BIOL2202 L26 Lecture notes PDF

Preview

Summary

Module 5: Population Genetics & Genomics
Lecture 26: Chance & Necessity...

Description

Lecture 26! Chance and necessity!

Daniel Ortiz-Barrientos, PhD!

Natural'selec+on

If there is variation, fitness differences, and inheritance, then the trait frequency distribution will differ among age classes or life-history stages beyond that expected from ontogeny Endler 1986

Varia+on

Differen+al'reproduc+on'

Heredity aa

Aa

X

50%

50%

Natural'selec+on After

Frequency

Frequency

Before

If there is variation, fitness differences, and inheritance, then the trait frequency distribution will differ among age classes or life-history stages beyond that expected from ontogeny Endler 1986

Natural'selec+on

Natural selection proceeds not for biological reasons, but from the laws of probability. Only variation, fitness differences, and inheritance contain biological content Endler 1986

Natural'selec+on

Allele$frequencies$change$systema3cally$in$popula3ons$ because$of$differen3al$survival$and$reproduc3on$among$ genotypes.

Natural'selec+on p(A)

Allele$frequencies$change$ systema3cally$in$popula3ons$ because$of$differen3al$survival$ and$reproduc3on$among$ genotypes.

Time

Fitness • Fitness,$symbolized$by$w,$is$the$ability$to$survive$and$ reproduce.$ • Each$member$of$the$popula3on$has$its$own$fitness$ value:$ 0$if$it$dies$or$fails$to$reproduce,$ 1$if$it$survives$and$produces$1$offspring,$ 2$if$it$survives$and$produces$2$offspring,$etc.$ • The$average$fitness$of$the$popula3on$is$calculated$by$ averaging$the$fitness$of$individuals.$

Average$Fitness$and$Popula3on$Size

A

Rela+ve'Fitness

a

• Example:$in$a$par3cular$species$of$insect,$fitness$is$ determined$by$a$single$gene$with$two$alleles,$A$and$a.$ • Allele$A$causes$insects$to$be$dark$and$allele$a$causes$ them$to$be$light.$A$is$completely$dominant$to$a.$ • In$forests,$dark$insects$(AA$and$Aa)$survive$beMer,$but$ in$open$fields,$light$insects$(aa)$survive$beMer.

Rela+ve'Fitness • In$each$environment,$the$fitness$of$the$ superior$genotype(s)$is$defined$as$1.$The$ fitness$of$the$inferior$genotype(s)$is$ expressed$as$a$devia3on$from$1.$ • The$fitness$devia3on,$s,$is$the$selec+on' coefficient,$and$measures$the$intensity$of$ natural$selec3on$ac3ng$on$the$genotypes$in$ the$popula3on.

Rela+ve'Fitness Genotype

AA

Aa

aa

Phenotype

Dark

Dark

Light

Rela3ve$fitness$ 1 in$the$forest

1

1;s1

Rela3ve$fitness$ 1;s2 in$the$field

1;s2

1

Natural'Selec+on'in'the'Forest'Habitat • Assume$that$ini3ally,$p$=$0.5$and$q!=!0.5$and$ that$s1$=$0.1.$ • Assume$that$the$popula3on$mates$randomly$ and$that$the$genotype$are$present$in$HardyU Weinberg$frequencies$at$fer3liza3on$in$each$ genera3on.

Natural'Selec+on'in'the'Forest'Habitat • The$Ini3al$gene3c$composi3on$of$the$ popula3on$is:$ • WAA)=$1,$and$the$frequency$of$AA'at$fer3liza3on$ is$p2$=$0.25$ • WAa)=$1,$and$the$frequency$of$Aa'at$fer3liza3on$ is$2pq$=$0.50$ • WAA)=$(1U0.1),$and$the$frequency$of$aa#at$ fer3liza3on$is$q2$=$0.25

Natural'Selec+on'in'the'Forest'Habitat • In$forming$the$next$genera3on,$each$ genotype$will$contribute$gametes$in$ propor3on$to$its$frequency$and$rela3ve$ fitness.$The$rela3ve$contribu3ons$of$the$ three$genotypes$will$be$ – For$AA,$(0.25)$×$1$=$0.25$ – For$Aa,$(0.50)$×$1$=$0.50$ – For$aa,$(0.25)$×$(0.9)$=$0.225

