section 5 evolution in small populations

Section 5Evolution in Small Populations

The central problems are losses of genetic diversity in small populations and changes in thedistribution of this diversity among populations.

Losses of genetic diversity can result in reducedevolutionary flexibility and decline in fitness.

Changes in distribution of diversity can destroylocal adaptations and break up co-adapted genecomplexes (outbreeding depressionoutbreeding depression).

Both of these problems can lead to a poorer“match” of the organism to its environment, reducingindividual fitness and increasing the probabilityof population or species extinction.

Therefore, conservation biologists should beconcerned with maintaining, as much as possible,natural levels of genetic variation and naturalpatterns of genetic diversity so that evolutionaryand ecological processes can continue!

Four factors that can reduce genetic variationand are a function of population size:

Founder EffectFounder EffectPopulation BottleneckPopulation BottleneckGenetic DriftGenetic DriftInbreedingInbreeding

The severity of these factors is dependent uponthe Genetically Effective Population SizeGenetically Effective Population Size (NNee)and not the absolute number of individuals orcensus size (NNcc).

Population size is defined in terms of the equivalent size of a standardized population calledthe “Idealized PopulationIdealized Population”.

We begin by assuming an infinitely large, randomlymating base population, from which we take a sample of N adults to form the “Ideal PopulationIdeal Population”.

The idealized population is maintained as arandomly mating, closed population in succeedinggenerations in which alleles may be lost by chanceand allele frequencies may vary due to samplingvariation.

Conditions of the Idealized Population:Conditions of the Idealized Population:No migrationGenerations are distinct & do not overlapNumber of breeding individuals is the same in all

generationsAll individuals are potential breedersAll individuals are hermaphroditicUnion of gametes is random, including the possibility

of selfing.No selection at any stage of life-cycleNo mutationNumber of offspring per adult averages 1 and has

a variance of 1

The Effective Population SizeEffective Population Size (NeNe) is the size ofan idealized population that would lose geneticdiversity, or become inbred, at the same rate asthe actual population.

In practice, the effective size of real populationsis usually much smaller than the number of breedingindividuals because real populations deviate instructure from the idealized population by having unequal sex-ratios, high variances in family size,variable numbers of successive generations, and in having overlapping generations.

Sex Ratio effective population size:

Ne = (4Nm X Nf)/(Nm + Nf)

ExampleExample: A census population of 500 individuals would have an Ne of 500 (with respect to sexratio) if all individuals bred and there was a 1:1sex ratio such as:

Ne = (4 X 250 X 250)/(250 + 250) = 500

However, if in this population of 500 with 250females and 250 males, only 114 and 63 femalesand males, respectively, bred, then Ne would be:

Ne = (4 X 114 X 63)/(114 + 63) = 28,728/117 = 162.31

This relationship can produce some surprising results.

For example, one male breeding with four femalesresults in an Ne of 3.2, not much different thanone male breeding with 9 females (Ne = 3.6)

Effective population size is also strongly affectedby the distribution of progeny among females(family size) and is estimated as:

Ne = (4N)/(2 + 2)

Where 2 is the variance in family size amongfemales.

Higher variances result in smaller effectivepopulation sizes.

Effects of variance in number of progeny amongEffects of variance in number of progeny amongfemales on Nfemales on Nee.

22 NNcc NNee NNee/N/Ncc

0 100 200 2.001 100 133 1.332 100 100 1.005 100 57 0.5710 100 33 0.33

If males can mate with more than one female, thevariation in family size (VVkk) for males and femalesmust be incorporated since VVkk is likely to be different for males and females.

NNee = 8N = 8Ncc/(V/(Vkmkm + V + Vkfkf + 4) + 4)

Large population fluctuations also reduce Ne

because every time a population crashes to smallsize, it experiences a demographic bottleneck.

