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Population genetics - genetics
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- 1why look at genes in populations?- to predict fate of genes over time (characteristics of the population affects the fitness of individuals carrying particular gene)
population genetics = the principals that explain how genetic variation changes in populations over space & time - 2population = a group of sexually interbreeding individuals 2 important atributes: - gene frequency (proportion of an allele in a population) - gene poolpopulation characteristic
allele frequency = frequency of homozygotes + 1/2(frequency of heterozygotes)
gene pool = sum of all alleles within a population with potential to contribute to next generation - 3what happens to genes: - Mendel's crosses involving dominant & recessive alleles -> 3 dominant: 1 recessive - speculation that similar ratios should be found in populations*many dominant traits extremely rare -> argument against Mendel
Hardy-Weinberg principle:
- allele frequencies don't depend on dominance/recessiveness but remain unchanged from generation to generation
- explains how genes are maintained in populations
*assumptions = large population, no mutations, no migrations, no selection & random reproduction - 4HW principle - explains why recessive genes don't disappear over time - in populations at equilibrium, both gene & genotype frequencies remain constant from generation to generation *its the gene frequency of the parent population that determine the genotypes of the next generationprinciple:
p2+ 2pq +q2 = 1
p= frequency one of allele
q= frequency of other allele
p+q=1
p2 = dominant homozygote genotype frq
2pq = heterozygote genotype frq
q2 = recessive homozygote frq - 5multiple alleles - when more than two alleles are present at a locus - HW principe still appliesin a population with 3 alleles at locus, genotype frequencies at equilibrium:
p2(A1A1)+ 2pq(A1A2)+q2(A1A3)+2qr(A2A3)+r2(A3A3) = 1 - 6rare recessives - when recessive phenotypes are rare in population - common to find carrier heterozygotes present at high frequencieschanges in gene frequencies - most natural pops not in equilibrium & gene frequencies alter from generation to generation according to:
- natural selection (modifies reproductive success)
- mutation (results in change in DNA sequence at specific locus)
- migration (gain/loss of individuals making up gene pool)
- genetic drift (random fluctuations in gene frequencies) - 7assumptions of H-W law - population at equilibrium - large population size - no mutations - no migration - random reproductionwhy do deleterious alleles remain?
- late onset -> no effect on reproductive success
- high mutation rates -> genes which accumulate mutations at very high levels
- heterozygote advantage -> recessives are fatal but heterozygote has advantage over dominating homozygote - 8applications of population genetics - deviations from H-W -> indicate whether selection, mutation, migration occurring - DNA fingerprinting -> forensic science, paternity issues - genetic counselling -> estimate risk of transmission of disease to offspring- -
--- - 9Darwin - 1809-1882 - evolution criticised by creationists theory = hypothesis that has been verified/supported by facts (empirical evidence) - evolution is predictiveevolution = process of temporal change by which organisms come to differ from their ancestors with respect to any heritable traits
heritable = characteristic passed on from parent to offspring (traits are genetic & expressed in phenotype) - 10evolutionary change = 2 processes - transformation - splitting/branching anagenesis vs cladogenesis anagenesis = between generation change within single lineage (transformation) cladogenesis = division of a single lineage due to genetic divergence (splitting/branching)Anagenesis:
- evolution within lineage
- usually ancestral form disappears as entire species changes to new form
Cladogenesis:
- division of lineage due to genetic divergence
- subsequent changes (anagenesis) occurring in both/either sister lineage results in further divergence
division events = 2 new lineages OR one new lineage in addition to original
branched evolutionary histories = phylogenies - 11Microevolution = evolution at population level - species comprised of one or more population of many individuals - refers to within-species evolution -> infraspecific evolution - includes processes leading to speciation speciation occurs mainly through cladogenesisMacroevolution = evolution at species level & above (genus & family)
- interspecific evolution
involves:
- speciation events (splitting of lineages)
- subsequent divergence of species & higher taxa - 12character = any measurable item on an organism character state = alternative variants of charactertraits:
genotype = genotypic traits -> info stored in DNA of one individual
- phenotypic traits - 13DNA - in nucleus of each cell, on linear chromosome - in mito -> within cytoplasm - on circular chromosome - mito derived from endosymbiotic bacterium to from first euk cellJunk DNA
- sequence within introns of genes
- 80% does have function (expression regulation)
- true junk DNA assumed not to be subject to natural selection - 14nucleotide base sequences making up same gene may differ between individuals SNP = single nucleotide polymorphism (A/G difference) - nucleotide sequences differ in this manner at same site = different alleles of same gene locus = polymorphicgenotype = information sotred in all genes of one individual
