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Nile Perch from Lake Victoria Genetic Diversity 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 all individuals will be susceptible to the same problems 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 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 Reductions in Polymorphism • Gene Flow - the movement of alleles from one population to another tends to maintain genetic similarity among populations African Wild Dog Rates of Gene Flow – Ne (effective population size) = 120 Minimum Viable Population • The smallest population for a species which can be expected to survive for a long time • Many factors effect MVP – the study of those factors is often called Population Viability Analysis – or Population Vulnerability Analysis – or PVA Factors that make populations vulnerable to extinction • • • • • Environmental fluctuations Catastrophes Demographic uncertainties Genetic problems Habitat fragmentation Environmental Fluctuations Kirtland’s Warbler Cheetah Habitat Fragmentation • Fragmentation is the transformation of large expanse of habitat into a number of smaller patches of smaller total area isolated from each other by a matrix of habitat unlike the original Habitat Fragmentation Habitat fragmentation occurs due to: • Natural climatic shifts • Human caused habitat loss: logging, agriculture, urbanization, dams, road construction, etc. • Overexploitation of species • Species introduction • Secondary effects due to extinctions Domesday Book – 1085-86 Selection from the Domesday Book Factors that make populations vulnerable to extinction • • • • • Environmental fluctuations Catastrophes Demographic uncertainties Genetic problems Habitat fragmentation Heath Hen – Extinction Vortex Minimum Viable Population Size • Another definition - often defined as 95% probability of 100 year survival, but can also plan for longer survival (500 or 1000 years) • MVP is usually determined by modeling Forces which may cause extinction 1) deterministic - something essential is removed (habitat loss) or something lethal is added (pollutant, disease, introduced species) - presumably we can act to minimize these risks Forces which may cause extinction 2) stochastic (random) - environmental, catastrophic, demographic and genetic - this is what we need to worry about and what is hardest to prevent • environmental randomness effects resources and conditions and we can't do much about it • catastrophic randomness - floods, fires, hurricanes, volcanoes - can't really prevent but can spread individuals around to minimize the impact • demographic - just natural random variation in birth and death rates can lead to extinction • genetic - lack of genetic variability can lead to problems of inbreeding and poor response to diseases and environmental change Bighorn Sheep and MVP Bighorn Sheep and MVP Grizzly Bear and 50/500 Rule MVP – 50/500 Rule? Reductions in Polymorphism Reductions in population size can lead to losses of genetic polymorphism Two special cases of reductions in population size are: 1. A few individuals move to a new area and start a new population that is isolated from other populations – founder effect 2. We can also experience a population bottleneck where a formerly large population is drastically reduced in size Founder Effect – Galapagos Tortoise Founder effect – Amish and Polydactyly Population Bottleneck – Northern Elephant Seal