The harmonic mean of population size in eachgeneration provides an estimate of Ne as follows:

1/N1/Nee = 1/t(1/N = 1/t(1/N11 + 1/N + 1/N22 + 1/N + 1/N33 + . . . . . + 1/N + . . . . . + 1/Ntt))

Where t is the time in generations.

The effects of a single population crash on the effective population size can be seen asfollows.

Year Nc

1 8002 10003 254 5005 1000

5-year mean of Nc = (800+1000+25+500+1000)/5= 665665

Ne = 1/5(1/800+1/1000+1/25+1/500+1/1000)= 110.5110.5

Two types of Genetically Effective Population Size

Inbreeding Effective SizeInbreeding Effective Size (NNeiei)--measures therate of loss of heterozygous individuals from alocal population (a loss of variation withinindividuals), or simply the increase in inbreeding.

Variance Effective SizeVariance Effective Size (NNevev)--measures the rateof loss of total genetic variation from a populationwhether the loss is experienced within individualsor among individuals.

Typically, Nei and Nev differ substantially onlywhen population size is significantly increasingor decreasing.

Unfortunately, no theories or equations exist thatsimultaneously handle multiple deviations from the ideal situation.

Thus, influences of bottlenecks, skewed sex ratios,and family size cannot at present be simultaneouslyestimated with these equations.

Real Meaning of Effective Population SizeReal Meaning of Effective Population Size

There is no such thing as the “effective populationsize” in the sense that many people think aboutthis.

Effective size is defined with respect to a geneticparameter of interest, and as the parameter ofinterest changes, the effective size can change.

Hence, a population can be characterized by several different “effective sizes” simultaneously.

Thus, the phrase “the effective population sizethe effective population size”is meaningless unless the genetic parameter ofinterest is also supplied.

The important point is that, due to propertiesassociated with sex ratio, family size, andpopulation fluctuations, Ne is nearly always significantly smaller than the census size.

Importance of Small Populations in ConservationImportance of Small Populations in ConservationBiologyBiology

Small or declining populations are more prone toextinction than large stable populations.

Population size is the most influential of the fivecriteria for listing species as endangered under theIUCN.

Species whose adult population sizes are less than50, 250, or 1,000 are listed as critically endangeredcritically endangered,endangeredendangered, and vulnerablevulnerable, respectively.

There are special evolutionary problems confrontedby small populations.

In small populations, the role of chancepredominates and the effects of selection are typically reduced or even eliminated.

Chance introduces a random, or stochastic, elementinto the evolution of populations.

Small populations become inbred at a faster ratethan do larger populations, as inbreeding isunavoidable.

When a small population reproduces, the subsequentgeneration is derived from a sample of parentalgametes.

Each offspring receives 1 allele, selected at random,from each parent.

Just by chance, some alleles, especially rare ones,may not be passed on to the offspring and may belost.

The frequencies of alleles that are transmittedto the next generation are likely to differ fromthose in the parental generation.

Over multiple generations, allele frequencieschange, or “driftdrift”, from one generation to thenext, a process termed “genetic driftgenetic drift”.

Chance Effects -- Genetic DriftChance Effects -- Genetic DriftBasic concept of genetic drift -- Evolution can bethought of as a change in allele frequency andfinite population size alone ensures that evolutionwill occur through sampling error.

Broadly speaking, there are four consequences ofrandom genetic drift.

These are not really consequences, but ratherdifferent ways in which the consequences maybe seen.

Random DriftRandom Drift -- Random changes of gene frequencywhich change in an erratic manner from generationto generation, with no tendency to revert to itsoriginal value.

Differentiation between subpopulationsDifferentiation between subpopulations -- Randomgenetic drift occurring independently in differentsubpopulations leads to genetic differentiationbetween subpopulations.

Uniformity within subpopulationsUniformity within subpopulations -- Geneticvariation within each subpopulation becomesgradually reduced, and the individuals become morealike genotypically.

Increased homozygosityIncreased homozygosity -- Homozygosity increasesin frequency at the expense of heterozygosity.

