population size and extinctionpeople.cst.cmich.edu/swans1bj/620 lect 02.pdf · extinction small...
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“Science is simplycommon sense at itsbest that is, rigidlyaccurate inobservation, andmerciless to fallacyin logic.”
Thomas Henry Huxley
Terminology Locus Allele Gene Chromosome Homozygote Heterozygote Polymorphic Monomorphic
Terminology Polygenic Epistatic Genotype Phenotype Quantitative trait Neutral Marker
Population Size andExtinction
Small populations aremore likely to goextinct than largepopulations Genetics Demographic
Population Size andExtinction
Population Size andExtinction
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Genetic Variation andPopulation Size Black-footed rock wallaby
Genetic Variation andPopulation Size
Genetic Variation &Population Size
Larger populationsizes have moreheterozygosity.
Halocarpus bidwilliPicoides borealis
Genetic drift andheterozygosity
Genetic drift results in a gradual loss ofgenetic diversity
Follow an individual locus and allelefrequency will drift until one allelebecomes fixed
Genetic drift
107 populations of Drosophila16 individuals per population
Genetic Variation& Fitness
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Inbreeding &Population Size Inbreeding
Inbreeding is related to populationstructure
Consider (a) one large panmictic population (b) lots of small, discrete populations
Identity by descent
assume every allele in initial population is differentIBD = probability of this individual carryingtwo copies of the same allele
=
Probabilities
Probability of event A happening ANDevent B happening = PA x PB
Probability of event A happening ORevent B happening = PA + PB (if A & B mutually exclusive)
Identity bydescent
• Probability of black allelebeing passed down left handside = 0.5 x 0.5 x 0.5 = 1/8
• Probability of black allelebeing passed down right handside = 0.5 x 0.5 x 0.5 = 1/8
• Probability of individual atbottom getting two blackalleles = 1/64
• Same argument for yellowallele = 1/64
• Probability of getting 2copies of black or 2 copies ofyellow = 1/64 + 1/64 = 1/32
Inbreeding in finitepopulations
Previous examplesshowed inbreeding inknown pedigrees throughIBD
What about in finitepopulations?
In a finite population,there is some probabilityof mating with a relative
This probability increaseswith subsequentgenerations
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Inbreeding in finitepopulations
Assume a population size of N, therefore 2N alleles inpopulation
Imagine eggsand spermreleasedrandomly intoenvironment(e.g. sea)
Inbreeding in finitepopulations
Assume a population size of N, therefore 2N alleles inpopulation
Imagine eggs and sperm released randomly intoenvironment (e.g. sea)
What is the probability of 2 gametes drawn randomlyhaving the same allele?
Why?2N allelesgen 0
gen 1
probability = 1/(2N)
You already have one allele withwhich you started, thus you only needto pick 1 additional allele
Inbreeding in finitepopulations
Therefore, after 1generation the level ofinbreeding is F1 = 1/2N
After t generations theprobability is
Why?
gen t-1
gen t
1/(2N) 1 - 1/(2N) Probability ofpicking 2nd allele
Probability ofpicking 2nd allele &it is already inbred
More generally
GeneticDrift
Ft = Probability thatany 2 alleles drawnrandomly from thepopulation are identicalby descent
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Measures of Inbreeding1. Inbreeding coefficient F
0.5Selfing (self-fertilization)0.0625First cousins0.125Half sibs (half brother to half sister)0.25Brother to sister, mother to so, etc
0Unrelated
OffspringF
Parents
Extinction Factors
Extinction Vortex Inbred individuals are less
fit then outbred individuals Deleterious recessive
mutations tend to becomeapparent in inbredindividuals
Inbred individuals mayhave lower reproductivesuccess, lowersurvivorship or lowerresistance to parasites
Inbreeding and Survival
Inbreeding increases juvenile mortality
Inbreeding and Survival
Inbreeding & Survival Inbreeding and Survival
Inbreeding doesn’t influence allspecies the same or does it?
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Inbreeding and Survival
Inbreeding doesn’t influence allspecies the same or does it?
