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1 Random Genetic Drift as an Evolutionary force affecting Genetic Variation, V Strength of Random Genetic Drift is determined by (1/2N), the inverse of.

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Presentation on theme: "1 Random Genetic Drift as an Evolutionary force affecting Genetic Variation, V Strength of Random Genetic Drift is determined by (1/2N), the inverse of."— Presentation transcript:

1 1 Random Genetic Drift as an Evolutionary force affecting Genetic Variation, V Strength of Random Genetic Drift is determined by (1/2N), the inverse of the 2N numbers of breeding adults in a population. Average Genetic variance WITHIN populations DECREASES with RGD. This limits Natural Selection. Average Genetic Variance AMONG populations INCREASES with RGD. This contributes to Speciation.

2 2 Ecological Opportunities for Random Genetic Drift. 1. Continuous drift; populations that are always small in size, N. a. Endangered species, like California condor, Florida panther b. Insular species (small islands, fragmented habitats) c. Polygynous or Polyandrous mating systems - many individuals but few breeders, e.g., polygynous elephant seals. 2. Intermittent drift: large fluctuations in population size, N t, from one generation to the next. It is the generations with LOW N that cause the most drift. 3. Bottleneck effects when populations are reduced to near extinction but then expand to large numbers. e.g. Northern elephant seals, cheetahs. Like (2), but LOW N infrequent occurrence. 4. Founder effects at colonization (e.g., Human religious isolates) a. a small group of individuals becomes geographically isolated from the remainder of the species. b. a small group of individuals colonize a new site.

3 3 Range of Florida Panther in 1,500 A.D. Range of Florida Panther in 2,001 A.D.

4 4 Continuous RGD and the endangered Florida Panther Population Bottleneck: hunted to near-extinction 50 years ago. Now, there are about animals. Evolutionary Genetic Consequences – Little Genetic Variation in the species: – Lower variation than other panther species. – Lower variation today than in museum samples from – Reproductive problems symptomatic of Inbreeding  80% of males have low sperm counts  93% of males have abnormal sperm, highest of all large cats.  High frequency of “kinked tail,” a recessive trait  Congenital Heart defect - a hole in the heart which is the result of inbreeding.

5 5 RGD and the endangered Florida Panther, Genetic Remedy: How do you counter act RGD? Answer: Artificial Migration or gene flow: bring in genetically different panthers from a Texas subspecies and OUTBREED! When Outbred to the Texas subspecies the hybrid individuals are heterozygous, and the reproductive problems are absent in the hybrid offspring.

6 6 Bottleneck and Mating System cause RGD in the endangered Northern Elephant Seal Hunted for the oil in their blubber, by 1890 there were fewer than 20 animals left: Bottleneck. 2005, there are more than 30,000 animals, but they have a Harem Polygynous mating system. Currently, there is Little Genetic Variation in the population at any gene. Now, the population experience massive dies offs owing to Increased Susceptibility to Disease owing to homozygosity of genes in the immune system. E.g., 40% of yearlings seals die of a skin disease.

7 7 Male, Northern elephant seal, with harem Harem size: 20+ females When 1 male has 20 mates, then 19 males have 0 mates = Extreme variation in male reproductive fitness.

8 8 Each Offspring gets (1/2) of its genes from Mother and the other (1/2) of its genes from Father. The Number of Mothers, N females, determines HALF the Strength of Random Genetic Drift. The Number of Fathers, N males, determines HALF the Strength of Random Genetic Drift. How do we determine the N for RGD in species with separate males and females? Random Genetic Drift and Mating Systems

9 9 The Number of breeding mothers = N females. The Number of breeding fathers = N males. How do you COMBINE these to measure RGD? 1] Arithmetic Mean number of breeders: N Average = {N females + N males }/2 2] Harmonic Mean number of breeders: 1/H = (1/2)(1/N females + 1/N males ) Important: H is always less than N Average Random Genetic Drift and Mating Systems

10 10 The Number of breeding Mothers = N females. The Number of breeding Fathers = N males. How do you combine these to measure RGD? 1] Arithmetic Average number of breeders: N Average = {N females + N males }/2 2] Harmonic Mean number of breeders: 1/H = (1/2)(1/N females + 1/N males ) H is always less than N Average Random Genetic Drift and Mating Systems

11 11 Same number of adult males and females but different amounts of RGD because of differences in the Mating System Population of N males = N females = 400 Monogamy: one mate per male, H = 400 1/H = (1/2)(1/ /400) = (1/2)(2/400) Polyandry: four mates per female, H = 160 1/H = (1/2)(1/ /100) = (1/2)(5/400) Polygyny: 20 mates per male, H = 38 1/H = (1/2)(1/20 + 1/400) = (1/2)(21/400)

12 12 RGD owing to Bottleneck in Cheeta, Acinonyx jubatus “King Cheeta” phenotype With a black stripe : homozygous for a recessive allele causing the stripe.

13 13 RGD and the endangered Cheeta, Acinonyx jubatus Population Bottleneck: 100,000 in the early 1900's but near-extinction 10,000 to 12,000 years ago. Today there are fewer than 10,000 animals. Little Genetic Variation in the species: a. genetically identical and homozygous at histocompatibility genes, the most variable genes in other mammals. b. skin grafts between two cheetahs are accepted. c. Blood samples from 55 cheetahs from 2 widely separated populations were almost genetically identical. Only in highly inbred strains of laboratory mice has such genetic uniformity ever been observed! Reproductive problems symptomatic of Inbreeding e.g., very low sperm counts and high incidence of abnormal sperm.