Natural'Selec+on'in'the'Forest'Habitat • To$obtain$the$propor3onal$contribu3ons$of$ each$genotype$to$the$next$genera3on,$ divide$their$rela3ve$contribu3ons$by$their$ sum$(0.25$+$0.50$+$0.225$=$0.975).$ • The$propor3onal$contribu3ons$to$the$next$ genera3on$are$ – For$AA,$0.25$/$0.975$=$0.256$ – For$Aa,)0.50$/$0.975$=$0.513$ – For$aa,$0.231$/$0.975$=$0.231

Natural'Selec+on'in'the'Forest'Habitat

• In$the$next$genera3on$all$of$the$alleles$ transmiMed$by$aa$homozygotes$are$a,$and$ half$the$alleles$transmiMed$by$the$Aa$ heterozygotes$are$a.$ • The$frequency$of$a$in$the$next$genera3on,$ symbolized$q'$will$be$ $ $q'$=$0.231$+$(1/2)(0.513)$=$0.487

Natural'Selec+on'in'the'Forest'Habitat

Natural'Selec+on'in'the'Field'Habitat

Natural'Selec+on'in'the'Field'Habitat • Exercise:$ • Assume$that$ini3ally,$p)and$q)are$equally$ frequent,$and$that$s2)=$0.1$ • Assume$random$ma3ng$and$HW$equilibrium$at$ fer3liza3on$every$genera3on$ • Calculate$the$ini3al$gene3c$composi3on$of$the$ popula3on$ • Calculate$the$frequency$of$a$a`er$one$and$two$ genera3ons$of$selec3on$in$the$field$

Natural'Selec+on'for'Quan+ta+ve' Traits • Fitness$can$be$influenced$drama3cally$by$different$ alleles$of$a$single$gene$but$is$more$o`en$influenced$ by$the$allele$of$many$genes$that$affect$quan3ta3ve$ traits$such$as$body$size,$disease$suscep3bility,$and$ fecundity.$ • Natural$selec3on$can$affect$the$distribu3on$of$a$ quan3ta3ve$trait$through$direc3onal$selec3on,$ disrup3ve$selec3on,$or$stabilizing$selec3on.

Types'of'Natural'Selec+on • Direc+onal'selec+on$favours$values$of$a$trait$at$one$ end$of$its$distribu3on.$ • Disrup+ve'selec+on$favours$extreme$values$of$a$trait$ at$the$expense$of$intermediate$values.$ • Stabilising'selec+on$favours$intermediate$values$of$a$ trait.

Key'Points • Natural$selec3on$occurs$when$genotypes$differ$in$ the$ability$to$survive$and$reproduce—that$is,$ when$they$differ$in$fitness.$ • The$intensity$of$natural$selec3on$is$quan3fied$by$ the$selec3on$coefficient.

Key'Points • At$the$level$of$the$gene,$natural$selec3on$changes$ the$frequencies$of$alleles$in$popula3ons.$ • At$the$level$of$the$phenotype,$natural$selec3on$ influences$the$distribu3ons$of$quan3ta3ve$traits.$ • Natural$selec3on$may$be$direc3onal,$disrup3ve,$or$ stabilizing.

Random'Gene+c'DriH Allele$frequencies$change$unpredictably$ in$popula3ons$because$of$uncertain3es$ during$reproduc3on.

Random'Changes'in'Allele' Frequencies

Random'Gene+c'DriH • For$every$pair$of$parents$segrega3ng$different$alleles$ of$a$gene,$there$is$a$chance$that$the$Mendelian$ mechanism$will$lead$to$changes$in$the$frequencies$of$ the$alleles.$ • When$these$random$changes$are$summed$over$all$ pairs$of$parents,$there$may$be$aggregate$changes$in$ the$allele$frequencies$even$without$the$force$of$ natural$selec3on.$

Simulating allelic frequency change over time # Wright-Fisher simulation # n = number of individuals # f = number of focal alleles at base population n=50 f=1 pop = as.matrix( c( rep(0,n-f), rep(1,f) ) ) pop = as.matrix( sample(pop, n, replace=T) )

Vary this number and re-run the simulation

Drift_graph = function(t,R){ N...