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Lecture10TakeHomePoints
• GeneticStochasticity
-Randomfluctuationinallelefrequenciescanleadtolossofparticulargenotypes
andfixationofparticulartraits.
-Smallfounderpopulationsinherentlycontainlimitedgeneticdiversity,and
inbreedingandgeneticdriftcanenhancethelossofdiversity.
-Inbreeding:Populationsthathaveahighfrequencyofcloselyrelatedindividuals
cansufferfrominbreeding.Ininbredpopulations,deleteriousallelescanbe
exposedinhomozygousindividuals,whichmayreducefitness.Theimportanceof
inbreedingvarieswithspecies.
PotentialGeneticConsequencesofInvasion
GeneticBottleneckandFounderEffect
Constrainedgeneticdiversity–smallpopulationsample
GeneticDrift
Randomlossofallelesinsmallpopulations
InbreedingDepression
Consequencesofdeleteriousrecessivegenesbecomingapparentthrough
inbreeding
GeneticHotspots
Repeatedintroductionsleadtohyperdiversityandthepotentialfornovel
invasivegenotypes
GeneticDiversityandInvasion
Invasionsofteninvolveasmallsampleoftheoriginalpopulation
Throughrandomchance,canoverorunderrepresentparticulargenotypes
AreBottlenecksBadforInvaders?
Yes
Reducedgeneticdiversity
Lessforselectiontoacton,thuslessresponsivetovariableenvironments
Expressionofdeleteriousallelesthroughinbreeding
Inbreedingdepression
andNo
Genetic‘purging’
Advantageofrelatedness
Manyinvasivespeciesarenotreallybottlenecked
Geneticpurging–moderatelyinbredpopulationscansheddeleteriousallelesover
severalgenerationsasselectionactsagainstthem.Afteranumberofgenerations,
thefitnessincreasestothatofoutbredpopulations.
Harmoniaaxyridis(Harlequinladybeetle)
Underwentabottleneckofintermediateintensity
Relatedindividualshadanincreasedprobabilityofmating,increasingthe
homozygosityofthepopulation.
Deleteriousrecessivealleleswerethenpurgedfromthefrompopulationafter~15
generationsbecauseonceexposed(asahomozygousrecessive),naturalselection
eliminatedthem
• GeneticConsequencesofInvasion
• Lowgeneticdiversity
• Mayhavenegative,neutral,orpositive(shortterm)effects
• Lowgeneticdiversity–canlimitresponsestoenvironmental
stochasticity.Canlimitabilityofapopulationtoadapttonovel
environments.Maycontributetoinbreeding.
• Insomesocialinsects,lowgeneticdiversitycanfacilitatemuch
higherpopulationdensitiesasindividualcoloniesfailto
recognizeotherindependentcoloniesasdistinctpreventing
theaggressionthatmaintainslowerpopulationdensitywhere
thespeciesarenative.
• Highgeneticdiversity
• Ininvasions,usuallytheproductofrepeatedintroductions
• Higherdiversitywithinpopulationsthanbetweenthem.
Thiscontrasttonativerangewhereindividual
populationsaregeneticallydistinctandmostvariation
isbetweenpopulationsasopposedtowithin.
• BrownanoleinS.Floridaisagoodexample.
• • Foundereffect–gypsymothinNorthAmerica(largelyasingle
introduction)butnotraccoonsinGermany(multipleindependent
introductions).
• Lagtimes–maybeaproductoflowgeneticvariabilityand
maladaptationtonovelenvironment
• Overcomethroughdenovomutation,geneflowfromanother
population,orselectionontheexistingvariationwithinthe
population.