3/9/13 Conservation genetics: The goal is to maintain as much genetic diversity as possible for long-term evolution -how to measure genetic diversity in populations? -% loci polymorphic (population measure) -% loci heterozygous (average individual measure) -what influences the maintenance of genetic diversity? -population size and effective population size -sex ratio -inbreeding depression versus purifying selection -two contrasting cases: -cheetah -northern elephant seal Why worry about genetic diversity? • Population’s ability to respond to future environmental change • Genetic resources – useful to humans (crops, biochemicals, source for transgenics) • Genetic diversity (individual or population) often is directly proportional to fitness – inbreeding depression if heterozygosity lost How to measure genetic diversity in pop’n? Per cent of loci that are polymorphic 1/3 to 1/2 of all enzyme loci are polymorphic a measure of what populations look like Per cent of loci that are heterozygous Average heterozygosity = to 15% = average % of an individual’s loci that are heterozygous or % of heterozygous genotypes averaged over all loci a measure of what individuals look like Vertebrates 648 species surveyed Invertebrates, 370 species surveyed Plants, 785 species surveyed [enzyme electrophoresis data; many sources, SJ O’Brien et al 1985, Freeman & Herron 2004] Group Loci exam’d Drosophila (43 species) 24 Mus musculus 46 Polymorphic loci (%) 43.1 20.5 Average heterozygosity (%) 14 8.8 3/9/13 Sources of genetic variation - mutation - immigration Processes that cause Losses – genetic drift = random loss over time – selection = deterministic [some selection increases diversity such as negative frequency-dependent selection; most decreases diversity] Loss of variation due to drift • Population heterozygosity ‒ Measures variation in genetic makeup (what average individual looks like) ‒ Loss over time can be estimated: Heterozygosity is inversely related to population size ⎞ ⎛ H t +1 = ⎜1 − ⎟ H t ⎝ Ne ⎠ Heterozygosity in the future “effective” population size (worry about what this means later) Effective Population Size (Ne) = a measure of how many individuals are contributing their genes to the next generation Ne few territories, only ~10% males mate >male genetic contributions come from few males >inbreeding [the females MATE RANDOMLY with these few males] àIt is hypothesized that there are zero lethal equivalents in elephant seal population because the mating system allowed natural selection to purify the gene pool by purging it of alleles that are good when in heterozygotes, but lethal when homozygous Is inbreeding always bad? No Some species regularly inbreed à wolves à lots of plants & some animals 3/9/13 Purging the deleterious alleles (ok when heterozygous, lethal when homozygous) through inbreeding Speke’s Gazelle à Had severe inbreeding depression (low birth weight, low survival) à Produced as many young as possible (AA – good, Aa – medium, aa – bad, some died) à Bred the good ones à Greatly reduced deleterious allels – inbreeding no longer a problem Is there hope for cheetahs? Maybe We hope yes