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5 - Population Persistance

3 bio 3115 conservation biology rarity risk and

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Unformatted text preview: Rarity, risk and genetic variability Species/populations with low levels of genetic variability have little ability to adapt to changing conditions… …and are therefore more prone (all other things being equal) to local extirpation E.g. the Furbish’s Lousewort, an endemic to Maine, is found only in four isolated populations M5.7 Bio 3115 Université d’Ottawa / University of Ottawa Intraspecific genetic variability Is composed of a between-population component (D) and a within-population component (H) For a species, the total genetic variability is given by the sum of these two components D12 H1 H2 D23 D13 H3 HT D H M5.8 Bio 3115 Université d’Ottawa / University of Ottawa M5 2009-2010.4 BIO 3115 Conservation Biology Ne is the number of individuals contributing genes to the next generation. (reproduction + survival of offspring) Ne may be much smaller than the actual population size due to skewed sex-ratios, mating behaviour, etc., which results in many reproductively mature adults not breeding. In general, genetic variability (heterozygosity) increases with Ne. Ne M5.9 Bio 3115 Université d’Ottawa / University of Ottawa Heterozygosity and population size in a New Zealand conifer, Halocarpus bidwillii In this species, heterozygosity (H) increases with effective population size H is the percentage of loci in a sample that have two or more alleles segregating in the population. 4 Heterozygosity (%) Heterozygosity Genetic variability and effective population size (Ne) 3 2 1 0 10 100 1000 10000 100000 1000000 Population Size M5.10 Bio 3115 Université d’Ottawa / University of Ottawa M5 2009-2010.5 BIO 3115 Conservation Biology Heterozygosity and population size in Redheaded woodpeckers Heterozygosity (%) 12 10 8 6 4 2 0 1 10 100 1000 10000 Population Size M5.11 Bio 3115 Université d’Ottawa / University of Ottawa Small samples from large gamete pool cause gene frequencies to fluctuate from generation to generation, leading eventually to fixation of one allele, extinction of the other. Consequently, genetic variation is lost over time (loci become homozygous) p1 g4 Frequency ( ) Consequences of small Ne I: genetic drift g1 p2 p4 Small Ne g2 g3 p3 Large Ne Generations M5.12 Bio 3115 Université d’Ottawa / University of Ottawa M5 2009-2010.6 BIO 3115 Conservation Biology Consequences of small Ne II: inbreeding In small populations, individuals are more likely to mate with genetically-related individuals. This results in an increase in homozygosity and a reduction in heterozygosity, i.e. a reduction in genetic variability. The inbreeding coefficient F is a measure of the degree of inbreeding in a population. Goes between 0 and 1. 1.0 Genotype frequency Outbred (F=0) Inbred (F=1...
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