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Information for the Term Paper • Goal – at least 10 pages long, will need at least 10 references • Use the primary literature for your references – reports of original research and ideas • Primary literature is usually published as research or review papers in scholarly journals, or in scientific books • Example journals – Science, Nature, Conservation Biology, Environmental Management, Ecology, Applied Ecology, Annual Review of Ecology and Systematics, etc. • Do not use secondary literature – general works based on primary literature – popular books and magazines – Discover, Natural History, National Geographic, Scientific American, Smithsonian, etc. Use of Webpages as Sources • You may need to use webpages for most up-to-date material but: • No more than 3 webpages for paper; or if more than 10 references – no more than 30% from webpages • Main value of webpages is they may provide very current information – but reader beware: • Webpages are not archived • Webpages are not peer-reviewed :VirtualSalt Evaluating Internet Research Sources Robert Harris Version Date: December 27, 2013 Previous Version: June 15, 2007 "The central work of life is interpretation." --Proverb http://www.virtualsalt.com/evalu8it.htm Genetic Diversity What is Genetic Diversity? • Genetic diversity is the diversity that occurs in genes of individuals and species • Genetic diversity originates with mutations • It is probably best to think of genetic diversity as occurring at 4 different levels: 1. among species 2. among populations 3. within populations 4. within individuals Measuring Genetic Diversity • Among and within populations we often measure genetic diversity by measuring polymorphism - the percentage of genes that are polymorphic (have several alleles) within a population • Polymorphism is sometimes also considered to be a process - the maintenance of genetic diversity within a population • We can also measure genetic diversity by measuring heterozygosity - the percentage of genes at which the average individual is heterozygous Importance of Genetic Diversity • Beginning with Darwin, most evolutionary biologists have thought that rather small heritable changes provide most of the variation on which natural selection acts • These small changes are most obvious when using measurable characters such as size or yield - typically they result in continuous variation seen when graphing size as a bell curve Continuous variation: Selection for white spotting in Dutch Rabbits Cause of Continuous Variation • The genetic cause for these changes are genes with small phenotypic effect called multiple factors or polygenes – • Polygenes are where several genes interact to produce a quantitative phenotypic effect on a character Mutation is the source of genetic variation Common mutation – black color in grey squirrels Mutation Rates Genetic Polymorphism • Polymorphism is the maintenance of genetic variability within a population • Polymorphism has been widely observed in many species Polymorphism in Grackles Direct Observation of Genetic Variation • Originally done by protein gel electrophoresis – still commonly used • DNA hybridization was another early method • Now mostly done by directly sequencing DNA – either in small fragments or entire genomes Protein Gel Electrophoresis Hominid relationships – based on gel electrophoresis DNA Hybridization Hominid relationships – based on DNA hybridization DNA Sequencing Hominid relationships – based on mitochondrial DNA sequencing Extent of Polymorphism • Electrophoretic studies have generally found about 25% of all loci tested to be polymorphic - humans - 28%, drosophila - 43% • It is generally thought that only one-third of genetic differences are detected by electrophoresis – this is due to the presence of silent substitutions – for example: GGU, GGC, GGA, and GGG all code for Glycine • Thus it is possible that two-thirds to three-fourths of all loci are polymorphic in a species and that an average individual is heterozygous at about 25% of its loci DDT resistance in mosquitoes DDT Resistance Mechanisms • an increase in lipid content that lets fat-soluble DDT separate from other parts of the organism • presence of enzymes that break down DDT into relatively less toxic products • reduced toxic response of the nervous system to DDT • differences in permeability of insect cuticle to DDT absorption • behavioral response that limits contact with DDT Comparing Heterozygosity • When comparing heterozygosity of different species we usually look at total genetic diversity symbolized Ht • Ht is composed of two aspects of heterozygosity – • Hs - genetic diversity within the populations that compose the species • and Dst - genetic diversity due to variability among the populations • thus Ht = Hs + Dst Some heterozygosity What do Hs and Dst tell us? • Species with widely scattered populations that don’t exchange genes (like desert pup fish in ponds in Death Valley) tend to have very high Dst • Species with populations that do exchange a lot of genes (like pines) tend to have high Hs – Humans also have high Hs Fitness • evolutionary fitness is a measure of the number of offspring an individual produces Loss of Fitness • Another important aspect of polymorphism is that it tends to maintain fitness • populations of animals in zoos, which are typically low in genetic diversity, often have low fitness - low fertility and high mortality among offspring Fitness of Zoo Animals Reasons for Loss of Fitness 1. increased incidence of deleterious recessive homozygous individuals 2. lack of heterosis – heterosis (hybrid vigor) is the phenomenon where heterozygous individuals have higher fitness than do homozygotes - often heterozygotes are more resistant to disease 3. lack of evolutionary potential - with all homozygotes there is lack of variation and thus limited ability to respond to environmental changes Inbreeding Depression • Inbreeding depression is the loss of fitness resulting from the breeding of closely related individuals - it occurs due to the three reasons listed before Ngorongoro Crater Lions at Ngorongoro Crater Vipera berus - adder Glanville Fritillary Butterfly Outbreeding Depression • The loss of fitness that occurs when distantly related individuals breed – • This occurs because certain populations may have been selected for traits that are successful in their environment, so that introducing novel traits may reduce fitness for that environment Austrian Ibex – Capra ibex ibex Turkish Ibex – Capra ibex aegagrus Nubian Ibex – Capra ibex nubiana Optimum outbreeding in Japanese Quail Fitness • evolutionary fitness is a measure of the number of offspring an individual produces Maintenance of Polymorphism without natural selection • random mating tends to maintain polymorphism – due to the benefits of sexual reproduction – recombination, independent assortment, and crossing over Maintenance of Polymorphism • The effects of nonrandom mating are variable - species may either mate assortatively (like with like) or disassortatively (like with unlike) • assortative mating results in many homozygous individuals • disassortative with many polymorphic, heterozygous individuals Assortative Mating - Three spined stickleback Disassortative Mating – Nonbreeding Ruff Disassortative Mating - Breeding male ruff and variations on head pattern Maintenance of Polymorphism • environmental variance - the environment may affect development of different genotypes so that which genotype dominates changes with the environment if the environment varies or different habitats exist within the species range, then different genotypes will exist Backswimmers – winged or wingless forms Maintenance of Polymorphism With Natural Selection • with selection, we would expect the most fit genotype to come to dominate the population, but polymorphism may still occur: 1. selection acts to maintain stable polymorphism so that different genotypes are most fit under different situations 2. fixation of a particular genotype is counteracted by mutation 3. fixation of a particular genotype in one population is counteracted by gene flow from another population Polymorphism under selection – in the Grove Snail - Cepaea Clines • in many species, local populations have little variation, but the entire species exhibits much variation as local populations are adapted to different conditions - if these changes in genes change in response to certain environmental variables, we may see a cline - a gradual change along a geographic transect Clines with Body Size • Bergmann's rule - many animals get larger in size as the species range approaches the poles - it is related to ability to keep warm larger bodies maintain warmth better • Allen’s Rule – size of extremities decreases towards the poles – heat is lost through things like large ears, long limbs Bergman’s Rule in same aged White-tailed Deer Allen’s Rule in Foxes Arctic Fox Desert (Kit) Fox Allen’s Rule in Hares Cline in Cyanide Production in White Clover Cline in cyanide production by white clover Greater Racquet-tailed Drongo cline in crest size