This coupled with the general tendency for deleterious alleles to be recessive, is the geneticbasis for the loss of fertility and viability thatalmost always results from inbreeding.

Genetic DriftGenetic Drift -- Random sampling of gametes withinsmall populations has three consequences of majorimportance in evolution and conservation:

Random changes in allele frequencies from onegeneration to the next.

Loss of genetic diversity and fixation of alleles within populations.

Diversification among replicate populations fromthe same original source (e.g. fragmented pops.)

FixationFixation -- Genetic drift will ultimately cause allexcept one allele to be lost and the remaining alleleis said to be “fixedfixed”.

The probability of losing an allele is dependent onits frequency and the population size.

In the case of two alleles, the probability of losingone allele is the probability of fixing the other allele.

The probability that a gamete does not containallele A1 is (1 - p).

Consequently, the probability that a randomlymating population losses allele A1 (all individualsbecome A2A2) is:

Pr(losing A1) = (1 - p)2N but, = (1 - p)= (1 - p)2Ne2Ne

The rarer the allele, the far greater the probabilityof being lost.

The gene frequencies in these samples will havean average value equal to that in the basepopulation and will be distributed about this meanwith a variance of:

variance = variance = 22 = p = p00qq00/2N/2Nee

Although genetic drift is random, we can calculatea 95% confidence interval for the magnitude ofchange as follows:

q ± 2q ± 2qq

ExampleExample: You have a population of 200 lionsconsisting of 100 males and 100 females,with an allele frequency of the slow allele at theADH locus of 0.33. The breeding structure ofthis group is that each male controls a pride of5 adult, breeding females.

Calculate the 95% confidence interval for thechange in gene frequency due to genetic drift.

Step I: Calculate effective population size.Step I: Calculate effective population size.

Ne = (4NfNm)/(Nf + Nm)= (4 X 100 X 20)/(100 + 20)= 8000/120 = 66.67

Step II: Calculate Variance in qStep II: Calculate Variance in q

2 = (p0q0)/2Ne

= (0.67 X 0.33)/(2 X 66.67)= 0.221 / 133.34= 0.0017

Step III: Calculate 95% CIStep III: Calculate 95% CI

q ± 2q

2q = 2 X (0.0017)0.5

= 2 X 0.04= 0.08

Thus, 0.33 ± 0.08

Therefore, in the next generation0.25 < q < 0.41

Founder EffectFounder Effect -- This occurs when a few individuals establish a new population, the geneticconstitution of which depends upon the genes of the founders.

Demographic BottleneckDemographic Bottleneck -- Occurs when a population experiences a severe, temporaryreduction in size.

The magnitude of loss of genetic variation dependson the size of the bottleneck and the growth rate of the population afterwards.

The proportion of genetic diversity remaining afterone generation, t, the next generation is:

1 - (1/2Ne)

This proportion can range from 0.5 (50% variation)with an Ne of 1 (the gametes of 1 individual carryon average 50% of the genetic diversity of thepopulation) to near 1.0 (100%) with a large Ne, (e.g., 1,000)

Generally, a bottleneck rarely has severe genetic or fitness consequences if population sizequickly recovers in a generation or two.

Population genetics theory tells us that perhapsmore important than depletion of quantitativegenetic variation by founder effects, bottlenecks,or genetic drift, is loss of rare alleles from thepopulation.

However, little empirical evidence supports thisidea.

We know that rare alleles contribute little tooverall genetic diversity but they may be important to a population during infrequentor periodic events such as unusual temperaturesor exposure to new pathogens, and may offerunique responses to future evolutionary change.

Impact of a bottleneck on heterozygosity:

H = -(1/2NH = -(1/2Nee)H)H00

Impact of a bottleneck on allelic diversity:

A = n - A = n - (1 - p(1 - pii))2Ne2Ne

i=1i=1

# alleles# alleles

where nn is the number of alleles before thebottleneck and pi is the frequency of the ith

allele.

