Week 4 Flashcards
What is a population?
group of organisms of the same species living in the same geographical area
What is population genetics?
application of genetic principles to entire populations of organisms
What is an allele?
One of the alternative forms (different nucleotides) of a gene.
When we talk about alleles in population genetics, what are we mostly concerned with?
the frequencies of alleles in a population.
What do allele frequencies measure?
- how common an allele is in a population
- they can be calculated for each allele in a gene pool.
Where is genetic variation stored?
- in a population’s gene pool.
- made up of alleles and allele combinations form when organisms have offspring
What does genetic variation lead to?
- Phenotypic variation
What is phenotypic variation necessary for?
- natural selection.
What is the basis of evolution?
- Changes in allele frequencies within or between populations
- selection, mutation, genetic drift, gene flow
What are the four main processes that cause changes in allele frequencies (genetic variation)?
- selection, mutation, genetic drift, gene flow.
What is the proximate mechanism for evolution?
- allele frequency changes
What are the four main components of natural selection?
- inheritance ( traits controlled by genes)
- variation (genetic variation between individuals)
- survival of the fittest (not all individuals mate)
- differential survival and reproduction
What does natural selection occur in response to?
- environmental pressures
Individuals do not evolve, ________ do
- populations do.
What is evolution measured as?
- change in relative proportions of heritable variations in a population over several generations.
Why is genetic variation important for evolution?
- genetic variation leads to phenotypic variation, which is required for natural selection.
What does natural selection cause happen in a population?
- different environmental pressures between populations so:
- greater between-population genetic variation but
- lower within-population genetic variation.
- sometimes resulting in new species.
What is a mutation?
- a random change in the nucleotides of a gene.
What is the ultimate source of genetic variation? How?
- mutation
- it increases genetic variation when it creates new alleles
What are the types of mutation?
- point mutations
What are point mutations? When can it change a phenotype?
- single base pair change in genome
- in a coding region
How can a point mutation be passed to offspring?
- if it happens during gamete mutation it can be passed on to offspring
What is genetic drift?
- a change in allele frequencies caused by random sampling of alleles over generations.
- completely random sampling.
- some individuals not surviving randomly and loss of alleles.
- random fluctuations
Where does genetic drift act more quickly?
- smaller populations
- can reduce variation in these populations through the loss of alleles.
What is fixation?
- in genetic drift where an allele becomes the only allele because all others have been removed.
Where does genetic drift usually happen?
- founder effects (as well as bottlenecks)
- reduced genetic variation from original population
- a different sample of the genes in the original population.
What is gene flow? When does this occur?
- the transfer of alleles from one population to another
- when individuals join new populations AND REPRODUCE
What does high gene flow do?
- keeps neighbouring populations similar and can maintain genetic variation within populations
What does low gene flow do?
- increases the chance that two populations will evolve into two different species and can lead to lower within-population genetic variation through inbreeding.
If an individual is inbred:
- has low heterozygosity
If high heterozygosity?
- high genetic variation.
Genetic drift and low gene flow over time produce a march towards
- homozygosity
What is the Hardy-Weinberg equilibrium model? What does it explain?
- a framework for understanding how populations evolve.
- explains how Mendelian segregation influences allelic and genotypic frequencies in a population
What does HWE describe? What are the 5 assumptions of hardy weinberg equilibrium?
- populations that are not evolving
- no natural selection
- No mutation
- No migration (gene flow)
- Infinitely large population (to avoid genetic drift)
Mating is random
What does the HWE predict? What can we then do with those?
- genotype frequencies in a population
- compare those with actual frequencies from populations
- used with simple mendelian inheritance (two alleles)
Describe HWE equation:
p + q = 1
Then p2 + 2pq + q2 = 1
If HWE assumptions are met, population will be in________
- genetic equilibrium AKA hardy-weinberg equilibrium
What are the two predictions of HWE?
- allele frequencies do not change over generations
- after on generation of random mating, genotypic frequencies will remain in the following proportions:
How do you do HWE with two numbers (allele counts)?
- first assign alleles
- prepare a punnet square of allele combinations
- we calculate the frequency of q2 because we know their alelle frequencies (both recessive - we don’t know this about p). Q2 = 12/(988+12) = 0.012
- calculate the frequency of q of the recessive blue allele (y) so q = 0.11
- we can minus that off 1 so p = 0.89
- then apply the formula to get the genotype frequencies.
Why do we use HWE?
- it’s a good null hypothesis compared to values from a real population to describe statistically significant deviations from the equilibrium. If different, evolution is occuring
- therefore either selection, mutation, or gene flow, or genetic drift is occuring to alter them.
- these are occurring and violating the HW assumptions and constitutes a null model and is fundamental to the study of evolution.