phenotype = observable/measurable characteristics of an organism
- comprises all observable non-gene traits: external appearance, internal structures
- easy to observe
* variation in most phenotypic traits is at least partly genetically determined (heritable) - 15variation due to non-gene factors: - wear/trauma (damage) - disease - somatic mutations (soma = cells other than those who produce sperm & egg)heritable intra-population variation = essential for evolutionary change to occur
- must be sufficient amount of variation - 16Potato blight & Irish famine - lack of sufficient heritable trait variation due to cloning by farmers - water mould capable of rapid evolution - caused >1M deaths from starvationBlack Sigatoka disease in bananas:
problem worsened by:
- cultivation only few banana varieties, most of which are vulnerable
- varieties propagated asexually
- genetic variability of fungus - 17continuous traits = intermediates exist between 2 extremes - many gene loci involved (polygenic) - many alleles involved - harder to discern contribution of individual allele to phenotype - environment plays rolediscontinuous/discrete traits = no intermediates exist
- one of few gene loci involved
- usually only few alleles involved at such loci
- allele-for-trait effect clearly evident in phenotype
- environment has little effect - 18quantifying population variation: sampling = representative (unbiased) samples needed - take random sample - take large samplemeasurement of variation
- measurement = continuous -e.g. height
- scoring = discontinuous - e.g. polymorphism - 19Microevolution = how populations change over time in relative abundance of genotypes/phenotypeseither: processes that create genetic variation:
- mutation
- recombination
OR: process that remove genetic variation:
- genetic drift
- selection - 20genetic drift - random changes in allele frequencies within a population - reproduction is random - bottleneck (random reduction in no. of breeders) - loss of heterozygosityinbreeding = mating between relatives
- loss of heterozygosity
- causes conditions such as cryptorchidism (testes fail to descend) - 21Linkage disequilibrium = non-random association of alleles between loci 2 loci (A&B) = 2 alleles per locus therefore: AB, Ab, aB, ab linkage equilibrium = random association between loci?
- 22adaptive evolution change = when subject to natural selection, heritable phenotypic traits, correlated with fitness, typically alter in frequency within a population - direct change in response to change in environmental variablesevolutionary change = proportion of variant within population as whole increases in next generation
- 23artificial selection mimics natural - with artificial, breeder/experimenter decides which trait has highest fitness - results in adaptive evolution under artificial conditions - doesn't correspond to differential fitness under natural conditions - evolutionary change in response to environmental changeselectively neutral genes =genes whose frequencies are not influenced by natural selection
- frequencies of variants of natural traits can change over time within a population (undirected evolutionary change)
- neutral genes know to hitch-hike with genes that have adaptive function - 24selectively neutral phenotypic traits: - marking on clover leaves - markings on cone shells (seem to have no adaptive significance)Pleiotropy = multiply phenotypic effects of an individual gene
- effect on one phenotypic trait can be adaptive
- effect on other trait may be neutral, deleterious or positive
example: reductive vs ageing
males: testosteorne -> increased fitness in early life, prostate disorders later on - 25three types of selection: 1. negative 2. positive 3. balancingfitness = adaptive traits correlated with fitness, neutral phenotypic traits are not
selection = adaptive traits subject to selection within population, neutral gene are not
random change = adaptive & neutral traits can be subject to random changes in frequency within a population (due to genetic drift) - 26Phenotypic plasticity - genotype produces different phenotype according to organism's particular environment - only some phenotypic expressions of genotype are beneficial, most = suboptimaleach different phenotypic expression of genotype is beneficial
- e.g. skin darkening & lighting in response to sun exposure in humans
- pale skin: admits UV light for vitamin D production
dark skin: protects against too much UV
*some forms of adaptive plasticity are fixed in the individual - 27constraints upon evolutionary change - insufficient genetic variations exist in gene pool - no selection pressure operating - sufficient selection pressure, but constraint upon changing the designdesign constraints:
- genome of organism is coevolved, integrated assemblage of genes
- cannot be simply unravelled to produce radically new structure to solve environmental challenge
e.g. insect small size for efficient respiratory system
limit on body size = phylogenetic constraint
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- Biology Karteikarten am Population genetics - genetics , erstellt von michaela minkin am 14/05/2018.
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