InbreedingInbreeding -- mating of individuals by commonancestry.

Probability of occurrence increases in smallerpopulations if mating occurs at random.

In a population of bisexual organisms, everyindividual has 2 parents, 4 grandparents,8 great grandparents, etc.

t generations back, an individual has 2t ancestors.

Not very many generations back, the number ofindividuals required to provide separate ancestorsfor all individuals in the population becomeslarger than any real population could attain.

For example, 50 generations back, would mean thatan individual would have 225050 = 1.2 X 10 = 1.2 X 101515 ancestors ancestors.

Therefore, any pair of individuals must be relatedto each other through one or more commonancestors in the more or less remote past.

The smaller the size of the population in previous generations, the less remote the commonancestor.

Thus, pairs mating at random are more closely related to each other in a small population than alarger population.

For this reason, the properties of small populationscan be treated as a consequence of inbreeding.

Inbred individualsInbred individuals -- offspring produced byinbreeding -- may carry two genes at a locus thatare replicates of the same gene in a previousgeneration.

There are two types of identity among allelicstates -- two types of homozygotes.

Identical by Descent or AutozygousIdentical by Descent or Autozygous -- two genesthat originate from the replication of a singlegene pair in a previous generation.

Independent by Descent or AllozygousIndependent by Descent or Allozygous --homozygous individuals not known to be autozygous.

It is the production of autozygosity that givesrise to increase of homozygotes as a consequenceof inbreeding.

The inbreeding coefficientThe inbreeding coefficient (FF) is the probabilitythat two genes at a locus in an individual are identical by descent and ranges from 0 to 1.

FF refers to an individual and expresses the degreeof relationship between the individuals parents.

If the parents of any generation have matedrandomly, then FF of the progeny is the probabilitythat 2 gametes taken at random from the parentgeneration carry autozygousautozygous genes at a locus.

This is the average coefficient of inbreeding ofall progeny.

Individuals of different families will have differentinbreeding coefficients because with randommating, some pairs of parents will be more closelyrelated than others.

The degree of relationship expressed in theinbreeding coefficient is a comparison between thepopulation in question and some specified orimplied base populations.

Without this reference point, it is meaninglessbecause all genes now present at a given locuswould be found to be identical by descent if tracedback far enough.

Therefore, FF only becomes meaningful if wespecify some time in the past beyond which ancestries will not be considered and at which time all genes in the populations are to beconsidered allozygous (= independent).

This point of reference is the base population and by definition, it has an inbreeding coefficient of zero (F = 0F = 0).

Inbreeding in the “Idealized PopulationIdealized Population”

We will deduce FF in successive generations,beginning with the base population.

Examine a hermaphroditic marine organism capableof self-fertilization, shedding eggs and sperm intothe sea.

There are N individuals, each shedding equal numbers of gametes at random.

Because it is the base population, all genes at alocus have to be considered as non-identical.

Therefore, considering only one locus, among thegametes shed there are 2N different sorts,bearing genes A1, A2, A3, . . . . , AN at the “A” locus.

The gametes of any one sort carry identical genes,those of different sort carry genes of independentorigin.

QuestionQuestion: What is the probability that a pair ofgametes taken at random carry identical genes?This is the inbreeding coefficient of generation 1.

AnswerAnswer: Any gamete has a (1/2N)th chance ofuniting with another of the same sort.

Therefore, 1/2N is the probability that uniting gametes carry identical genes, and is thus the coefficient of inbreeding of the progeny in generation 1.

QuestionQuestion: What is F in the second generation?

AnswerAnswer: There are now two ways in whichidentical homozygotes can be produced.

New replication of genesNew replication of genesReplication of genes from previous replication.Replication of genes from previous replication.

The probability of newly replication genes comingtogether is again (1/2N1/2N) with the remaining portion(1 - 1/2N1 - 1/2N) of zygotes carrying genes that are independent in their origin from generation 1 but may have been identical in origin in generation 0.

