other species
What is phylogenetics?
The study of evolutionary relationships between species
What are phylogenetic trees useful for?
The main aim is to illustrate the relatedness of different species/strains/sequences. The distance between two entities on a tree is usually related to how similar they are. The distance is normally related to both evolutionary pressures and to time.
What do strong evolutionary pressures result in?
Strong evolutionary pressures can result in changes in organisms even if there is not a great distance between them.
How did they show that Polio Vaccine did not cause HIV?
- It was theorised that contaiminated polio vaccine introduced SIV into humans in DRC, Africa.
- Some polio vaccines used to be produced using cultured chimpanzee cells, which could have been infected with SIV (which is closely related to HIV).
- However Worobey et al (2004) investigated this using phylogenetics. The tree demonstrates that the SIV genome in DRC wild chimpanzees was completely distinct from all HIV genomes.
- Other SIV strains were likely to be the source as they are closely related to HIV.
- Showed that the polio vaccine was not contaminated with anything that caused HIV.
