Population genetics Flashcards
lecture 14 and 15 week 7
What is population genetics
population genetics is the study of the genetic variation in a population, and the evolutionary factors that explain this variation
What are evolutionary processes
evolution: a change in allele and/or genotype frequency over time
evolutionary processes included mutation and recombination, natural selection, gene flow, genetic drift and non-random mating
What is the Hardy-Weinberg equilibrium
- describes situations in which allele and genotype frequencies do not change
- based on Mendelian genetics but at a population level
What are the assumptions of the Hardy-Weinberg equilibrium
- no selection
- no migration
- no mutation
- large population size (no genetic drift)
- random mating occurs
no evolution occurs
Example of Hardy-Weinberg equation
(need to use known genotype so normally recessive)
In the UK, 1 in 2500 babies are born with CF. What is the frequency of heterozygous carriers in the population
q squared = homozygous recessive with CF
1/2500
q = root of 1/2500 = 0.02
p + q =1
p = 1 - 0.02 = 0.98
p2 + 2pq + q2 = 1
2pq = 1 - 0.964 - 0.0004 = 0.0392
frequency of heterozygous carriers of CF is 0/0392
What are mutations
mutations are needed in order for evolution to occur
- source of new alleles and genetic variation
- provides ‘raw material’ for other evolutionary forces to act on
What is natural selection
variation in average reproductive success among phenotype
-based on overproduction in natural phenotypes
- individuals compete for resources, phenotypic variation occurs within the population and some phenotypes may have better chance of reproducing than others
- evolution occurs if phenotype with different reproductive success has a genetic basis
fitness: measure of the relative reproductive success of a genotype
adaptation: a consequence of selection - prevalent or maintained in a population because of natural selection for that function
What are the different types of selection
-positive selection
- negative selection
- balancing selection
What is positive selection
natural selection that increases the frequency of a favourite allele
What is negative selection
natural selection that decreases the frequency of a deleterious allele
What is balancing selection
natural selection that acts to maintain two or more alleles in a population
eg. sickle cell anaemia
heterozygote advantage: AA vulnerable to malaria, SS affected by sickle cell anaemia
- S and A alleles are maintained in the population at intermediate frequencies in regions of malaria
What are the different types of selection that effect a particular trait
- stabilising selection
- directional selection
- disruptive selection
What is stabilising selection
maintain the status quo and acts against extremes
eg. babies size at birth
What is directional selection
directional selection leads to a change in trait over time
eg. a drought occurred and only plants with large seeds survived
- birds with a bigger beak were then more likely to survive and the population changed so birds had a bigger beak size
artificial selection is where humans choose a trait which drives selection
What is disruptive selection
operates in favour of extremes and against intermediate forms
eg. evolution of two distinct populations of apple
- maggot flies in North America after apples were introduced from Europe
- this can lead to the formation of new species
What is sexual selection
sexual selection promotes traits that increase an individuals access to reproductive opportunities
intersexual selection: members of one sex compete for the attention of another sex
intrasexual selection: members of one sex compete for access to the other sex
What is gene flow
- the movement of alleles from one population to another (caused by migration)
-increases genetic diversity in the receiving population but makes populations more similar to each other - can be maladaptive
What is genetic drift
random changes in allele frequency from generation to generation (effect is stronger on smaller populations)
- in extreme cases a population bottleneck occur where there is a catastrophic reduction in population, this causes a loss of genetic variation
founder effect: a few individuals of a population starts a new population
- leads to a loss of variation
What is non-random mating
assortative mating: individual with the SAME genome more likely to mate than individuals with different genotypes
disassortative mating: individuals more likely to mate with individuals of a DIFFERENT genotype
non-random mating leads to a change of genotype frequencies, but not a change in allele frequencies
assortative mating: excess of homozygotes (inbreeding)
disassortative mating: excess of heterozygotes