Thus, the total probability of identical homozygotesin generation 2 is:

FF22 = [(1/2N) + (1 - 1/2N)F = [(1/2N) + (1 - 1/2N)F11]]

An incrementincrementattributable tonew inbreeding

“remainderremainder”, attributableto the previous inbreedingand having the inbreedingcoefficient of the previousgeneration.

We designate the “increment” or new inbreedingas: F = 1/2NF = 1/2N then,

FFtt = 1 - (1 - = 1 - (1 - F)F)tt where F where F00 = 0 = 0

We can also calculate the probability of pullingout two genes that are NOTNOT identical bydescent, or heterozygous as: HHtt = 1 -F = 1 -Ftt

Then by subtracting Ft, we can obtain the rate ofchange in heterozygosity from random genetic drift as: HHtt = (1 - = (1 - F)F)tt ≈ H ≈ H00ee-(t/2N)-(t/2N)

Note: IF EFFECTIVE POPULATION SIZE ISNote: IF EFFECTIVE POPULATION SIZE ISKNOWN OR CAN BE CALCULATED, IT WOULDKNOWN OR CAN BE CALCULATED, IT WOULDBE MORE APPROPRIATE TO USE NBE MORE APPROPRIATE TO USE Nee IN THE IN THEPREVIOUS EQUATIONS.PREVIOUS EQUATIONS.

Example 1Example 1: In a population of 200 randomly matingwhite rhinos, p0 = 0.73, what will heterozygositybe in 50 generations?

Step I: Calculate qStep I: Calculate q00..

q0 = 1 - 0.73 = 0.27

Step II: Calculate HStep II: Calculate H00,,

H0 = 2p0q0 = 2 X 0.73 X 0.27 = 0.394

Step III: Use equation HStep III: Use equation Htt = H = H00ee-(t/2N)-(t/2N)

H50 = (0.394)e-(50/400)

H50 = (0.394)e-0.125

H50 = (0.394)(0.88)

H50 = 0.3477

Example 2Example 2: In a fish hatchery, all eggs (an infinitely large number) are fertilized by the spermof a single male in each generation. Calculate rateof inbreeding.

Step I: Calculate NStep I: Calculate Nee

Ne = (4NmNf)/(Nm + Nf)

Ne = (4 X 1 X 10,000)/(1 + 10,000)

Ne = 3.9996

Step II: Calculate rate of InbreedingStep II: Calculate rate of Inbreeding

F = 1/2Ne

F = 1/(2 X 3.9996) = 1/7.9992 = 0.125

In small populations, matings among relatives isinevitable not as a result of deliberate inbreedingbut simply as a consequence of small numbers offounders and the small population size.

Inbreeding also becomes inevitable in largerpopulations, but it takes longer.

For example, a population of size 100 over 57generations becomes, on average, as inbred asthe progeny of brother-sister matings.

Inbreeding decreases heterozygosity and increases homozygosity thus altering genotypicratios from that expected based on Hardy-Weinberg expectations.

However, inbreeding does NOT change allele frequencies.

The reduction in heterozygosity due to inbreedingis directly related to the inbreedingcoefficient.

We can estimate the level of inbreeding bycomparing observed (Ho) heterozygosity withexpected (He) under random mating as follows:

F = 1 - (He/Ho)

Effects of population size on the level of inbreedingcan be determined by considering the probabilityof identity by descent in the idealized randomly mating population.

When the initial population is NOTNOT inbred (F0 = 0),the inbreeding coefficient in any subsequentgeneration t is:

FFtt = 1 - [1 - 1/2N = 1 - [1 - 1/2Nee]]tt

Thus, inbreeding accumulates with time in allclosed finite populations at a rate dependent upontheir population size.

If population sizes fluctuates among generations,as occurs in real populations, the expression forthe inbreeding coefficient at generation t is:

FFtt = 1 - = 1 - [1 - (1/2N[1 - (1/2Neiei)])]i=1i=1

tt

Where Nei is the effective size of the ith

generation.