Inbreeding and conservationgenetics
Inbreeding has greatest effects in small populationsor populations that have suffered a bottleneck
Loss of genetic diversity inthe cheetahsevere bottleneck in its past
Inbreeding example 1Keller et al Nature 372:356-357
Sparrows on Mandarte Is., BC Inbreeding associated with poor survivorship Paternities determined by genetic fingerprinting Population crash Survivors less inbred than those that die
0.0156 (n=3)0.0025 (n=7)After crash0.0363 (n=132)0.0189 (n=74)Before crashJuvenilesAdults
inbreeding coefficient
Distribution of inbreeding coefficients forsurvivors/non-survivors
survivornon-survivor
Inbreeding example 2:Soay sheep on St. Kilda(Coltman et al Evolution 53:1259-1267)
Soay sheep subject toparasitism by intestinalnematodes
Use markers distributedthrough the genome
Heterozygous individualsappear best able to resistinfection and to survive
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Heterozygosity & parasitism
filled, low-densityyears
open, high-densityyears
(FEC = fecal eggcount of strongylenematodes)
Heterozygosity & survivorship
filled, low-densityyears
open, high-densityyears
(survivorship oflambs over winter)
Factors acting againstgenetic drift
No matter how big a population, geneticdrift will eventually drive all loci tohomozygosity (i.e. all alleles at all loci inall indvs. identical by descent)
Mutation and migration counters geneticdrift
Analysis of effect of these processesidentical.
“It requires a veryunusual mind toundertake the analysisof the obvious.”Alfred North Whitehead
“We see only what weknow.”
Johann Wolfgangvon Goethe
EVOLUTIONARY PROCESSESAND PATTERNS
Theories of species creationsand diversity prior toevolutionary theory
Theory of Special Creation Species are unchanged
through time and areindependent of one another
All species were createdindependently by “…theTrinity on the October 26th4004 B.C. at 9:00 in themorning” Archbishop JamesUssher 1664.
EVOLUTIONARY PROCESSESAND PATTERNS
Theories of species creations and diversity priorto evolutionary theory
Theory of Spontaneous Generation New organisms (species) may suddenly appear
wherever conditions are suitable Some new life-forms arise spontaneously from
streams, soils, totting meat, and other nonlivingmaterials; not all live arises directly from livingorganisms
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1620’s
VanHelmont’s evidence forspontaneous generation
“…for if you press a piece of underware soiled with sweattogether with some wheat in an open mouth jar, afterabout 21 days the odor changes and the ferment comingout of the underwear and penetrating through the husksof the wheat, changes the wheat into mice. But what ismore remarkable is that mice of both sexes emerge (fromthe wheat) and these mice successfully reproduce withmice born naturally from parents…But what is even moreremarkable is that the mice which came out were notsmall mice… but fully grown.”
EVOLUTIONARY PROCESSESAND PATTERNS
Theories of species creationsand diversity prior toevolutionary theory
Prior to Darwin and Wallace -Lamarck New simple life forms arise by
spontaneous generation andchange over time into morecomplex life forms
Individuals change in response totheir environment and thechanges are passed to the nextgeneration.
EVOLUTIONARY PROCESSESAND PATTERNS
Theories of Evolutionary Darwin and Wallace Species are related to one another, and they
change over time, thus species existing todayhave descended, with modifications, from otherpreexisting species.
Natural selection acts on individuals; individualswith certain favorable characteristics will producemore offspring.
Darwinian Selection All natural selection results in evolution, but not
all evolution is the product of natural selection. What is evolution?
What is natural selection?
What is an adaptation?
Darwinian Selection All natural selection results in evolution, but not all evolution is
the product of natural selection. What is evolution?
Evolution is the change in allele frequencies (or traits) over time. Selection
• Natural• Sexual
Genetic drift Mutation Migration
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Darwinian Selection All natural selection results in evolution, but not all
evolution is the product of natural selection. What is evolution? Evolution is the change in allele frequencies (or traits)
over time. What is natural selection?
What is an adaptation?
Darwinian Selection All natural selection results in evolution, but not all
evolution is the product of natural selection. What is evolution? Evolution is the change in allele frequencies (or traits)
over time. What is natural selection? Natural selection is the differential reproductive success
resulting from an adaptation. What is an adaptation?
Darwinian Selection All natural selection results in evolution, but not all
evolution is the product of natural selection. What is evolution? Evolution is the change in allele frequencies (or traits)
over time. What is natural selection? Natural selection is the differential reproductive success
resulting from an adaptation. What is an adaptation? An allele (or trait) which provides superior reproductive
success to an individual possessing it relative to anindividual with a different allele (or trait).
Darwinian Selection
Sum it all upNatural selection for anadaptation results inevolution.
What can selection do to atrait?
If a trait is underselection, what are thepossible results?
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Chimp Lucy Human
Outlet
Midcavity
Inlet
Female Mimicrya - unpaired male (m) assumes female coloration and posture while approaching the female (f) as the consort male (c) fights with an approaching male (m2)
m2
b - the female accepts a mating by the female mimic while the consort continues fighting with the approachingmale
c - The consort male allow the female mimic to finishmating without interruption, even when he is notdistracted by the other male
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Insects, spiders, nectar
Tools use to get insects
Leaves and fruit
Ticks off of iguanas etc.
