Genetic Drift and Gene Flow Flashcards
what is the only evolutionary force that can cause adaptation by refining the fit between species and their environment?
natural selection
what are 2 main evolutionary forces?
genetic drift and gene flow
what are genetic drift and gene flow both infleunced by?
- the demography of populations
- movement and reproductive behaviour of individuals
- occur relentlessly
- independent of natural selection
what is genetic drift?
a set of population processes where sampling error (biases that occur when sample sizes are too small) affects allele frequencies
when does the bias caused by sampling error decrease?
when sample size increases
what are the three ways sampling error can occur?
- founder effect
- population bottlenecks
- when populations are constant in size, but small
what is founder effect? explain and give an example.
- occurs when a new population is founded from a small sample of colonists that usually come from a large source population
- the large source population has lots of genetic diversity
- but the small sample that leave to start the new population, do not have the allele frequencies that are repersentative of the population that they came from
- most likely when a species is undergoing a major shift in distribution (ex. from volcanic stuff)
- could occur when humans move to parts of the world they never previously existed
- major cause: climate change. reduced genetic variability in a newly formed population might be incapcable to adapting to changing environments if they contain low genetic variation due to founder effect
what is an example of founder effect?
- founder effect can cause high frequencies of alleles that cause medical conditions in humans
- Porphyria amongst white Afrikanners in South Africa
- caused by an autosomal dominant allele that inhibits the production of haem, that is incorportated into haemoglobin, which carries oxygen in red blood cells
- the build up of porphyrins can cause blood look in teeth, calling it vampire disease
- dominant alleles common among Afrikanners (the diseased allele) but rare in the population of the Netherlands that settled there
- the founder event seems to come from a small sample of immigrants that arrived in south africa from the netherlands
what is population bottleneck?
- where a population crashes to a small number of survivors, from which it might regrow
- the small sample of survivors might have allele frequencies different from the original population, and those allele frequency biases will be transmitted to future generations as the population regrows
- Example: greater pairie chicken: population was reduced drastically as humans converted their grassland
what was low genetic variation in the prairie chickens linked with?
low hatching success of eggs, which suggested the bottlenecked populations suffered from the genetic effects of inbreeding
how does a population being constant in size, but small, cause sample error? (genetic drift)
- individuals within a population make uneven reproductive contributions to the next generation
- the small sample of breeders and the few number that survive will not have the allele frequencies that repersent the population in the previous generation
when do we say an allele is fixed?
when it goes to a frequency of one
when do we say an allele is lost?
when it goes to a frequency of zero
if drift is strong in small populations, what correlation should we see?
a positive correlation between genetic variation within populations and population size.
what is loci polymorphic?
containing more than one allele