Indirect Estimates of Inbreeding CoefficientsIndirect Estimates of Inbreeding Coefficients

In most populations, levels of inbreeding areunknown.

However, an estimate of the average inbreedingcan be obtained from the effective inbreedingcoefficient (FFee):

FFee = 1 - (H = 1 - (Htt/H/H00))

Gray wolves became establishedon Isle Royale in about 1949during an extreme winterwhen the lake froze.

The wolf population is assumed to have beenstarted by a single pair of individuals.

Population rose toabout 50 in 1980.

Population crashedto 14 in 1990.

Decline could have been due to:reduced availability of preyreduced availability of preydiseasediseasedeleterious effects of inbreedingdeleterious effects of inbreedingcombination of factorscombination of factors

Suggested that the island population must beinbred due to the low number of founders.

All individuals have the same rare mtDNAhaplotype.

DNA fingerprint data suggests that island wolves are as similar as sibs in a captive populationof wolves.

Allozyme heterozygosity, based on 25 loci, was3.9% for Isle Royale wolves compared to 8.7%for a captive population.

Fe = 1 - (Hisland/Hmainland) = 1 - (0.39/0.87)= 0.55

Thus, this endangered island population is highly inbred.

Gray wolves suffer reproductive fitness dueto inbreeding -- the Isle Royale population hassmaller litters and poor juvenile survival!

Pedigree Path AnalysisPedigree Path Analysis -- Once a pedigree of anindividual (say “XX”) is obtained, we can calculateits inbreeding coefficient, FFXX.

Step IStep I: Draw the pedigree so that the commonancestors appear only once.

A common ancestor is any individual related toboth parents of X, the individual for whom wewish to determine FX.

If there are no common ancestors, then FX = 0

AStep IIStep II: Determine the inbreeding coefficient.

If there is no pedigree information on the commonancestors, it is often assumed to be non-inbred

If the common ancestor is inbred, then its inbreeding coefficient, FCA, must be calculatedbefore calculating FX

Calculate FCA as you would FX as described below.

Once FCA is determined, FX can be calculated.

Step IIIStep III: Look for loops in the pedigree

A loop is a path that runs from X, through oneparent, to the common ancestor, through the otherparent, and back to X without going through anyindividual more than once.

Determine the number of steps in each path.

Step IVStep IV: Calculate the contribution of each loopto the inbreeding coefficient.

The contribution of each loop to FX is determinedas follows:

(1/2)(1/2)ii X (1 + F X (1 + FCACA))

Where FCA is the inbreeding coefficient of thecommon ancestor and i is the number of steps ineach loop as defined in step III.

Step VStep V: Sum the contribution of each loop. The summation of all the contributions will be the inbreeding coefficient of individual X.

Example I: Half-Sib Matings

X

AA B AA C

D EPedigree

AA

D E

X

Path

AA

D E

X

Loop FCA i Contribution to FX

D - A - E 0.0 3 (1/2)3 X (1 + 0.0)

FX = 0.125

Example 2 -- Full-Sib Mating

A B A B

X

AA

BB

X

C D

Loop FCA i Contribution to FX

C--AA--D 0.0 3 (1/2)3 X (1+0.0) = 0.125C--BB--D 0.0 3 (1/2)3 X (1+0.0) = 0.125

FX = 0.25

A B

C D E F

G H I J

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WHAT IS FO?

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What is FWhat is FMM??

