section 4 evolution in large populations: mutation, migration & selection genetic diversity lost...

Section 4Evolution in Large Populations: Mutation, Migration & Selection

Genetic diversity lost by chance and selectionregenerates through mutation.

When genetic diversity is lost in small threatenedpopulations, it can be recovered by migration fromother genetically distinct populations.

Migration often reverses effects of inbreeding.

Many rare species are being hybridized out ofexistence by crossing with common related species.

Mutation and migration are often importantdeterminants in the maintenance of geneticdiversity.

Balance between deleterious mutations & selectionresults in an ever-present but changing genepool of rare deleterious mutations (mutation loadmutation load)in the population.

Inbreeding exposes these mutations, resulting inreduced reproduction & survival which in turnincreases the extinction risk in threatened species.

Genetic diversity is the raw material required foradaptive evolutionary change.

However, genetic diversity is lost by chance insmall populations and as a result of directionalselection.

Mutation is the ultimate source of genetic diversitywhile recombination can produce new combinationsof alleles.

If genetic diversity is lost, it can be regeneratedvia mutation, but this is a very slow process.

Alternatively, genetic diversity can be restoredby natural or artificial immigration betweenpopulations with different allelic content.

Mutations are sudden changes in an allele or chromosome.

All genetic diversity originates from mutations.

Patterns of genetic diversity in populations are theresult of a variety of forces that act to eliminateor increase & disperse mutations among individuals and populations.

Conservation Concerns with regards to mutations:Conservation Concerns with regards to mutations:

How rapidly mutations add genetic diversity to populations.

How mutations affect the adaptive potential andreproductive fitness of populations.

How important are the accumulation of deleteriousalleles to fitness decline in small populations.

The most important mutations are those at lociaffecting fitness traits, most notably, lethal ordeleterious mutations.

G

C T

ATransition

Substitutions

TransversionTransversionSubstitutionsSubstitutions

Silent SubstitutionSilent Substitution: Base substitution that DOES NOTDOES NOT change an amino acid.

These probably have little or no impact on fitness and therefore are also referred to as Neutral MutationsNeutral Mutations.

Neutral mutations are important as molecular markers and clocks that provide valuable information on genetic differences amongindividuals, populations, & species.

Rate of mutation is critical to its role in evolution.

Mutation rates differ for different classes ofloci.

Although spontaneous mutations are consideredto be nearly constant over time, mutation ratesmay be elevated under stressful conditions andby particular environmental agents (radiation,mutagens).

Mutation is normally a recurrent process wheremutations continue to arise over time.

Mutation RateMutation Rate:A1 A2

Initial Allele FrequencyInitial Allele Frequency: p0 p1

p = -p0

The time taken to regenerate genetic diversity isa major issue in conservation biology because itmay take thousands to millions of generations toregenerate genetic diversity at a single locus.

Time to regenerate genetic diversity due to mutation:

pptt = p = p00(1 - (1 - ))tt or p or p00ee--tt t = (lnpt = (lnp00 - lnp - lnptt)/)/

ExampleExample: How long will it take a microsatellite locusto regenerate a frequency of 0.5 for an allele thathas been lost?

p0 = 1.00 pt = 0.5 = 1 X 10-4

t = [ln 1.00 - ln 0.50]/1 X 10-4 = 6,931 generations!6,931 generations!

Mutations typically occur in both directions andsince there are two opposite forces, this usuallyresults in an equilibrium.

A1 A2

Stable Equilibrium: q = / ( + )ˆ

Most mutations not occurring in functional lociare expected to be neutral or nearly neutral.

Mutations within functional loci will predominantlybe deleterious and some are lethal.

While selection can remove deleterious alleles from the population, the time taken is so longthat new deleterious mutations will arise beforeprevious deleterious mutations have been removed,especially for recessive alleles.

Eventually, an equilibrium is reached between theaddition of deleterious alleles by mutation and their removal by selection.

This is known as mutation - selection balancemutation - selection balance.

Consequently, low frequencies of deleterious allelesare found in all naturally outbreeding populationsand this is known as the mutation loadmutation load.

Mutation Loads:

Mutational loads are found in essentially ALLspecies, including several threatened & endangered.

Deleterious alleles are normally found only atlow frequencies, typically much less than 1% atany locus.

Deleterious alleles are found at many loci.

Deleterious alleles increase due to mutation rate(pp) and are removed by selection at a rate of:(-spq(-spq22)/(1-sq)/(1-sq22)) therefore:

q is approximately p - spqp - spq22

At equilibrium q = 0, so p ≈ spq2 and q2 ≈ /s

Therefore, the equilibrium frequency is:

q ≈ (q ≈ (/s)/s)0.50.5ˆ̂

MigrationMigration: Gene pools of populations divergeover time due to chance events and selection.

Such divergence may be reduced by migration whichcan have very large effects on allele frequencies.

Change in allele frequency due to migration:

q = m(qq = m(qmm - q - q00))

Where mm = migration coefficient, qqmm = allele freq.in migrant population, qq00 = allele frequency inoriginal population.

ExampleExample: You have a mainland population of 1,000bats with an allele frequency (qm) of 0.75.

200 individuals from the mainland migrate to anearby island that contains a population of 150 individuals with an allele frequency (q0) of 0.40.

Of the 200 migrants, only 100 are able to breed.

What is the new allele frequency in the islandpopulation in the generation following themigration event?

q = m(qm - q0)qm = 0.75 q0 = 0.40m = migration coefficient = 200 migrate but only

100 breed thus, m = 100/250 = 0.4q = 0.4(0.75 - 0.4) = 0.14q1 = q0 + q = 0.4 + 0.14 = 0.54

n = 1,000qm = 0.75

n = 150q0 = 0.40

200 migrate

Rearrangement of this equation allows examinationff the effect of Introgression.

ExampleExample: Ethiopian wolves are genetically distinctfrom domestic dogs but hybridization occurs inareas where they co-occur, as in Web Valley,Ethiopia.

The population for the Sanetti Plateau is relativelypure.

Extent of admixture from domestic dogs in theweb population can be estimated using allelefrequencies at a particular microsatellite locus.

Dogs lack the “J” allele while “pure” Ethiopianwolves are homozygous for it.

Sanetti population q0 1.00 (“old”)

Web population q1 0.78 (“new” -- containsdog)

Domestic Dog qm 0.00 (“migrants”)

m = (q1 - q0)/(qm - q0) = (0.78 - 1.0)/(0 - 1.0) = 0.22

Based on this, the Web Valley population of Ethiopian wolves contains about 22% of its geneticcomposition from Domestic dog.

It is important to realize that this is anaccumulated contribution, not a per generationestimate.

Migration-selection equilibrium depends only uponthe migration rate (mm), the selection coefficient (ss)and the allele frequency in the migrants (qqmm).

Thus, equilibrium is NOTNOT dependent upon the allelefrequency in the initial population.

When migration rates are high and selection isweak, migration dominates the process and canerase local adaptation.

Conversely, when migration rates are low andselection is strong, there will be local adaptation.

At equilibrium q = 0 and:

q = (2m + s) ± [{2m + s)2 - (8s m qm)}/2s]0.5ˆ

Although there are 2 solutions to this equation, because the allele frequency has to be between0 and 1, only one solution will be correct.

Migration-selection balance can arise betweenwild and captive populations when there is regularmovement of wild individuals into captivity orvice versa.