evolutionary processes Flashcards
history of population genetics and evolution
the modern synthesis (1900s-60s) put together Darwin and Wallace’s (1859) theory of evolution with Mendel’s (1850s-60s) principles of genetics to describe natural selection in terms of allele frequencies
genetic equilibrium
- allele and genotype frequencies stay the same from one generation to another
- testing for genetic equilibrium involve calculating the statistical difference between observed and expected allele frequencies of offspring
7 conditions required for genetic equilibrium
- organisms are diploid
- organisms only reproduce through sexual reproduction
- no overlap between generations
- mating is random
- population is large
- no migration, mutation or selection
- allele frequencies are equal in both sexes
hardy-weinberg equation
- allows for calculation of allele and genotype frequencies within a population at genetic equilibrium
- p^2 + q^2 + 2pq = 1
- p= frequency of dominant allele
- q = frequency of recessive allele
non random mating
- assortative mating = similar genotypes mate e.g. pigeons with size
- dissasortative mating = opposite genotypes mate e.g. pigeons with colour
Assumption of no net mutation
mutations of dominant to recessive alleles occur at the same frequency as mutations from recessive to dominant alleles e.g. mutations from wild type to albino in rabbits occur more frequently than mutations from albino to wild type, but there is a selection balance as albino rabbits are more likely to be predated
relative fitness (w)
absolute fitness of each genotype divided by absolute fitness of fittest genotype
absolute fitness
- viability * fertility
- viability is the probability of surviving to reproductive age
- fertility is the number or offspring that survive to reproductive age
natural selection, directional selection
- one allele is always at an advantage e.g. peppered moths
- dominant allele advantage means allele will spread very quickly in a population (AA=Aa>aa)
- codominant dominant allele advantage means adv. allele spreads, disadv. allele gets selected out (AA>Aa>aa)
- recessive advantageous allele means allele will spread very slowly to start with, then quickly becomes only allele in population (aa>Aa=AA)
natural selection, overdominance/balancing selection
- heterozygous advantage where there is codominance (AA<Aa>aa)</Aa>
- should maintain both alleles (stable polymorphism), but equilibrium frequency depends on relative fitness of each homozygote
- e.g. sickle-cell anaemia, homozygote is malaria resistant
natural selection, underdominance
- heterozygous disadvantage where there is codominance (AA>Aa<aa)
- produces unstable equilibrium
- allele frequencies tend to move away from equilibrium point
- least common allele declines and eventually becomes extinct (less common means more likely to be heterozygous)
e.g. chromosomal rearrangements in drosophilia
genetic drift
- random changes in allele frequencies
- often makes populations less fit
- occurs more often in small populations as offspring produced are at a much lower frequency than alleles
- extreme cases caused by a temporary dramatic reduction in population size
founder effect
- small group of individuals leave original population to found new one
- e.g. Eastern Pennsylvanian Amish population, founded by 200 German immigrants and married strictly within community. One individual carried recessive allele for Ellis van Creveld syndrome rare in German population but now common in Amish population
genetic bottleneck
- random event wipes out most of population, leaving only a small group
e.g. typhoon on Pingalap island in 1800s left 30 survivors, one carrying rare recessive allele for achromatopsia (complete colour blindness), 10% population are now colourblind
measuring genetic diversity
- heterozygosity = likelihood individual will be heterozygous at a given locus, usually averaged over many genetic loci
- allelic diversity
Number of alleles averaged across loci = total number of alleles over all loci / number of loc