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Loop FCA iK-G-C-AA-D-H-L 0.0 7 (1/2)7 0.0078

A

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Loop FCA iK-G-C-AA-D-H-L 0.0 7 (1/2)7 0.0078K-H-C-AA-D-G-L 0.0 7 (1/2)7 0.0078

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Loop FCA iK-G-C-AA-D-H-L 0.0 7 (1/2)7 0.0078K-H-C-AA-D-G-L 0.0 7 (1/2)7 0.0078K-G-D-AA-C-H-L 0.0 7 (1/2)7 0.0078

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Loop FCA iK-G-C-AA-D-H-L 0.0 7 (1/2)7 0.0078K-H-C-AA-D-G-L 0.0 7 (1/2)7 0.0078K-G-D-AA-C-H-L 0.0 7 (1/2)7 0.0078K-H-D-AA-C-G-L 0.0 7 (1/2)7 0.0078

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Loop FCA iK-G-C-AA-D-H-L 0.0 7 (1/2)7 0.0078K-H-C-AA-D-G-L 0.0 7 (1/2)7 0.0078K-G-D-AA-C-H-L 0.0 7 (1/2)7 0.0078K-H-D-AA-C-G-L 0.0 7 (1/2)7 0.0078K-G-CC-H-L 0.0 5 (1/2)5 0.0313

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Loop FCA iK-G-C-AA-D-H-L 0.0 7 (1/2)7 0.0078K-H-C-AA-D-G-L 0.0 7 (1/2)7 0.0078K-G-D-AA-C-H-L 0.0 7 (1/2)7 0.0078K-H-D-AA-C-G-L 0.0 7 (1/2)7 0.0078K-G-CC-H-L 0.0 5 (1/2)5 0.0313K-G-DD-H-L 0.0 5 (1/2)5 0.0313

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Loop FCA iK-G-C-AA-D-H-L 0.0 7 (1/2)7 0.0078K-H-C-AA-D-G-L 0.0 7 (1/2)7 0.0078K-G-D-AA-C-H-L 0.0 7 (1/2)7 0.0078K-H-D-AA-C-G-L 0.0 7 (1/2)7 0.0078K-G-CC-H-L 0.0 5 (1/2)5 0.0313K-G-DD-H-L 0.0 5 (1/2)5 0.0313K-GG-L

A

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Loop FCA iK-G-C-AA-D-H-L 0.0 7 (1/2)7 0.0078K-H-C-AA-D-G-L 0.0 7 (1/2)7 0.0078K-G-D-AA-C-H-L 0.0 7 (1/2)7 0.0078K-H-D-AA-C-G-L 0.0 7 (1/2)7 0.0078K-G-CC-H-L 0.0 5 (1/2)5 0.0313K-G-DD-H-L 0.0 5 (1/2)5 0.0313[C-AA-D 0.0 3 (1/2)3 0.125]K-GG-L 0.125 3 (1/2)3 X (1.125) 0.1406

A

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Loop FCA iK-G-C-AA-D-H-L 0.0 7 (1/2)7 0.0078K-H-C-AA-D-G-L 0.0 7 (1/2)7 0.0078K-G-D-AA-C-H-L 0.0 7 (1/2)7 0.0078K-H-D-AA-C-G-L 0.0 7 (1/2)7 0.0078K-G-CC-H-L 0.0 5 (1/2)5 0.0313K-G-DD-H-L 0.0 5 (1/2)5 0.0313[C-AA-D 0.0 3 (1/2)3 0.125]K-GG-L 0.125 3 (1/2)3 X (1.125) 0.1406K-HH-L

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Loop FCA iK-G-C-AA-D-H-L 0.0 7 (1/2)7 0.0078K-H-C-AA-D-G-L 0.0 7 (1/2)7 0.0078K-G-D-AA-C-H-L 0.0 7 (1/2)7 0.0078K-H-D-AA-C-G-L 0.0 7 (1/2)7 0.0078K-G-CC-H-L 0.0 5 (1/2)5 0.0313K-G-DD-H-L 0.0 5 (1/2)5 0.0313[C-AA-D 0.0 3 (1/2)3 0.125]K-GG-L 0.125 3 (1/2)3 X (1.125) 0.1406[C-AA-D 0.0 3 (1/2)3 0.125]K-HH-L 0.125 3 (1/2)3 X (1.125) 0.1406

