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Env 200 march 7 lec 13.docx

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School of Environment
Karen Ing

Env 200 march 7 lecture 13  Issues with small populations o Fitness and genetic load  Each of us carries 5-8 lethal alleles which if homozygous, would likely cause death.  The more lethal alleles found in a population, the greater its genetic load (the difference between the theoretical fittest genotype of a population and the avg fitness of that population)  Inbreeding among individuals carrying a genetic load similar deleterious alleles mating- increasing homozygosity of deleterious alleles  As homozyhosity rises through inbreeding, a positive feedback loop known as inbreeding depression sets in-characterized by reduced survival of offspring, low birth rates, and infertility among other things. o Ex: hemophilia in European royal families  Queen Victoria arranges marriages for her children and grandchildren with royal families of Europe (to strengthen political ties)  In spain and Russia, the plan backfires leading to political unrest as the royal children are discovered to be hemophiliacs-anti-British sentiments are fueled as the blood of Britain is considered “tainted”  Interestingly this particular mutation seems to have started with Victoria as hemophilia was unknown in her ancestors  In small populations, gene frequencies can change randomly in a process known as GENETIC DRIFT o Reach into a pile of pennies and pull out six. If there were 5 heads and one tail or four heads and two tails tou would not be particularly surprised o However if u pulled out 600 pennies we would expect the results to be closer to 300 heads and 300 tails (ex: we don’t expect 400 heads and 200 tails) o In a small sample chance can cause a departure from the expected result  In the next generation... o This process of random fluctuation continues generation after genetion because the population has no genetic memeory of its state o Each generation is an independen event o However as an allele frequency decreases it could become less likely to be samepled and a positive feedback process sets in where the allele continues to spiral down in frequency (assume a population of 100)  50 heads/50 tails (pull out of 10, 6 heads and 4 tails)  Next generation 60 heads and 40 tails  Next generation same thing  Next generation 70 heads and 30 tails  Next generation 80 heads and 20 tails o It is possible that an allele could disappear completely simply as the result of random chance  Small populations subject to drift can arise from founder effect-particularly of concern when we try to re- constitute a pop form just a few individuals (or when only a few individuals survive some severe impact) o Imagine you had 9 others are the only survivors of the human race o Your group cannot possibility contain the full genetic diversity of all humans on the planet o Nevertheless (assuming the group has both sexes) u cud form a breeding population o After many generation there might be millions of ppl o But this second human race would be substantially genetically diff from our current human race would b substantially genetically diff from our current human race reflecting the genetics of the founding individuals  Founder effect can be important when a small group of individuals leaves to found a new pop in a new env but it can also be important when a catastrophic event reduces a pop to a few survivors (a subset of original genetic diversity) the event is termed a genetic bottleneck o A genetic bottleneck  Original population composed of red and blue genetic members  Bottleneck event in which the population is greatly reduced  Only a few red individuals survive to pass their reduced # of genes to the new red population  Examples of species that have passed through genetic bottlenecks o Lions of ngorongoro crater, Tanzania o In 1962 a plague of biting flies killed almost all the lions in the park leaving 9 females and one male o This population is geographically restricted ot the crater which cuts off emigration/immigration o While the pop has rebuilt to approximately 125 individuals their allelic diversity is diff and much lower than that of lion pop in other locations  The elephant seal was hunted almost ot extinction in the 1800s o By the 1890s only about 20 survived o Elephant seals breed in harems with a single male mating with a group of females so one male may have fathered all the offspring at the extreme bottleneck point o The pop today has expanded to about 30,000 but biochemical analysis shows that all elephant seals are virtually genetically identical  EVOLUTION o Change overtime o Characteristics and organism o Pop evolve (not individuals) o Natural selection  Mechanisms of evolution o Sexual selection o Genetic drift  Founder effect  Biodiversity o Biodiversity-number of earth’s organisms-richness of biological variation  Possible partly b/c the earth has many diff combinations of env con
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