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Evolutionary Biology
Genetic drift and inbreeding
| Question | Answer |
|---|---|
| Genetic drift | random fluctuations in allele frequencies occur as a result of ‘sampling error’ between generations in finite populations null hypothesis for evolutionary change |
| Vs natural selection | not random – favours mutations that give an adaptive advantage – adaptive evolution |
| Non-adaptive evolution | the replacement of old alleles by new alleles (and sometimes the trait they confer) |
| Genetic drift as sampling error | Sampling alleles in each generation produces random changes in allele frequencies Sampling error alone can cause frequency of alleles to go up or down Genetic drift is most important in small populations |
| Random variance (V) in allele frequency between one generation and the next | P = allele frequency N = number of gene copies in population (2 x number of individuals for diploids) V=p(1-p)/2N |
| Random variance (V) in allele frequency between one generation and the next | As the population gets smaller, the variance gets higher …. i.e. allele frequencies are less similar from one generation to the next |
| Examples | P = 0.5 population size = 10 V=0.5(1-0.5)/20=0.25/20=0.0125 P = 0.5 population size = 1000 V=0.5(1-0.5)/2000=0.25/2000=0.000125 |
| Random genetic drift in populations of different sizes | 1=fixation 0=extinction Each line is the same locus in a different population If left long enough, all populations will reach fixation – allele frequency either 0 or 1 |
| Loss of heterozygosity (H) | As allele frequency drifts towards fixation or loss, the frequency of heterozygotes decreases Heterozygosity will fall over time as alleles move towards being fixed or lost |
| Using the hardy-weinberg equation | p2+ 2pq + q2 = 1 Freq of H = 2pq ... where p = freq of allele A1, q = freq of allele A2 Frequency of H is highest when p = 0.5 ( 2 x 0.5 x 0.5 = 0.50) As freq of A1 moves towards 0 or 1 the freq of H falls ( e.g. 2 x 0.9 x 0.1 = 0.18 ) |
| Heterozyogosity of populations | over all loci - Heterozygosity falls more rapidly in small populations than in large populations - alleles are fixed or lost more rapidly |
| Predicting the speed of decline in heterozygosity (H) | Averaged across populations, the frequency of heterozygotes (H) obeys the relationship |
| Predicting the speed of decline in heterozygosity (H) | Where Hg+1 is heterozygosity in the next generation N = number of individuals in the population (2N = number of gene copies) Hg is heterozygosity in the present generation |
| Predicting the speed of decline in heterozygosity (H) | The value of (1-1/2N) is always between 0.5 (when N = 1) and 1 (when N = infinite) |
| Predicting the speed of decline in heterozygosity (H) | So expected H in the next generation is always less than in current generation • If N is large the decrease in H is small - if N is small the decrease in H is large! • What will happen to all populations in the end? |
| Population bottlenecks and drift | Big decrease in population size – a specific case of drift Usually results in loss of genetic variation |
| Bottlenecks and founder effects | If a new population is formed by a small number of colonists, the genetic drift which ensues is known as the FOUNDER EFFECT Founders will suffer loss of genetic variation – rare alleles are likely to be lost |
| Genetic drift in real populations | • Genetic drift is the predominant force at the genetic and phenotypic level • Genetic drift, unlike natural selection, acts on genetic variation in a predictable manner, in relation to past and present population size |
| Genetic drift in real populations | • So if we can measure variation at genes not under natural selection, we can compare patterns of DNA variation from current populations to reconstruct their population history |
| Genetic drift in real populations: example | 107 experimental populations of D. melanogaster heterozygous for eye colour (A1 A2). In all populations the starting frequency of Allele A2 was 0.5 |
| Genetic drift in real populations: example | Selected 8 males and 8 females randomly from each population in each generation to start the next generation of that population |
| Genetic drift in real populations: example | By generation 19 30 populations lost allele A2 29 had fixed allele A2 |
| Genetic drift enables us to reconstruct population history: seychelles warbler | Endangered when discovered. Was it ever widespread and abundant? Used simulations to model genetic drift over time and reconstruct population history Seychelles warblers existed in 10,000’s across the region a few hundred years ago |
| Effective population size | Number of individuals in a population = CENSUS SIZE But, genetic drift and loss of heterozygosity will be greater than expected… because not all individuals contribute genetically to the next generation! |
| Effective population size | The population is effectively smaller than it really is Ne = effective population size ‘The size of an ideal theoretical population that would lose heterozygosity at the same rate as the actual population’. |
| Factors influencing effective population size | 1) Variation in the number of progeny If some individuals have more offspring than others, Ne will be reduced |
| Factors influencing effective population size | 2) Overlapping generations • Individuals mate over multiple generations • Offspring may mate with parents •They carry identical copies of the same genes, so the effective number of genes in the population is reduced |
| Factors influencing effective population size | 3) Unequal numbers of males and females |
| Factors influencing effective population size | 4) Fluctuations in population size All populations fluctuate in size – the rate of genetic drift is higher in small populations, so Ne is more strongly affected when population is small |
| Genetic drift and conservation | Small Ne leads to drift, leads to: 1) Loss of heterozygosity – and associated benefits a) Hz advantage, b) ‘hiding’ deleterious mutations 2) Loss of genetic diversity - loss of adaptive potential |
| Inbreeding | Mating between related individuals A degree of inbreeding is inevitable in small populations – members of the population share ancestors |
| Inbreeding | F = coefficient of inbreeding Probability that two alleles in an individual are identical by descent (descended from the same allele in a single ancestor) |
| Inbreeding coefficient increases more rapidly in small populations than in large populations | - inbreeding coefficient can be predicted as a function of the population size |
| Inbreeding depression - reduces fitness/survival | Reduces heterozygosity and thus any heterozygote advantage Exposes rare deleterious recessive alleles (as homozgotes) i.e. Ellis-van Creveld syndrome |
| Inbreeding depression | Loss of genetic diversity (Adaptive potential) inability to adapt to new challenges - e.g. new diseases Population level effect – not an individual effect |
| Inbreeding depression | Example: Florida panther, Puma concolor coryi • Tail kink, Cryptorchidism in males, deformed sperm, disease susceptibility |
| Inbreeding and genetic drift | Both have similar effects in terms of reducing genetic diversity Both increase risk of extinction in small populations |
| Consequences | Small pop - inbreeding/genetic drift - loss of genetic variability - reduction of fitness/adaptability - lower reproduction/increased mortality - smaller population |
Created by:
reub8n
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