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Loop FCA iK-G-C-AA-D-H-L 0.0 7 (1/2)7 0.0078K-H-C-AA-D-G-L 0.0 7 (1/2)7 0.0078K-G-D-AA-C-H-L 0.0 7 (1/2)7 0.0078K-H-D-AA-C-G-L 0.0 7 (1/2)7 0.0078K-G-CC-H-L 0.0 5 (1/2)5 0.0313K-G-DD-H-L 0.0 5 (1/2)5 0.0313[C-AA-D 0.0 3 (1/2)3 0.125][C-AA-D 0.0 3 (1/2)3 0.125]K-GG-L 0.125 3 (1/2)3 X (1.125) 0.1406K-HH-L 0.125 3 (1/2)3 X (1.125) 0.1406

FM = 0.375

Inbreeding is of profound importance inconservation biology as it leads to:

reductions in heterozygosityreduced reproduction & survivalincreased risk of extinction

Inbreeding results in predictable increase inhomozygosity and may be manifested asInbreeding DepressionInbreeding Depression such as:

reductions in fecundityreductions in offspring sizereductions in growthreductions in survivorshipchanges in age at maturityphysical deformities.

Characteristics of Inbreeding Depression

AnimalsAnimals PlantsPlantsOffspring # Pollen QualityJuvenile Survival Number of OvulesLongevity Amount of SeedInterbirth interval Germination RateMating Ability Growth RateMaternal Ability Competitive AbilitiesSperm Quantity & QualityCompetitive AbilityDevelopmental Time

Despite overwhelming evidence from laboratory& domesticated species, there has beenconsiderable skepticism that inbreeding depressionoccurs in wildlife.

Ralls & Ballou found higher mortality in inbredversus outbred progeny for 41 of 44 species ofmammals.

On average, progeny of full-sib matings displayeda 33% reduction in juvenile survival.

Data on inbreeding depression in the wildare difficult to compile, as the level of inbreedingis not easily determined.

Data from domesticated animals indicate F of 10%will result in a 5% -- 10% decline in individualreproductive traits such as clutch size or survivalrates; in aggregate, total reproductive attributesmay decline by 25%.

Remedy for inbreeding depression is to outcrossthe inbred population to another unrelatedpopulation.

In many cases of conservation concern, no otheroutbred population exists.

If other, independent, inbred populations exist,they will usually allow recovery of fitness.

Population IaaBB

Population IIAAbb

Hybrid PopulationAaBB

X

Fitness will be restored or even enhanced aboveoriginal non-bred levels. Known as Heterosis.

fitness reduced in subsequent generations assegregation of alleles produces homozygotes.

Two competing hypotheses for the mechanismleading to inbreeding depression.

Dominance HypothesisDominance Hypothesis -- Inbreeding results inmore instances of deleterious recessive allelesappearing in homozygous form, where they areclearly expressed, rather than being masked bydominance in the heterozygous state.

This hypothesis suggests that inbred populationshave already experienced exposure of deleteriousrecessive alleles, and most have probably been purged from the population.

Further inbreeding should not then have largeeffects on fitness.

Overdominance HypothesisOverdominance Hypothesis -- focuses on the lossof genome-wide heterozygosity and its presumedfitness advantages.

This hypothesis predicts that further inbreedingshould result in continued loss of fitnessthrough further heterozygosity loss.

Many species are known to avoid inbreeding in thewild, further evidence that inbreeding depressionis real and important.

However, not all inbreeding is cause for alarm.

Some natural populations apparently haveexperienced low levels of inbreeding for many generations with no ill effects.

Fundamental points of this section:Fundamental points of this section:

Small isolated populations will loose some fractionof their original genetic diversity over time, approximately at a rate of 1/2N1/2Nee per generation.

Small population numbers over prolonged periodsof time are to be avoided in conservationprograms whenever possible.

There should be concern about low genetic variation in small populations, but it is by no meansa universal pattern.