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Genetic omelettes and the death of evolution of new species

Genetic omelettes and the death of evolution of new species. Maladaptation. Genetic consequences of inbreeding. 1) decrease in heterozygosity, no change in P (allelic diversity) (the more related the individuals, the faster the loss of H)

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Genetic omelettes and the death of evolution of new species

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  1. Genetic omelettes and the death of evolution of new species Maladaptation

  2. Genetic consequences of inbreeding 1) decrease in heterozygosity, no change in P (allelic diversity) (the more related the individuals, the faster the loss of H) 2) increases the probability of a zygote receiving identical alleles (homologous alleles), which will result in increased expression of recessive alleles. 43e-1

  3. Genetic consequences of inbreeding 3) increased phenotypic expression of deleterious alleles (strongly selected against) - often results in decreased size, reproduction, vigor, etc., which decrease fitness (i.e., inbreeding depression) -e.g., sickle cell anemia, cystic fibrosis, Tay-Sachs, hemophilia, phenylketonuria, etc. - Genetic load 4) increase in phenotypic variability resulting from a deviation from the mean genotypes in non-inbred individuals 43e-1

  4. Inbreeding coefficient Sewall Wright (1923) F = the probability that an individual will receive two equal alleles, at a specific locus, that are from the same ancestor. Autozygous = alleles that are identical by descent allozygous = not identical by descent F = probability that an individual will be autozygous at a given locus 1 - F = probability that an individual will be allozygous at a given locus 43e-2

  5. Calculate Junior’s inbreeding coefficients from this pedigree: Mom Dad AB CD C = .5 C = .5 AC Sis C = .5 Junior (or could be DD from Dad) CC Probability of C from Dad to Sis to Junior = .25 Probability of C from Dad (through Sis) to Junior = .50 Probability of Jr. inheriting CC from Dad = .25 X .50 = .125 Probability of Junior inheriting DD from Dad = .125 F = .125 + .125 = .25 = probability of Jr. being autozygous 31

  6. Calculation of F from sib mating parents AB CD A = .5 A = .5 sibs A = .25 A = .5 A = .5 What is F? -- -- Identical by descent Probability AA .25 x .25 = .0625 BB “ CC “ DD “ F = 4 x .0625 = .25 31e

  7. Calculating F in a non-inbred population C1C2 1 grandparent Non-inbred gene pool, F1 generation Inbred F2 C1C1 Non-inbred C1C2 autozygous allozygous Ne = number of breeding individuals 2 Ne = number of alleles in the gene pool Probability of drawing any first allele, say C1, = 100% Probability of drawing the same allele again = F = 1 2Ne 51-1

  8. Calculating F in a Non-inbred population, cont. Fnoninbred = 1 which is approximately 0 in an ideal pop. 2Ne Probability of drawing autozygous alleles = 1 = Ft 2Ne = p (C1) * p (C1) Probability of drawing allozygous alleles = 1 - 1 2Ne 51-2

  9. Relationship of F and H When H0 = 1 (i.e., no initial inbreeding), F = 0 so: Ft = 1 - Ht I.e., inbreeding and heterozygosity are inversely related. * * • Bottom line: all real-world populations tend to become • completely homozygous because of genetic drift • AND completely inbred 50

  10. Outbreeding depression due to regional adaptation Hunting results in extinction of Czech ibex Translocation of ibex from nearby Austria IbexTurkey X IbexCzech-Austria (fall rut) (spring rut) fertile hybrids that rutted in fall, gave birth in February (coldest month) extinction of population 51A

  11. OUTBREEDING: • Outbreeding = crossing of unrelated invididuals. • Hybrid vigor = Heterosis = increased fitness due to outbreeding. • which is why: • stray dogs look like mutts and not like AKC poodles • you see wild-type fruitflies on your rotting apple

  12. Consequences of inbreeding:Results of an early experiment on inbreeding in rats (Ritzema-Bos 1894) 55-top

  13. Juvenile mortality increases after 1 generation % juvenile mortality * * F=0.25; e.g., wild-caught male x daughter 55-bottom

  14. Ralls and Ballou: Examination of zoo pedigrees Infant mortality in 41 of 44 species was higher in the inbred animals (7 orders, 21 families and 36 genera)

  15. Summary Inbreeding: 1) Inbreeding depression a) decrease in fertile matings b) decrease in litter size c) increase in juvenile mortality 2) Inbreeding does not always result in inbreeding depression a) selfing plants b) Tamil tribes of India c) European Bison 3) Positive aspects a) derive offspring without deleterious alleles b) fix alleles (domestic stock) 43f3

  16. Usual outcome of inbreeding: THE F VORTEX increased F declining Ne (decreased H) (increased genetic drift) Inbreeding depression decreased N decreased r (reproductive rate) Extinction 75

  17. How much inbreeding is tolerable? If F = 1 and Ne = 4 M F 2Ne M + F Then F= 1 F =1 + 1 2 4 M F8F 8M M + F Important! 60a

  18. How much inbreeding is tolerable? F =1 + 1 8F 8M Research on domestic farm animals: natural selection for performance can balance inbreeding depression if the ΔF is no more than 1% per generation. So, F = 0.01 is a tolerable level of inbreeding 60a

  19. How much inbreeding is tolerable? F =1 + 1 8F 8M If F = 0.01 is a tolerable level of inbreeding, then .01 = 1 + 1 so F = 25 and M = 25 8F 8M or, Ne = 50 Magic number! 60a

  20. What happens to the ‘magic number’ when sex ratios are unequal? 1 1 8Nm 8Nf F = + Conclusion: 15 = smallest number of effective individuals of one sex Number of females .005 tolerance 25 .01 tolerance 15 Number of males 60

  21. Population bottlenecks bottleneck Population size Time H = 1 - 1 = expected proportion of Ho retained after a 2Ne 1-generationbottleneck Ht = Ho 1 - 1 t = proportion of Ho retained t generations after 2Ne a bottleneck if Ne at t=0 = 4, then Ht=1 = 1 - 1 = 7 i.e. 1/8 of original H 2 x 4 8 was lost in 1 generation 61A

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