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 incyanideproductionby whiteclover
Greater Racquet-tailed Drongocline 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
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