Seeds
Darwinian Selection
Is there variation about a trait?
Darwinian Selection
Is the variation heritable and not the result of maternal effects?
Darwinian Selection
Is there an excess of individualsso that only some animals liveto reproduce?
Is there an excess of individualsso that only some animals liveto reproduce?
Darwinian Selection
Is reproduction nonrandom?
The drought of 1977 eliminatedseed set by most of the plantsproducing small soft seeds.Tribulus cistoides seeds are largeand hard and became the dominantfood item. Only large birds withdeep beaks could defend resourcesand access the resources
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Darwinian Selection
Is reproduction nonrandom?
Darwinian Selection
Did evolution occur?
The El Niño of 1983 produced1359 mm of rain and lavish seedset by the small soft seeded plants.Birds with shallow beaks harvestthese seeds more efficiently andthus reproduced better than birdswith deep beaks, undoing theselection shown here. Fluctuatingenvironmental conditions maintainboth phenotypes.
At what level does selectionoccur?
Gene … for the good of the gene.
Individual … for the good of the individual.
Group … for the good of the group.
Population … for the good of the population.
Species … for the good of the species.
Darwinian Selection
THEINDIVIDUAL
Darwinian Selection
NOT THEGROUP!!!*
*In silico (computer simulations) do show specific conditions where group selection can work but This has never been seen in nature
Darwinian Selection
NEVERTHE
SPECIES!!!
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Darwinian Selection Darwinian Selection
Darwinian Selection Darwinian Selection
Darwinian Selection
P.S.Darwinian Selection
THERE ISNO
ALTRUISM!
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Darwinian Selection
Why does it appear that some animalshelp others at a cost to themselves?
Darwinian Selection Hamilton’s inclusive fitness
Direct Fitness Personal reproduction
Indirect Fitness Relatives reproduction
Br - C > 0 What are the units of B and C?
Fitness must be in addition tothat which you would havegotten from the relativeregardless of the behavior!
Darwinian Selection
Who will you save from afire? Brother Cousin 1 Cousin 2 Father Friend Mother Sister Spouse
Darwinian Selection
Who will you save from a fire? Brother - Age 21 Cousin 1 - Age 19 Cousin 2 - Age 20 Father - Age 65 Friend - Age 23 Mother - Age 63 Sister - Age 22 Spouse - Age 25
Darwinian Selection Darwinian Selection
Who will you save froma fire? Brother - Age 21
Cousin M - Age 23 Father - Age 65
Mother - Age 63 Sister - Age 22
Cousin F - Age 19 Spouse - Age 25
Cousin F - Age 20
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Darwinian Selection
Who will you save froma fire? Brother - Age 21
Cousin M - Age 23 Father - Age 65
Mother - Age 63 Sister - Age 22
Cousin F - Age 19 Spouse - Age 25
Cousin F - Age 20
Darwinian Selection
Who will you save froma fire? Brother - Age 21
Cousin M - Age 23 Father - Age 65
Mother - Age 63 Sister - Age 22
Cousin F - Age 19 Spouse - Age 25
Cousin F - Age 20
Darwinian Selection
Who will you save froma fire? Brother - Age 21
Cousin M - Age 23 Father - Age 65
Mother - Age 63 Sister - Age 22
Cousin F - Age 19 Spouse - Age 25
Cousin F - Age 20
Darwinian Selection
Who will you save froma fire? Brother - Age 21
Cousin M - Age 23 Father - Age 65
Mother - Age 63 Sister - Age 22
Cousin F - Age 19 Spouse - Age 25
Cousin F - Age 20
Darwinian Selection
The Modern Synthesis Combines genetics, geology, and natural
selection Gradual evolution results from small genetic
changes acted upon by natural selection Speciation and macroevolution can be
explained by microevolution given the age ofthe earth
Darwinian Selection
Modern Synthesis - Darwin’s postulates restated Mutation is the ultimate source of all variation,
segregation and independent assortment createnovel combinations of alleles
Alleles are passed between generations More offspring are produced than can survive Individuals with the most advantageous allelic
combination for the current situation produce moreoffspring.
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God does not playdice with theuniverse.
Albert Einstein
Who are you totell God whatto do?
Neils Bohr
Not only does Godplay dice, but hethrows them wherewe cannot seethem.
Stephen Hawkings
Happy is he whogets to knowthe reason forthings.
Virgil
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