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

Ecological Opportunities for Random Genetic Drift. 1. Continuous drift; populations that are always small in size, N.

a. Endangered species, like California condor, Florida pantherb. 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, Nt, 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

Range of Florida Panther in 1,500 A.D.

Range of Florida Panther in 2,001 A.D.

4

Continuous RGD and the endangered Florida Panther

• Population Bottleneck: hunted to near-extinction 50 years ago.• Now, there are about 50-80 animals. • Evolutionary Genetic Consequences

– Little Genetic Variation in the species:– Lower variation than other panther species. – Lower variation today than in museum samples from 1900.– 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.

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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.

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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.

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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.

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Each Offspring gets (1/2) of its genes from Mother and the other (1/2) of its genes from Father.

The Number of Mothers, Nfemales, determines HALF the Strength of Random Genetic Drift.The Number of Fathers, Nmales, 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

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The Number of breeding mothers = Nfemales. The Number of breeding fathers = Nmales.

How do you COMBINE these to measure RGD?1] Arithmetic Mean number of breeders:

NAverage = {Nfemales + Nmales}/2

2] Harmonic Mean number of breeders:1/H = (1/2)(1/Nfemales + 1/Nmales)

Important: H is always less than NAverage

Random Genetic Drift and Mating Systems

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The Number of breeding Mothers = Nfemales. The Number of breeding Fathers = Nmales.

How do you combine these to measure RGD?1] Arithmetic Average number of breeders:

NAverage = {Nfemales + Nmales}/2

2] Harmonic Mean number of breeders:1/H = (1/2)(1/Nfemales + 1/Nmales)

H is always less than NAverage

Random Genetic Drift and Mating Systems

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Same number of adult males and females but different amounts of RGD because of differences in

the Mating System Population of Nmales = Nfemales = 400

Monogamy: one mate per male, H = 400 1/H = (1/2)(1/400 + 1/400) = (1/2)(2/400)

Polyandry: four mates per female, H = 160 1/H = (1/2)(1/400 + 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)

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RGD owing to Bottleneck in

Cheeta, Acinonyx jubatus

“King Cheeta” phenotype With a black stripe :

homozygous for a recessive allele causing the stripe.

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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.

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The Effect of a Population Bottleneck is long lasting.

GenerationNumber of

Breeding Pairs

1 100

2 10,000

3 1,000,000

4 100,000,000

ArithmeticMean N

25,252,525

When population size varies:

1/H = (1/4)(1/N1 + 1/N2 + 1/N3 +1/N4)

1/H = 0.005050

H = 198

H <<< Naverage

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Founder Effect causes RGD in the Lancaster, Pennsylvania

Amish: a religious isolate population

• Founder Effect: Founded in in 1720-1770 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 1800. Converts into the religion are like migrants into a population; they represent gene flow.

• (Pedigree data exist for nearly entire group.)

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• 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 1991-92 in Amish in 6 states.

Founder Effect and Subsequent Inbreeding in the Lancaster Amish

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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 1744.

2. Pyruvate kinase deficiency: a blood disorder; all cases trace back to Strong Jacob Yoder, who joined group in 1742.

3. 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.

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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

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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 affectedby 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.

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Inbreeding in the flour beetle, Tribolium castaneum

femalemale XEight Generations ofBrother-Sister Mating

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Generation of Inbreeding

Surv

ivin

g Fa

mili

es

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8 9 10

C2-1

C2-2

C2-3

C2-4

100% Families Survive

Highly InbredOnly 25% Families Left75% have gone extinct!

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Generation of Inbreeding

Ln(

Fam

ily S

ize)

2.5

3

3.5

4

4.5

5

5.5

0 1 2 3 4 5 6 7 8 9 10

C2 Treatment

At the start, the average was 150 offspring/family with no inbreeding

Highly Inbred35 offspring/family

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In flour beetle, experimental inbreeding by Brother-

Experimental inbreeding in flour beetles byBrother-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.

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How can Obligate Inbreeding species exist?Dandelion, Taraxacum officinalis

Clarkia xantiana parviflora

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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 intopopulations and because they can become fixed

by Random Genetic Drift, Natural Selection simply cannot produce the

best of all possible worlds.

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Fundamental Differences about the Evolutionary Process

S. Wright R. A. Fisher

Important Forces:Selection, Random Drift,

Mutation

Population Size:Small Populations

N ~ 500 - 10,000

Important Forces:Selection and Mutation

Population Size:Large Populations

N ~ 1,000,000,000,000

27MetapopulationLarge, Panmictic Populations

R. A. Fisher’s World S. Wright’s World

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s, selective effectof an allele

0.1000

0.0100

0.0010

0.0001

Natural Selection ~ s

Random Genetic Drift0.00001

Population SizeThresholdNe = 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

+/-

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s, selective effectof an allele

0.1000

0.0100

0.0010

0.0001

Natural Selection

Random Genetic Drift

0.00001Population Size

Threshold Ne > >1,000,000

Alleles with s in this range have an evolutionary history

determined largely by natural selection

+/-

R. A. Fisher’s View

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s, selective effectof an allele

0.1000

0.0100

0.0010

0.0001

Natural Selection

Random Genetic Drift0.00001

Population SizeThreshold

Ne = 50

Genes with s in this range are effectively neutral

And have an evolutionary history determined

largely by RGD

+/-

S. Wright’s View