Chapter 6Evolution in Large Populations I: Natural Selection & Adaptation

Species have to cope with a plethora ofEnvironmental changes.

Red Queen HypothesisRed Queen Hypothesis -- Adaptations in competitors, parasites, and pests are so common that species have to continually evolve to avoid falling behind competing organisms.

There are limits to physiological adaptations.

If environmental changes are greater than anyindividual can cope with then the species becomes extinct.

Evolutionary change through natural selection isan alternative means (vs. physiological adaptation)for adjusting to environmental change.

This is Adaptive EvolutionAdaptive Evolution!

Natural SelectionNatural Selection -- differential reproduction &survival of different genotypes.

When adaptive evolutionary changes occur overlong periods of time, they may allow a populationto cope with conditions more extreme than anyindividual could originally tolerate.

Adaptive evolution is observed when large genetically variable populations are subjected toaltered biotic or physical environments.

Conservation Importance of Adaptive EvolutionConservation Importance of Adaptive Evolution

Preservation of ability of species to evolve inresponse to new environments.

Loss of adaptive evolutionary potential in smallpopulations.

Most endangered species exist on the periphery oftheir historic range so they must adapt to what was previously marginal habitat.

Genetic adaptation to captivity and its deleteriouseffects on reintroductions.

Adaptation of translocated populations to theirnew environment.

Conservation biology is concerned with preservingspecies as dynamic entities that can evolve to cope with environmental change.

Retaining the ability to evolve requires thepreservation of genetic diversity.

Consequently, we must understand the factorsthat influence the evolution of natural populations.

An evolving population is a complex system influenced by mutation, migration, selection, andchance operating within the context of thebreeding system.

To understand this complexity, we use modellingwith no factors, one factor, two factors etc.

In its simplest form, evolution involves a changein gene frequency and its importance can be summarized as:

Mutation is the source of all genetic diversity but is a weak evolutionary force over theshort-term.

Selection is the only force causing adaptiveevolutionary change.

Migration reduces differences between populationsgenerated by mutations, selection, and chance.

Chance effects in small populations lead to lossof genetic diversity and reduced adaptiveevolutionary potential.

Fragmentation and reduced migration lead to random differentiation among subpopulationsderived from the same original source population.

Selection arises because different genotypes havedifferent rates of reproduction and survival(reproductive fitness) and such selection changesallele frequencies.

Selection operates at all stages of life-cycle.

In animals this involves mating ability and fertilityof males and females, fertilizing ability of sperm,number of offspring per female, survival or offspring to reproductive age and longevity.

The most intensive selection that can apply againsta recessive allele is when all homozygotes die(lethal).

For example, all individuals homozygous for Chondrodystrophic dwarfism (dwdwdwdw) in endangeredCalifornia condors die around time of hatching.

Modeling impact of selection against Chondrodystrophy in California condors.

Genotype++++ +dw+dw dwdwdwdw totaltotal

Zygotic p2 2pq q2 1.0FrequencyRelative 1 1 0(lethallethal)FitnessAfter p2X1 2pqX1 q2X0 1 -q2

SelectionAdjusted 0 1Frequency p2

(1-q2)2pq

(1-q2)

The frequency of the dwdw allele in the next Generation (q1) is:

q1 = q/(1 + q)

The change in frequency (q) = -q2/(1 + q)

Thus, the lethal allele always declines in frequency.

ImportanceImportance: it becomes progressively harder to reduce the frequency of the deleterious recessiveallele as its frequency declines!

In conservation genetics we are concerned withboth selection against deleterious mutations andselection favoring alleles that improve the abilityof a population to adapt to changing environments.

Prior to industrial revolution, its peppered wingsprovided camouflage as it rested on lichen-coveredtree trunks.

Sulfur pollution killed most lichen and soot darkenedtree trunks. Previously rare dark variants (melanics) were nowbetter camouflaged.

Melanic form was first reported in 1848 but by1900 they represented 99% of all moths in thispart of England.

Simple model for this type of selection:

Beginning frequencies of melanic (MM) and typical (mm)Alleles with frequencies of pp and qq, respectively.

Assumptions:Large random mating populationno migrationno mutationselection occurs on adults but before reproductionmm individuals have a relative fitness of 1 - swhere s is the selection coefficient.

MM Mm mm totalZygoticFreq. pp22 2pq2pq pp22 1.01.0

RelativeFitness 11 11 00

AfterSelection pp22 2pq2pq qq22(1 - s)(1 - s) 1 - sq1 - sq22

AdjustedFreq.

pp22

(1 - sq(1 - sq22))2pq2pq

(1 - sq(1 - sq22))(q(q22 - sq - sq22))(1 - sq(1 - sq22))

Frequency of M after selection (p1):

Change of M (p):

Since the sign of p is positive, melanic allele increases in frequency.

Rate of increase depends upon the selectioncoefficient (s) and allele frequencies.

pp(1 - sq(1 - sq22))

(spq(spq22))(1 - sq(1 - sq22))

1848, frequency of M = p = 0.005 and typicals hadOnly 70% survival of melanics (s = 0.3) then:

p1 = p/(1 - sq2) = 0.005/(1 - (0.3 X 0.9952)] = 0.0071

Change in frequency (p) =

p1 - p = 0.0071 - 0.005 = 0.0021

Models of 4 different degrees of dominance aregiven in Figure 6.5Figure 6.5

In each case, the selection coefficient (ss) represents the reduction in relative fitness of thegenotype compared to that in the most fitgenotype (Fitness = 1.0Fitness = 1.0).

Values of s range from 0 to 1.

Additive CaseAdditive Case -- heterozygote has a fitnessintermediate between the two homozygotes.

Completely Dominant CaseCompletely Dominant Case -- heterozygote has afitness equal to the AA11AA11 homozygote.

Partial Dominance CasePartial Dominance Case -- heterozygote has a fitness nearer one of the homozygotes than theother with its position on the scale depending onthe value of hh.

Overdominant CaseOverdominant Case -- heterozygote has higherfitness than either homozygote.

The length of time it takes for an allele frequencyto change by a given amount of selection dependsupon the intensity of selection and on the modeof inheritance.

For a recessive lethal allele we can determine thenumber of generations to change an allele freq. from q0 to qt as:

t = 1/qt = 1/qtt - 1/q - 1/q00..

Directional SelectionDirectional Selection:

Captive populations are likelyto show adaptation to captivity.

Typically results in decreasedfitness when returned to the wild.

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Stabilizing SelectionStabilizing Selection:

Favors intermediate phenotype.

Expected to reduce geneticvariation.

May cause phenotypic stabilizingselection, leading to retentionof genetic diversity.

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Disruptive Selection:Disruptive Selection:

Favors both phenotypic extremes

May lead to increased variationIn future.

In fragmented habitats this leadsTo adaptation in each local Environment.

Speciation is possible.

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