14 14 The Effect of a Population Bottleneck is long lasting. Generation Number of Breeding Pairs ,000 31,000, ,000,000 Arithmetic Mean N 25,252,525 When population size varies: 1/H = (1/4)(1/N 1 + 1/N 2 + 1/N 3 +1/N 4 ) 1/H = H = 198 H <<< N average

15 15 Founder Effect causes RGD in the Lancaster, Pennsylvania Amish: a religious isolate population Founder Effect: Founded in in by fewer than 200 people. 2001, there are now 18,000 Amish in Lancaster County, PA. Tend to marry within the religion with few converts to the religion after Converts into the religion are like migrants into a population; they represent gene flow. (Pedigree data exist for nearly entire group.)

16 16 Inbreeding is defined as matings between genetically related individuals. It increases homozygosity and leads to a high incidence of recessive, genetic diseases.  Recessive genetic disease: homozygotes are the affected individuals. Lack of Allele Variation at Histocompatibility Genes: Human immune system genes function best when heterozygous. Increased homozygosity at these genes causes the population to be more susceptible to communicable diseases. Rubella Outbreak in in Amish in 6 states. Founder Effect and Subsequent Inbreeding in the Lancaster Amish

17 17 Founder Effect and Subsequent Inbreeding in the Lancaster Amish Inbreeding: high incidence of recessive, genetic diseases: 1. Ellis-van Creveld syndrome (dwarfism, heart trouble, extra digits). Only 50 cases observed in the entire world in past 100 years: 43 of these cases are among Amish of Lancaster. All 43 cases trace back to lineage of Mr. and Mrs. Samuel King, who joined group in Pyruvate kinase deficiency: a blood disorder; all cases trace back to Strong Jacob Yoder, who joined group in Hemophilia, sex-linked bleeding disorder in which it takes a long time for the blood to clot; all cases trace back to 2 sisters who converted in 1820.

18 18 Inbreeding has occurred frequently in royal families. Royal incest was common in ruling families of Egypt, Incan Mexico, Hawaii, and Europe. Connections between royalty and inbreeding 1) Kings are often polygynous. 2) Inbreeding preserves royal blood line. 3) Inheritance of material possessions. 4) Inheritance of position both matrilineal and patrilineal. 5) Prescribed in origin legends. E.g., Incan kings married sisters because the Sun married his sister, the Moon

19 19 Inbreeding is the cause of the large number of cases of hemophilia, X-linked recessive blood disorder, in royal families of Europe. Queen Victoria of England was the original carrier for the hemophilia allele, 20 members of her family inherited the disease over several generations. The disease spread through Europe, as her children and grandchildren married into different royal houses to create political alliances, e.g. Russia and Spain. However, the birth and subsequent death of male heirs affected by hemophilia lead to political unrest. In Russia, to the firing-squad execution of the royal family. In Spain, generated anti-British sentiment for ‘polluting’ the Spanish blood line and contributed to origins of World War I.

20 20 Inbreeding in the flour beetle, Tribolium castaneum female male X Eight Generations of Brother-Sister Mating

21 21 100% Families Survive Highly Inbred Only 25% Families Left 75% have gone extinct!

22 22 At the start, the average was 150 offspring/family with no inbreeding Highly Inbred 35 offspring/family

23 23 In flour beetle, experimental inbreeding by Brother- Experimental inbreeding in flour beetles by Brother-sister mating results in: 1)Loss of 75% of all family lineages in 8 generations. 1)A 77% Decrease in family size of the surviving lineages in 8 generations.

24 24 How can Obligate Inbreeding species exist? Dandelion, Taraxacum officinalis Clarkia xantiana parviflora

25 25 Interaction of Mutation, Selection, and RGD Only naive theories about evolution assume that Natural Selection leads a population to achieve an optimal level of adaptation. Because Mutation introduces harmful alleles into populations and because they can become fixed by Random Genetic Drift, Natural Selection simply cannot produce the best of all possible worlds.

26 26 Fundamental Differences about the Evolutionary Process S. WrightR. A. Fisher Important Forces: Selection, Random Drift, Mutation Population Size: Small Populations N ~ ,000 Important Forces: Selection and Mutation Population Size: Large Populations N ~ 1,000,000,000,000

27 27 Metapopulation Large, Panmictic Populations R. A. Fisher’s World S. Wright’s World

28 28 s, selective effect of an allele Natural Selection ~ s Random Genetic Drift Population Size Threshold N e = 500 Genes with s in this range are effectively neutral have a history determined largely by RGD Alleles with s in this range have a history determined largely by natural selection +/-

29 29 s, selective effect of an allele Natural Selection Random Genetic Drift Population Size Threshold N e > >1,000,000 Alleles with s in this range have an evolutionary history determined largely by natural selection +/- R. A. Fisher’s View

30 30 s, selective effect of an allele Natural Selection Random Genetic Drift Population Size Threshold N e = 50 Genes with s in this range are effectively neutral And have an evolutionary history determined largely by RGD +/- S. Wright’s View


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